Meadow Vole Control, Guardian Pest Solutions

Meadow Vole

Description

Meadow voles also known as “meadow mice” or just plain “voles” are blunt-nosed, short-tailed mice that love to eat bulbs, hostas and tree roots, but when those are not available they will eat grass roots. Voles are heavy vegetation eaters and considered the most abundant rodent, with an ability to multiply from two to 100 within a single year.

Importance

Meadow voles will move from empty fields or uncultivated areas to adjacent lawns, gardens and landscaping in search of food and shelter.

Voles are not a health risk to humans and pets because they typically are not found indoors, but are considered a nuisance pest when they create trails across yards and destroy expensive landscaping. They can cause severe damage especially in the winter months when they are active under snow cover. The damage is noticeable in the spring after the snow melts.

Control Methods

Voles are a difficult pest to eradicate. Due to the complexity of treatment, voles are generally not a pest many people can eliminate on their own. Our solution includes a thorough inspection, highly effective control measures and active monitoring.

Guardian Pest Solutions uses the least amount of material possible to solve your meadow vole problem. We utilize natural products, baits and mechanical means as forms of treatment. When conventional materials are required, we follow very stringent guidelines to pose no threat to people, pets or plants. All products are EPA approved, and our technicians are highly trained and will inform you of any specific safety measures that need to be taken.

Because meadow voles are so difficult to prevent and control, contacting Guardian Pest Solutions will be the best choice in meadow vole prevention.

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Control measures

Selecting the most appropriate control measures

Table 1.2 shows where and when the different groups of biting Diptera are active. Personal protection measures, such as repellents and protective clothing, are effective against all of them. Bednets are effective against Diptera that bite at night. Measures to make houses and shelters insect-proof work against species that enter houses to feed and rest. Mosquitos can usually be prevented from breeding in and around houses by simple, long-lasting measures. However, some biting may continue because of mosquitos flying in from adjacent plots of land where they still breed. Cooperation between neighbours is therefore important in order to achieve good control.

The self-protection methods can be selected without knowing exactly which species one wishes to control. They are mainly used to protect individuals, families or small groups of people living together. Methods such as the use of insecticide-treated bednets, house improvement and the prevention of breeding may also be used to reduce diseases in a community if the majority of the people participate.

The methods for disease control in the community are usually implemented on a large scale and require at least some support and participation by a local health care organization. Health workers with experience in the control of vector-borne disease should be consulted for the selection and implementation of the most appropriate control strategy for the local situation.

Table 1.2. Selection of control measures for biting Diptera a

Disease control in the community

Insectproofing of houses

Prevention of breeding in and around houses

Other control methods
(adults)

Prevention of breeding in field

Other control methods
(adults)

a + + = effective; + = usually effective; +/- = sometimes effective; — = not effective.

b In the case of epidemic outbreaks, ultra-low-volume insecticide space-spraying can be considered.

c Anopheles does not usually breed near houses in urban areas, with the exception of A. stephensi in southern Asia. In Africa, malaria transmission occurs in the semiurban fringes of cities with prevailing rural conditions.

d It may be possible to obtain some additional protection by diverting mosquitos to domestic animals (see p. 105).

e Against Culex quinquefasciatus.

f Siting animal shelters far away from rice fields was effective in Japan (14).

g Control of the larvae of Culex tritaeniorhynchus in rice fields in Asia is difficult but may sometimes be achieved by intermittent irrigation, the use of larvivorous fish, and the application of bacterial larvicides.

h To control pest mosquitos breeding in rural areas, such as tidal salt marshes, granular insecticides are sometimes used which only release the active agent after flooding with water, which coincides with the hatching of the eggs. Other methods include the control of water levels and the improvement of irrigation and drainage systems.

i Sometimes by removing or destroying the aquatic vegetation to which the larval and pupal stages are attached (see p. 18).

j By application of larvicides to streams and rivers (see p. 45).

k By avoiding places where sandflies are known to rest and breed.

l Sometimes biting midges enter houses or tents.

m Where feasible the draining or filling of marshy areas is highly effective but is often too costly. In some cases, aerial spraying of such places with insecticides provides effective but temporary control by killing the larvae.

n Protection from bites is possible with thick clothing. Commonly available repellents are moderately effective against tabanids.

o Methods that reduce or stop feeding on domestic animals not only benefit them but also the people living near their quarters. Commercially available insecticide-impregnated eartags for animals are highly effective against Stomoxys calcitrans for between one and two months.

p See Chapter 2.

q Includes the use of traps and screens and spraying of daytime resting places of the flies with residual insecticides.

Personal protection

Personal protection methods, used by individuals or small groups of people to protect themselves from biting insects and the diseases they may carry, act by preventing contact between the human body and the insects. The equipment is small, portable and simple to use. The methods may offer significant protection against infection to individuals and sometimes have an impact on disease transmission in communities when a large proportion of people use them.

Repellents

Repellents are among the most commonly used methods to prevent mosquitos and other blood-sucking pests from biting. They are applied directly to the skin or to clothing and other fabrics such as bednets and anti-mosquito screens. Repellents evaporate much more quickly than most insecticides. Insecticides last longer and act by killing or knocking down insects after contact, whereas most repellents act by preventing human — insect contact and do not knock down or kill. The duration of protection by a repellent applied to skin may range from 15 minutes to 10 hours; on clothing and other fabrics the effect lasts much longer. The effectiveness and duration depend on the type of repellent (active ingredients and formulation; see Fig. 1.37), the mode of application, local conditions (temperature, humidity, wind), the attractiveness of individual people to insects, loss due to removal by perspiration and abrasion (1517) and the sensitivity of the insects to repellents, each species having its own specific sensitivity (1820). The biting density also plays an important role: the more mosquitos there are, the more one is likely to be bitten.

Under certain conditions the user may be completely protected while in other situations the protection is limited. People working or travelling in humid tropical forests are likely to need repeated application of repellents to the skin because of quick removal by perspiration (21). Because of the short period of action, repellents are mostly applied when insects start biting. For mosquitos this is very often around sunset.

Fig. 1.37. Repellents are available as sprays, lotions, creams, sticks and wipe-on applicators or tissues.

When and where to use

Repellents are valuable for people in situations where other protective measures do not work, are impractical or are prohibited: people who must be outdoors at night; plantation workers at risk during daytime; people crossing or approaching areas such as tundras, swamps, grasslands or forests infested with mosquitos or other biting insects; and so on. Repellents may be preferred for use indoors if the screening of a house is impossible or considered unpleasant because it reduces ventilation too much in hot climates. Travellers often favour repellents as they are easily transportable and they can be applied anywhere at any time. Repellents can play an important role in combination with other methods for the control of mosquitos or biting flies; for example their use in the early evening can be followed by the use of bednets against indoor night-biting mosquitos.

Repellents are widely available but their retail price may be too high for daily use by many people. The various types differ in effectiveness according to their composition.

Instructions for use

Whatever repellent is used, it should be applied sparingly to all exposed skin, especially the neck, wrists and ankles. The surroundings of the eyes or mucous membranes (nose, mouth) should not be treated. Repellents should not be sprayed on the face; instead they can be applied by spraying on to the hands (Fig. 1.38) and then rubbing on to the less sensitive parts of the face as necessary. If an allergic skin reaction is observed the treated skin should be washed with water and a physician consulted and shown the can or other packing material (Fig. 1.39). A repellent can be checked for adverse skin reactions by applying a small quantity to the back of the hand.

Fig. 1.38. Spray cans are used to apply repellent to exposed body parts.

Types of repellent

Traditional or natural repellents

Various substances and methods of application have been used since ancient times to repel blood-sucking insects (22). Smoke from an open fire repels insects, especially in still air or a poorly ventilated dwelling. The repellent effect of smoke may be increased by burning certain materials such as aromatic wood containing resins or various types of plant. In southern India, leaves of Vitex negundo (“nochi”) are burned to repel mosquitos from houses.

The oils of some plants, such as citronella, are repellent when applied directly to the skin or clothing but their protective effect is very brief. It has sometimes been prolonged by mixing the volatile repellent with animal fat or oil to reduce the rate of evaporation. Many traditional repellents have the disadvantages that:

— they last a very short time;
— they are unpleasant to use (strong odours, irritating);
— they may have unhealthy side-effects (e.g. smoke).

However, their advantages are that:

— they are easily available;
— they are locally known and acceptable;
— they are inexpensive.

In this manual it is impossible to mention all the locally used traditional repellent substances and their application methods. Many of these substances have never been tested by scientists and their effectiveness remains to be confirmed.

Some plant products used as repellents which are safe for humans

Oil from the citronella plant is widely used as a repellent. Industrially produced citronella is an active ingredient in some commercial repellents. When freshly applied to human skin, citronella is about as effective in repelling some biting insects as the chemical repellents, but for only about an hour.

In Africa, Asia and Latin America, leaves of the neem tree (Azidarachta indica) are sometimes burnt, producing an unpleasant odour, or hung dried inside houses. Some people believe that neem trees near a house keep mosquitos away but there is no scientific evidence of this. Extracts of neem seeds are used as agricultural insecticides.

The wood or the extracted resin of certain aromatic trees is sometimes burnt as a mosquito repellent. In some African countries such wood is sold in local markets (23).

Modern repellents for application to the skin

During the latter half of the twentieth century, several synthetic repellents have been produced which are long-lasting, nontoxic, cosmetically acceptable on the skin, and effective against a wide variety of insects. The most successful substances for skin application, developed in the first half of this century, were dimethyl phthalate, indalone and ethyl hexanediol. These substances are still among the active ingredients of some commercial repellents.

Fig. 1.39. A repellent-impregnated tissue taken out of its airtight package. The tissue is saturated in a mixture of deet and alcohol. It is used for wipe-on application.

A breakthrough came in 1954 with the discovery of N,N-diethyl-3-toluamide or deet, a colourless, oily liquid with a slight odour. It is still the best available product, repelling a wide variety of insects, ticks and mites and generally lasting longer than the other repellents (1820, 2427). Deet is also effective against blood-sucking terrestrial leeches (28, 29).

Deet is available as a pure liquid and in 5 — 90% solutions. To make deet and other repellents more convenient to apply and cosmetically attractive, they are often prepared as lotions, creams, foams, solid waxes (sticks) or spray-on preparations in pressurized containers. The repellents are often mixed with an oily or alcoholic base and a pleasant smelling perfume. The mixtures are spread, wiped or sprayed onto the exposed skin.

In some mixtures the base material (oils, silicones, polymers) reduces the evaporation rate of the repellent, thus extending the duration of efficacy (1517). In some formulations of deet the repellent effect may last up to 12 hours, although 4 — 6 hours is more common. A disadvantage of some extended-duration formulations is that they may feel sticky when applied to the skin; this does not happen with an ethanol solution of deet.

Allergic or other serious reactions to deet, such as the development of rash, have rarely been reported (3032). The compound is considered safe for adults, except following prolonged exposure to high concentrations. Since children appear more sensitive, it is recommended that their skin exposure be kept to a minimum whenever possible and that deet should be applied to their clothing, rather than to their skin (33). Some plastic materials (e.g., pens, watch faces, spectacle frames, car seat covers) and painted surfaces may be dissolved or damaged by deet.

Data from India suggest that N,N-diethylphenylacetamide (DEPA) is as effective as deet but less expensive (34). Citronella is often used because it is inexpensive and some people think that it has a more pleasant smell. Less commonly available are dimethyl phthalate and some carboxylic compounds. They are mixed with deet in some commercial formulations. Mixtures of different repellent substances may be effective against a wider variety of insects than single repellents.

This is a recently developed inexpensive personal repellent that provides relatively long-lasting protection. It is made of materials used in soap production, such as coconut oil, and contains 20% deet and 0.5% permethrin. The bar is used by wetting it (or the skin) and producing a lather that is rubbed on exposed parts of the body (Fig. 1.40). The face can be protected by application to the neck, forehead and ears. After application a white lotion-like film remains on the skin for a short time. The residual film feels sticky and some users find it unpleasant. It is not easily removed by contact with clothes but can be removed by rinsing or rubbing. The method is considered safe but care should be taken to avoid sensitive skin areas when it is used on small children. However, it is not yet recommended by WHO for long periods of repeated daily usage, pending a full safety evaluation.

The repellent bar should be applied at sunset to provide protection during the evening. Depending on the local mosquito species and other factors, the repellent soap protects for 4 — 8 hours. Under optimal conditions, protection lasting up to 12 hours may be achieved. The amount and duration of protection have been reported to vary for different species of insects and different conditions of use (3539).

Fig. 1.40. The repellent bar.

A 40-gram bar, used daily and sparingly on arms, legs and other exposed areas, lasts approximately 20 days. Although the bar is patented, the patent holder permits local production for noncommercial purposes. The procedure and ingredients are similar to those for the production of soap.

Crude raw coconut oil

Antioxidant, such as butylated hydroxyanisole (BHA)

Pharmaceutical-grade permethrin (25/75 cis/trans permethrin)

Perfume base (e.g., rose, oil of lavender)

Caustic soda solution

The ingredients should be obtainable from most pharmacies. Deet can be obtained from most chemical suppliers. Technical-grade permethrin is a suitable alternative to the pharmaceutical-grade compound.

Mix the permethrin with deet at room temperature and add to the coconut oil in which the antioxidant has been dissolved. Heat the resultant blend to 40 °C and add the perfume base. To this blend, add the caustic soda solution at ambient temperature, with rapid stirring. When all the caustic soda has been added, sprinkle the clay in and pour the emulsion into moulds, where the reaction continues for 12 hours. The following day, cut the blocks into 40-g bars. If the bars are wrapped in polypropylene film and placed in an airtight box the product will retain its effectiveness for more than two years. If they are packaged in a small plastic sandwich bag, or placed unwrapped in an airtight box, the shelf life is one year. If the product will be used up within a few weeks of manufacture the lower-cost packaging is sufficient.

Protective clothing

Clothing can offer protection from biting insects when it is of a thickness and texture through which insects cannot easily bite. Lighter colours generally attract fewer insects than darker colours. Boots can protect the ankles from biting insects. Thick socks in combination with long trousers offer protection when the bottoms of the trousers are tucked into the socks. Some protection is also offered by long-sleeved shirts, headnets, collars and hats. However, some insects can bite through socks or other clothes; the treatment of clothing with an insecticide or repellent can deter this.

The small biting midges, sandflies and blackflies are unable to bite through clothes, even if these are made of thin material (40). People active during daytime can best protect themselves by wearing thin clothing over as much of the body as possible and applying repellents to the parts of the body left exposed (26, 41). Repellents are only partially effective against swarms of biting midges. Headnets or hooded wide-mesh jackets impregnated with a repellent offer good protection (22, 4244).

A vest has been developed in the former USSR which is too thick for mosquitos to feed through and which allows the user sufficient aeration of the body. It consists of an undervest, with long sleeves made from a wide-mesh material of which the fibres are about 0.5 cm thick, covered with a long-sleeved conventional shirt (45).

Treated clothing

Clothing can be treated with repellents to prevent insects from landing or feeding, or with quick-acting insecticides of the pyrethroid group, such as permethrin. These latter compounds do not repel the insects but allow them to make contact with the fabric and irritate or kill them before they manage to feed. The application of repellents to clothing and other fabrics is preferable to skin application because it reduces the likelihood of allergic reactions. Limited contact with the human skin and strong adherence to fabric fibres make it possible to use higher doses of repellents and insecticides.

Synthetic pyrethroid insecticides are generally preferred to volatile repellents for treating clothing because:

— they act quickly and repel or kill biting insects;

— they are long-lasting and to some extent withstand weathering, sunlight and washing in cold water;

— they are more pleasant to use (little or no odour, colour or greasiness);

— they are safe and do not irritate human skin if applied at the correct doses (46);

— they do not affect plastic products;

— they are cheaper than repellents, only infrequent applications of small amounts being required.

However, if the clothing is treated with a non-repellent pyrethroid, flying insects may feed on uncovered skin, necessitating the application of a repellent to the bare skin. Because of the vapour effect, clothing freshly treated with a volatile repellent offers more protection to uncovered skin than that treated with a pyre-throid insecticide.

Impregnated socks can give effective protection against blackflies, which often bite around the ankles. Impregnated trousers and stockings provide effective protection from ticks and mites (47). Treated clothing is also effective against mosquitos, sandflies, biting midges, fleas and body lice (4752). Repellents may remain effective for up to a week when applied to clothing. An extended efficacy can be obtained by sealing the impregnated fabric in a container or airtight bag when not in use to prevent evaporation of the repellent. A repellent applied to clothing normally retains its effect longer than on skin because there is:

· no loss by abrasion;
· no loss due to skin absorption;
· no removal of the active compound by sweating;
· slower evaporation because of lower temperature, except when clothing is exposed to sunlight;
· better adherence to cotton and synthetic fibres.

Clothing treated with permethrin can remain toxic to insects and ticks for several weeks or months, depending on wear and exposure to washing and rain. Treated clothing may remain effective after up to 10 rinses with cold water and soap. However, more permethrin is lost after washing in hot water and soap (50, 52).

Which repellent or pyrethrold?

Any of the repellents considered safe for skin application may be used to treat clothing. Permethrin has been extensively tested and is still considered the insecticide of choice for clothing treatment (46). Some of the other pyrethroids, e.g. cyfluthrin, may also be suitable but most of the safe pyrethroids degrade quickly in sunlight.

How to treat clothing

Clothing can be treated with permethrin by spraying the insecticide from a pressurized can or by soaking in an aqueous emulsion. The recommended dosage for coats, jackets, long-sleeved shirts and trousers is 1.25 g/m 2 (0.125 mg/cm 2 ) and for short-sleeved shirts it is 0.8 g/m 2 (0.08 mg/cm 2 ). A pressurized spray can containing deet may be more easily available. The recommended dosage for deet is 20 g/m 2 (2 mg/cm 2 ), or about 70 g of active ingredient for one piece of clothing. Technical-grade deet suitable for the treatment of fabrics by dipping is available as 30% and 95% mixtures with alcohol. Treatment procedures are described on p. 85.

People sleeping out of doors in places where the nights are cool, and for whom mosquito nets are unaffordable or impractical, could consider covering themselves at night with sheets or other fabrics treated with insecticide or repellent. This method has not yet been tested but it can be expected to be as safe and effective as use of treated clothing. For complete coverage of the body in hot climates it would be possible to use thin, open-weave fabrics that allow unobstructed breathing.

Insect-repellent wide-mesh netting jackets

Special jackets made of wide-mesh netting, with a hood to protect the head, may provide sufficient protection from biting insects when impregnated with deet or other repellents (Fig. 1.41; 43, 5356). They are especially suitable for people on brief visits to areas infested with high densities of mosquitos and other biting insects, as in northern Siberia, Scandinavia and Alaska. Open-mesh material offers the advantages that it can be used in combination with normal clothing or with no clothing beneath and that it is relatively cool.

A disadvantage is that the netting easily gets entangled in dense vegetation; it is most practical in areas with little vegetation. The jackets can be made of strong wide-mesh cotton or a mixture of polyester/cotton or nylon. Mesh jackets sold in Canada and the USA are made of polyester netting containing strands of cotton. Cotton is required to absorb the desired treatment level of 0.25 g of deet per gram of netting (or 10 — 15 g of deet per m 2 ). The jackets should be stored in an airtight plastic bag when not in use.

Wide-mesh netting similar to that used in the jackets described above can be employed to protect the head and neck (Fig. 1.42; 57,58). It is preferably used in combination with a hat or other head covering. The netting allows good visibility and ventilation.

Insect-repellent bands and anklets

Many species of bloodsucking insects bite predominantly around the ankles and wrists. Strips of cotton fitted around the extremities and impregnated with a repellent reduce biting substantially (Fig. 1.43; 18,59). The cotton strips are about 10 cm wide and 35 cm long and can be provided with buttons and buttonholes or can be elasticized (like sweat bands) so that they remain in place.

Fig. 1.41. Wide-mesh netting jackets impregnated with repellent provide protection from mosquitos and other biting insects.

The bands are used with a repellent rather than an insecticide because repellent vapour action protects nearby uncovered areas of the body. When not in use, the anklets should be stored in an airtight plastic bag or tin to reduce evaporation of the repellent. The recommended dosage of deet concentrate (95%) for one band is 4 ml, or the band may be saturated in a 30% deet/alcohol mixture. If used for about 2 hours each evening, deet-impregnated bands remain effective for at least 50 days.

Insect-repellent detachable patches of fabric

The treatment of clothing can be avoided by using detachable patches of fabric impregnated with repellent. The patches can be attached by, for example, press buttons or Velcro strips. In one study (59), four 15-cm × 15-cm pieces on the front of a shirt and one on the back were found to reduce mosquito bites considerably over a period of more than two months when used twice a week. The patches can be treated by soaking in a 10% solution of deet or DEPA and should be stored in airtight plastic bags when not in use. The advantages of the treated patches are that they do not come into direct contact with the skin, they can be removed when clothing has to be washed, and they provide more economical and simpler treatment.

Fig. 1.42. Headnets impregnated with repellent can be used to protect the head and neck from mosquitos and other biting insects.

Fig. 1.43. Anklets impregnated with repellent stop insects from biting the ankles, feet and lower legs.

Insecticide vaporizers

Unlike repellents, only a few insecticides, such as dichlorvos, have a spatial effect at normal room temperature. However, some insecticides kill or repel insects at a distance through an airborne effect when vaporized with a heating device. Insecticides can also be released into the air as aerosols, for example when sprayed from pressurized spray cans.

Dispensers releasing insecticide into the air help to protect people nearby. Traditionally, plants or wood containing repellent or insecticidal substances have been burned (23, 60). More modern devices include mosquito coils, vaporizing mats, dichlorvos dispensers and aerosol spray cans; these are relatively inexpensive and may protect several people at a time. However, their use is confined to houses and other places with limited ventilation. They may be effective in dense vegetation where the repellent is not too diluted by air movements. The compounds used are mostly quick-acting knockdown insecticides with both a killing and a repellent effect, for instance the allethrins, a group of pyrethroid insecticides. The allethrins are considered to be safe to humans if used properly.

Insecticide vaporizers protect against mosquitos and biting flies by:

— preventing them from entering a room (deterrent effect);

— irritating and disturbing them after contact (excito-repellent effect) and preventing them from biting;

— paralysing or killing them (insecticidal effect).

Coils (Fig. 1.44) are among the most popular and widely used insecticide vaporizers because they are easy to use, effective (6166) and inexpensive. Once lit, coils smoulder at a steady rate for 6 — 8 hours, steadily releasing insecticide into the air.

Fig. 1.44. Mosquito coils are among the cheapest and most commonly used insecticide vaporizers.

Originally, mosquito coils consisted of a mixture of pyrethrum powder (see box), a combustible filling material, such as sawdust, and a binder, such as starch. Some of the synthetic pyrethroids, especially knockdown agents like the allethrins, are now commonly used in coils. They are more effective and more easily obtainable than pyrethrum (61). DDT is an ingredient of some brands of coil in China but is ineffective when used in this way (61). To make the smoke more acceptable the coil sometimes incorporates a fragrance. The shelf-life of coils is at least three years if they are packed in paper or plastic and stored in boxes, protected from light and moisture.

The pyrethrum plant (Chrysanthemum cinerariaefolium) contains several active substances (pyrethrins) that are toxic to insects. The active material can be extracted with a solvent from the dried flowers (Fig. 1.45) and stems and has commonly been used in sprays for quick knockdown of flying insects. Dried pyrethrum flowers, ground to a powder, or the extract obtained from them, are used to produce anti-mosquito sticks and coils. However, because of the uncertainty of supplies and the introduction of more effective synthetic pyre-throids, the use of pyrethrum has declined.

Fig. 1.45. The pyrethrum flower (© WHO).

How to use

The coil is placed on a suitable stand and the free end is lit. A metal stand is normally provided in a box of coils. The stand ensures that the coil does not touch or rest on a surface, which might cause it to go out or to set fire to nearby flammable objects. When used indoors, coils mounted on stands should be placed on a fireproof base, such as a saucer or plate and as low as possible in the immediate vicinity of the people to be protected.

The coils should be lit just before mosquitos become active. One coil is sufficient for a normal bedroom (35 m 3 ). In confined areas such as a closed tent or a small closed room, the smoke may cause irritation to the eyes and lungs. For larger spaces, several coils should be placed at different points. If rooms are ventilated or if the coils are used outdoors it is important that they are upwind of the people to be protected.

If lit in the evening, a coil can provide protection until early morning. However, a strong draught in a room with an open door or windows, or windy conditions outdoors, may significantly speed up the rate of burning while dispersing the insecticide and diluting its effect. For better protection a coil should be used during the early evening hours indoors (or a repellent should be applied to exposed skin or clothing outdoors) and a mosquito net should be used indoors during the remaining part of the night.

The efficacy, convenience and safety of coils can be improved by placing them in special containers or holders. Holders may prolong the burning time by up to 20%. Holders also protect the burning coil from wind and rain, and prevent flammable objects from making contact with it. Various models of coil holder are widely available in Asia (Fig. 1.46). They can also be easily made from used cans with the metal stand soldered to the bottom. The can itself is perforated with small holes in the side and top.

Portable coil holder

People working in forested areas where there is not much wind (woodcutters, rubber-tappers, plantation workers, gold-miners) can obtain some protection from biting mosquitos and phlebotomine sandflies by attaching one or two smouldering coils in special holders to their belts (Fig. 1.47). Each coil is kept in place between two pieces of metal or non-flammable fibre glass gauze. The advantages of coil holders over skin repellents are that they are cheaper, do not elicit any skin reactions when used frequently, and are not washed off by perspiration.

Fig. 1.46. Commercially available coil holders. The holders are commonly used in Asia, especially in crowded rooms. They improve the performance, convenience and safety of smoul-dering mosquito coils.

Coils can be made cheaply from an insecticide and a flammable base material (67).

1.3% pyrethrum powder

Water-soluble glue (starch gel)

Filler (coconut shell flour, sawdust, jute)

Fungicide (benzoic acid, sodium dehydroacetate)

More effective alternative insecticides are (+)-allethrin (0.2 — 0.3%) and (+)-trans-allethrin (0.10 — 0.15%). If one of these is used the quantity of filler is increased to 60 — 80%. To regulate burning, commercially produced coils often contain potassium nitrate. The sawdust particles have to be of the correct dimensions, otherwise the coil does not burn well. This has to be determined by trial and error. Mix the ingredients thoroughly and add an equal weight of water to produce a uniform and homogeneous paste. Compress the mix in a mould of the desired shape and place on a rack to dry. A suitable mould can be carved out of a piece of wood. If the device is meant to burn for many hours a coil shape is the most convenient. For shorter periods (3 — 4 hours) it is possible to give it the shape of a long thin stick.

Fig. 1.47. A rubber-tapper with a special portable coil holder attached to his belt.

A cheaper alternative to mosquito coils has been developed in India (68): ropes soaked in a solution of a suitable insecticide, when burnt, produce a smoke that kills and repels mosquitos and biting flies. The recommended material, widely available in India, consists of jute fibres, is about 0.9 cm in diameter and weighs about 28 g/m. Esbiothrin was used in India, but other insecticides used in mosquito coils would also be suitable. A 1.2-m impregnated rope will burn for 10 — 12 hours if hung indoors from a ceiling. The ropes are preferably burned inside cylinders of wire mesh to prevent them from making contact with flammable materials.

How to impregnate ropes:

If esbiothrin is used for impregnation the recommended dosage is 1 ml/kg: 1 ml of technical-grade esbiothrin is dissolved in 1.15 litres of kerosene, and a 1-kg jute rope is dipped into the solution until saturated. The rope is dried in the shade and stored in a box or bag until required.

Vaporizing mats

Where electricity is available, small electric heating plates can be used to vaporize volatile insecticides from mats (Fig. 1.48). This popular method has the advantage over coils that no visible smoke is produced. The mat is often a porous paper pad measuring 35 × 22 × 2 mm, impregnated with an insecticide. The mats are packed in foil to prevent evaporation of the insecticide before use. The insecticides are usually allethrin pyrethroids, e.g. bioallethrin, esbiothrin and esbiol, which are considered to be safe to humans but have a rapid killing and repellent effect on mosquitos and biting flies (62, 69).

The mats contain an indicator dye that changes colour from blue to white in about the same time that it takes for the insecticide to evaporate. If used in a room of about 35 m 3 , a mat containing, for example, 40 mg of (+)-allethrin or 20 mg of (+)-trans-allethrin will last for 8 — 10 hours. However, towards the end of the period less insecticide will be released. In larger rooms more than one mat, or mats containing more insecticide, should be used.

Several types of electrical heater are sold with the mats. All have a flat pad-like resistance unit (5 to 6 watt) mounted in a ventilated plastic case. Some models are directly plugged into a power point. The heater normally produces a temperature of 160 °C between the mat and the heater and 125 °C on the upper surface of the mat. A mat temperature of about 145 °C is needed for vaporizing the insecticide. Some heaters on the market do not achieve this temperature and therefore do not vaporize the insecticide sufficiently. A heater takes about 30 minutes to reach its operating temperature.

Fig. 1.48. An electrical heater unit for vaporizing insecticide from mats.

Electric liquid vaporizer

This device is a technological improvement on vaporizing mats. The insecticide is evaporated by an electric heater through a porous wick from a reservoir bottle containing the liquid (Fig. 1.49). The liquid insecticide lasts for up to 45 periods of 8 — 10 hours. Many models are controlled by a switch and have a pilot lamp.

This method is more convenient and more effective than the mat heater because the amount of insecticide released remains constant over time, but for the moment it is more expensive.

Dichlorvos is a volatile liquid whose vapour is highly toxic to flying insects. Liquid dichlorvos impregnated into a special absorbent material, such as polyurethane, slowly evaporates without the need for a heating device. A dispenser usually consists of a piece of polyvinyl chloride plastic or a resin saturated with liquid dichlorvos, mounted in an open plastic support (Fig. 1.50). Some dispensers are strips measuring 5 × 25 cm, while others have the shape of a small box. They are sealed in an airtight package to avoid premature vaporization of the insecticide.

The dispenser in its plastic support is placed at a height of 1 — 2 m above the floor or is suspended from the ceiling. Most models contain sufficient dichlorvos to treat a room of 15 — 30 m 3 for 1 — 2 months. A strong draught will shorten the period of effectiveness. The advantages of this method are the long period of effectiveness and the lack of a need for electricity, making it especially suitable for use in rural houses, tents or caravans.

The continuous exposure of young children and sick or elderly people to dichlorvos in poorly ventilated rooms should be avoided. Some reports suggest that continuous exposure to dichlorvos may have caused health problems in a few people.

Fig. 1.49. Two models of electric liquid vaporizer.

Fig. 1.50. A dichlorvos dispenser releases a volatile insecticide continuously without heating for 1 — 3 months.

Pressurized spray cans

Pressurized cans provide a convenient method of spraying insecticidal aerosols in rooms, on mosquito nets, vehicles and so on, to obtain rapid knock-down of mosquitos and other flying insects. The spray cans contain a concentrate of the insecticide in an organic solvent or water together with a liquefied or compressed gas propellant. Pyrethrum used to be the common ingredient in many different brands of aerosol sprays. Today, however, the synthetic pyrethroids and to a lesser extent the carbamates (propoxur and bendiocarb) and organophosphorus compounds (dichlorvos) are the main active ingredients. The spray may contain a “knock-down” agent to give a rapid effect, a slow-acting agent that actually kills the insect, and a synergist — usually piperonyl butoxide — to increase the activity of the ingredients. In view of worldwide concern about the use of chlorofluoro-carbons, which may affect the ozone layer of the atmosphere, most brands now contain other propellants.

The spray can is operated by briefly pressing a valve incorporating a nozzle on top of the container. The spray can be directed against flying or crawling insects or sprayed into a room (Fig. 1.51). Rooms should then be kept closed for about 15 minutes in order to kill as many insects as possible. The hiding and breeding places of cockroaches, fleas, lice and bedbugs can be sprayed directly from a distance of about 20 cm.

Space sprays have a very short residual effect: once the aerosol has settled out of the atmosphere insects can again enter the area with impunity. Furthermore, the active ingredients (commonly (+)-allethrin or (+)-trans-allethrin) are rapidly degraded by light. An advantage of short-lasting insecticides is that they do not leave any toxic residues on beds, furniture or other surfaces. This method works best in screened spaces and can be repeated daily or several times a day.

The spray can is under pressure and should not be exposed to direct sunshine or temperatures over 50 °C. Most sprays contain the flammable substances propane or butane and should not be directed at fires or smouldering objects, e.g. cigarettes.

Fig. 1.51. Aerosol sprays containing rapidly acting insecticides are used for immediate killing of flying or crawling insects.

Water-based aerosol spray

Water-based aerosol sprays have recently been developed and are claimed to offer the following advantages over oil-based aerosols: they leave no oil residues or stains on surfaces, do not produce an unpleasant smell or an irritant effect, and are not flammable. However, the droplets of oil-based aerosols are usually finer and more effective. The cans must be shaken well before use.

Spray gun

Before the invention of the pressurized disposable spray can, a hand-compressed spray pump was commonly used. This spray pump has a reservoir which can be filled with a solution of pyrethrum or other insecticide (Fig. 1.52). It is cheaper to use a spray gun than to buy pressurized spray cans. However, the droplets in the aerosol from a spray can are finer, stay in the air longer and are usually more effective. Spray guns are nowadays used mainly against crawling insects.

Spray guns and the liquids to fill them are commercially available in some countries. The liquids can be based on equal parts of kerosene and alcohol, to which are added a small quantity of one or two quick-acting insecticides and a perfume.

An example of a standard insecticide mixture is:

Fig. 1.52. Spray guns are pumped by hand in order to deliver aerosols.

White spirit (or pure alcohol)

Propoxur and dichlorvos are among the many other insecticide mixtures that can be used.

Battery-operated electronic devices that produce a high-pitched buzz have been widely sold as mosquito repellents. Some manufacturers have claimed that they simulate the sound of a male mosquito and that this sound is repellent to mated females. Others claimed that the buzzers simulate the sound of the dragonfly, thus inducing mosquitos to fly away. However, several independent scientific investigations in different countries have convincingly demonstrated that these electronic gadgets provide no protection from biting mosquitos (70, 71). An apparently positive test by producers was faulty in design. In the United Kingdom some companies have been fined for making unsubstantiated claims in their advertisements for buzzers.

Protection measures in hammocks

Hammocks are used in many parts of the world for sleeping and resting. They are often used in jungle areas and offer the following advantages over other sleeping places:

— they are not easily accessible to crawling insects, scorpions, snakes and other small animals;
— they are well ventilated and suitable for use in hot climates;
— they provide dry sleeping places and are not in contact with damp soil;
— they are light and easily folded and are therefore easy to transport.

However, they do not protect the user from flying insects. Mosquitos often settle and feed where the body touches the lower part of the hammock (Fig. 1.53). At night the use of hammock mosquito nets (see p. 79) can offer protection but during daytime the use of nets is often considered inconvenient for various reasons, among them poor visibility and reduced ventilation.

Suggestions for protection in the absence of a mosquito net

· Application of a volatile repellent such as deet to the lower part of the hammock at a dose of about 20 g/m 2 . The repellent persists for only a few days and some mosquitos may try to feed from above.

· Placing a burning mosquito coil close to the hammock. If used in a coil holder it is safe to place the smouldering coil under the hammock.

· A method that provides longer-lasting protection is the impregnation of the whole hammock or the lower part of it, using a sponge, with a quick-acting pyrethroid insecticide. Mosquitos making contact with the treated part of the hammock are killed or incapacitated. Because of the thickness of the hammock material this method requires a relatively high dose of insecticide (1.5 g of permethrin or more per m 2 ).

· A more economical method, requiring far less insecticide and probably equally effective, is that of protecting the lower surface of the hammock with an impregnated piece of netting or cloth (Fig. 1.54). This material can be loosely attached to the hammock with a few pins or with stitches. It should be attached close to the hammock so that mosquitos are more likely to settle on it and be killed. However, the netting should not touch the hammock except where it is pinned or stitched on, because this would enable some mosquitos to feed before being killed. The advantages of using removable material are that it is easily impregnated, can be removed when the hammock is washed, and can be stored in an airtight box when not in use.

Fig. 1.53. Mosquitos often attack the occupant of a hammock from below, where the body presses against the lower part of the hammock.

Fig. 1.54. A piece of cloth or netting impregnated with an insecticide or repellent and loosely attached to the lower part of a hammock can provide partial protection from biting mosquitos.

Mosquito nets

Mosquito nets (Fig. 1.55) have been in use since very early times to protect people against bloodsucking insects at night; they also help to protect against other creatures, such as spiders, cockroaches, beetles, lizards, snakes and rats. When made of thicker opaque sheeting they also protect against cold and dust, and provide privacy.

Mosquito nets normally have a mesh size of 1.2 — 1.5 mm, which is sufficiently small to prevent mosquitos from entering. Very small insects, however, such as phlebotomine sandflies and biting midges may enter. Only opaque sheeting, very fine-mesh jersey netting (mesh size less than 0.2 mm), and impregnated netting (see p. 82) offer protection against these insects. In hot climates, poor ventilation through fine-mesh netting is a serious disadvantage. The wider the mesh size the better the ventilation, but if the mesh is more than 2 mm most mosquitos can enter.

Traditional netting materials are linen, raffia (palm fibre) and hemp. Nets are now made of cotton or synthetic fibres (nylon, polyester or polyethylene). The quality of a mosquito net depends on the thickness and strength of the threads and on the production process. The threads in a mosquito net can be woven or knitted (Fig. 1.56). A disadvantage of woven nets is that the threads can slide over each other, thus creating enlarged holes through which mosquitos can pass. However, in woven nets made of stiff, polyethylene fibres this does not seem to be a problem.

Synthetic nets usually cost less and are less likely to rot than cotton nets. Inexpensive nets of cotton, nylon or polyester often contain starch, which gives a less flimsy, more attractive appearance. The starch dissolves when the nets are washed.

Fig. 1.55. A rectangular mosquito net.

Fig. 1.56.(a) The mesh of a mosquito net is traditionally indicated by the number of holes to the square inch. The netting shown here has a mesh of 156 (12 × 13) (actual size).

Fig. 1.56.(b) Close-ups of knitted and woven netting material.

Some terms for characteristics of netting material

Mesh: the number of holes per square inch. For example, mesh 156 has 12 × 13 holes per square inch (see Fig. 1.56).

Mesh size: the size of the openings in a net. It is determined by the number of holes per square inch (the mesh) and the thickness of the threads with which the netting is made. The mesh size recommended for most tropical countries is between 1.2 and 1.5 mm.

Denier: an indication of the weight (and therefore the strength) of the thread. It is defined as the weight in grams of 9000 metres of a single thread. Commonly used mosquito net threads have a denier between 40 and 100 but denier 40 is easily torn and 70 or more is recommended.

Strength: an indication of the pulling strength of a thread, expressed in grams per denier. If 1 metre of 40 denier thread breaks with a load of 160 g, the strength is 4 g per denier.

Monofilament/multifilament fibre: the thread of a mosquito net consists of one or more fibres. A nylon or polyester fibre is multifilament (consisting of many filaments), while polyethylene fibres are monofilament.

Sheeting border: nets are often provided with a strong border of cotton sheeting or synthetic jersey. This protects the net from wear due to daily tucking in of the net under the mattress. If the border is wide enough (30 cm) the extra material will also reduce bites from insects that may make contact with the lower part of the net whilst the occupant is asleep.

Ceiling: fine-meshed jersey or other opaque material is often used as a ceiling for the net to prevent dust from falling through.

Colour: white material is most commonly preferred but other colours are available. In a white net it is easier to see any mosquitos that have entered. A darker colour may be preferable because nets are less likely to appear soiled.

Mosquito net models

Mosquito nets are produced in different sizes and shapes. A net should cover the sleepers completely and should be sufficiently spacious for them to avoid contact with the fabric. Sufficient length is needed so that the net can be tucked in under the mattress or sleeping mat. Various models have been developed for specific circumstances. They differ in convenience for daily use, and prices vary widely. The method of suspension is an important consideration.

This is the most popular and practical model, normally used over a bed or sleeping mat. It is suspended from four or more loops along the upper edges. This model can be provided with an overlapping entrance flap of about 60 cm on one of the long sides to facilitate entering or leaving without pulling out the part of the net tucked in under the mattress (Fig. 1.57). Care should be taken to ensure that the overlap is properly closed to keep mosquitos out.

Dimensions vary: most nets have a height of about 150 cm and a length of 180 — 190 cm. A single-size net has a width of 70 — 80 cm, contains about 9 m 2 of netting material, and is used to cover one person on a single bed or sleeping mat. Double nets with a width of 100 — 110cm (10 — 11 m 2 of netting) and family-size or large double nets with a width of 130 — 140 cm (12 — 13 m 2 of netting) are used for larger beds. Extra-large nets with a width of 180 — 190cm (14 — 15 m 2 of netting) are used for very large beds and where several family members sleep together on a sleeping mat in one room. The optimal size depends on sleeping habits and available space.

Very large nets are sometimes used by groups of people (e.g. in Mauritania) who spend the early evening hours together. These nets are used in shelters that provide shade during daytime but do not have walls.

Special supports for rectangular bednets

Indoor supports Where it is customary to rearrange and use beds for seating during daytime, nets should be supported using detachable poles or mosquito net supports attached to the ceiling or wall (Figs. 1.58 and 1.59).

Fig. 1.57. A rectangular net with an overlapping entrance flap.

Fig. 1.58. A support system for a rectangular net which enables quick and easy overhead storage during the day. The components can be made of bamboo, wood or plastic.

Fig. 1.59. A support system for a rectangular net which can be used indoors or outdoors (adapted from 72).

Fig. 1.60. Flexible wooden poles can be placed in a crossed position at the ends of the bed with the lower ends tied or attached to the legs of the bed. The poles and net are easily removed during daytime.

Outdoor supports Where people habitually sleep outdoors during the hot season nets are best supported by a frame that can be easily detached from the bed (Fig. 1.60) (72).

Circular, or conical, nets are often preferred because they can be hung from a single support (Fig. 1.61). The top is suspended by a loop attached to a sleeved hoop of rattan or plastic. The nets are mostly available in double size. Compared with the rectangular net, more care has to be taken to avoid contact between the body and the net, which would allow mosquitos to feed.

Fig. 1.61. A circular net suspended from a single support.

Wedge-shaped nets are available only in single size. They are much cheaper than rectangular bednets because only about half the netting material is needed. The head end is suspended by a loop attached to a sleeved wooden bar. It can be hung from any suitable fixing point above the head of the bed or sleeping place. The foot end, which is made of thick material so that mosquitos cannot feed on the feet, must be firmly tucked under the mattress or otherwise secured (Fig. 1.62). Because of its small volume when folded and because it can be suspended from a single point, the wedge-shaped net is convenient for travellers and campers.

These nets are available in small sizes. Usually marketed for the protection of food from flies, they are also used to protect babies and infants (Fig. 1.63). Because the nets are self-supporting they are easy to set up indoors and outdoors.

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Fig. 1.62. A wedge-shaped net.

Fig. 1.63. Two self-supporting nets used to protect babies and infants. The collapsible “umbrella” net (left) is commercially available. The other model (right) is also collapsible and consists of U-shaped pieces of wire; it is easy to make.

Camp bed with protective cover

A collapsible camp bed with a self-supporting cover has been developed for use by workers in rainforest areas, for instance gold-miners in the Amazon region. The cover is a detachable part of the bed and consists of waterproof (polypropylene) sheeting with built-in ventilation openings of mosquito gauze and a door of mosquito netting closed with a zipper (Fig. 1.64). It is more comfortable than a hammock with a mosquito net, but it is also more expensive and more bulky.

Mosquito nets for hammocks

Special mosquito nets are available for hammocks; these are similar to rectangular bednets but have sleeves for the hammock ropes at each end. In some areas these nets are made from opaque cotton cloth which offers privacy, provides additional protection from the cold and is more sturdy. To prevent mosquitos from entering, the nets can be left hanging down to touch the ground.

If the ground surface is dirty or if there is a need to prevent small animals from climbing up the net, it can be closed by pulling up one side under the hammock with strings and tucking the other side into it. The sleeves are tightly closed around the hammock ropes by means of strings. The net is suspended from four points, as shown in Fig. 1.65, or from two points if horizontal pieces of wood are used in the roof to keep the two long sides apart. In the latter case the net is suspended from a single string tied between the two hammock rope ends.

Unfortunately, the net is often tight around the hammock, and direct contact between the body and the net or between the lower part of the hammock and net may occur, enabling mosquitos to feed. To avoid this a larger net should be used.

A special military or expedition model for use in jungle areas has the netting attached to the sides of the hammock. At the two ends the hammock is extended with pieces of wood. It is covered by a waterproof roof. Entry is via an opening with a zipper. To prevent mosquitos biting from below, the hammock is made of impenetrable material which also provides insulation from the cold. However, in hot climates these nets trap sweat, making them uncomfortable to lie in.

Fig. 1.64. A camp bed with a self-supporting cover gives protection from rain and insects.

Fig. 1.65. A hammock mosquito net.

Instructions for use of mosquito nets

Any holes developing in the net should be mended as soon as possible. It is important to use a net sufficiently large to cover the entire bed or sleeping place, so that contact between the body and the netting is avoided and mosquitos cannot bite through the net.

In some areas it is customary for several people, especially children, to share one net. This may result in overcrowding, and parts of people’s bodies may protrude from the net during the night (Fig. 1.66). A bigger net or an extra one could be used to prevent overcrowding. Alternatively, the net can be impregnated with an insecticide to repel or kill mosquitos before they land on the unprotected skin.

A net can be closed by:

— tucking it in under the mattress or sleeping mat;
— lowering it around the sleeping place until it makes complete contact with the floor; a border of heavy material ensures good contact, or weights can be put on the border or inserted in the hem to keep it in place.

The net should be let down before darkness falls. Mosquitos that manage to enter can be killed by swatting or by spraying with insecticide before the people concerned go to sleep.

Fig. 1.66. Mosquito nets provide poor protection if shared by too many people.

Open floor with slits

Sometimes mosquitos enter a net from underneath. This problem often occurs in stilt houses with bamboo floors or containing beds with mattresses made of woven string. Sleeping mats can give protection when used with the mosquito net tucked underneath. However, mosquitos may continue to feed through openings in mats that are too thin. A quick-acting safe insecticide on the mat or string mattress may prevent this. This will also help to kill bedbugs. More permanent protection can be obtained by the use of an impenetrable surface under the sleeping mat or the bed. Cloth or plastic sheeting may well serve this purpose.

Obtaining a net

Bednets are widely available in different models, sizes and qualities. They can also be made locally from a length of netting material. The advantage of local manufacture is that quality, design and shape can be chosen to suit personal preferences. Opaque sheeting nets can be made from locally available textiles used in clothing manufacture. Open netting is commonly available as curtain material. Any type of strong opaque sheeting can be used for the borders, ceilings and suspension loops. The seams to which the loops are attached should be reinforced. The durability and effectiveness of cheap factory-produced net can be improved by adding a border to its lower edge.

Problems with mosquito nets

The protection provided by mosquito nets will be reduced if they are not used properly or holes are left unrepaired. In addition, contact may occur with the net during sleep, allowing mosquitos to bite through the net. Furthermore, hungry mosquitos may remain in the room and feed when the occupant leaves the net. They may also be diverted to unprotected people sleeping in the same room (Fig. 1.67).

Fig. 1.67. Disadvantages of a standard, untreated mosquito net.

Individual or community protection with untreated nets

If a small num b er of people in a community use mosquito nets, they will probably benefit because hungry mosquitos can easily find an alternative blood-meal nearby on unprotected people or domestic animals. However, if all inhabitants of a community use nets and there are no attractive domestic animals to feed on, hungry mosquitos are likely to persist until (1) they find holes in nets, (2) they find places where they can feed through nets, or (3) occupants leave nets. In this situation, the use of nets may not result in a reduction of malaria in a community (73, 74). On the other hand, diverted hungry mosquitos can easily obtain blood-meals if there are animals on which they can feed (75). In areas where malaria transmission is low or moderate this may be sufficient to reduce malaria among community members.

Insecticide-treated mosquito nets

The above-mentioned problems of standard mosquito nets can be solved by impregnating them with a quick-acting pyrethroid insecticide (74,7678) which irritates or kills mosquitos on contact, preventing them from finding openings (Fig. 1.68). An impregnated net with holes that are not too large is as effective as an undamaged net (79 — 81). Insecticide treatment thus extends the useful life of a net. Mosquitos that land on an impregnated net and attempt to feed through it on part of the body in contact with the net are likely to be killed (44). The behaviour of a mosquito that survives contact with the insecticide is so disturbed that it is unlikely to attack again (79, 80, 82, 83). People without a net and sleeping near someone with a treated net may receive some protection from bites (79). A person leaving such a net during the night or in the morning runs a reduced risk of being bitten.

Fig. 1.68. Advantages of mosquito nets treated with an insecticide.

Insecticide-treated nets serve as human-baited traps when somebody is sleeping inside by attracting and killing mosquitos and other biting insects.

These factors make the widespread use of treated mosquito nets particularly important in the control of malaria. When employed by all members of a community the practice kills many Anopheles and reduces the chance that any will live long enough to transmit malaria parasites. People outside their nets early in the night or before dawn, or people not using nets, thus receive some protection against the risk of infection (84 — 88).

Treated nets can also be used to protect the most vulnerable groups in a community, such as pregnant women, children and old and sick people, from infection with malaria or other insect-borne diseases. Young children, going to sleep early, receive most protection (89).

The use of impregnated bednets may lead to the disappearance or reduction of other pests that are sensitive to the insecticide used, such as bedbugs, head lice, chicken ticks and houseflies (90, 91). The nets are probably also effective against fleas and triatomine bugs.

Which nets can be treated?

All types of bednet are treatable, including old nets with holes and nets of synthetic or natural fibre. However, multifilament nets are better than monofilament nets at holding the insecticide. The insecticide particles are easily dislodged from monofilament nets by abrasion or washing. For more information on the insecticides that can be used and on how nets should be impregnated see p. 85.

Control of malaria in a community with treated nets

Insecticide-treated bednets have been successful in reducing the number of malaria infections in villages where the transmission of malaria is low or moderate, for instance in China and the Gambia (88,92). In villages where the transmission of malaria is intense (holoendemic malaria), community use of impregnated mosquito nets was found to have little impact on the number of infected people. However, people received 90 — 95% fewer infective bites from malaria-carrying mosquitos, and were apparently better able to overcome the disease and to develop immunity (76, 85, 93 — 96).

Alternative materials for treatment

Other materials, such as fabrics of wide-mesh netting and bed curtains made of loose single-strand fibres, can also act as a physical barrier to the entry of insects, if treated with insecticide.

Treated nets with a mesh size slightly less than the wing span of a flying insect will force it to land before passing through, and on contact with the net it will be killed or repelled (97 — 102). Treated netting with a mesh size of approximately 4 mm protects against most mosquito species (81) and a mesh size of 2 mm would probably be effective against biting midges and sandflies (103). Such nets allow good ventilation in hot climates.

The advantages of wide-mesh nets (74) include:

· increased ventilation in hot and humid climates;

· reduced cost, even though stronger fibre is required;

· weight and volume when folded are less, making the nets easy to distribute and practical for travellers and nomadic people.

The disadvantages of such nets are:

· the nets offer no protection once the insecticide has lost its activity; prompt re-treatment is particularly important with this type of net;

· wide-mesh nets are more easily torn than standard nets;

· they are not yet commercially available but can be made out of curtain or other wide-mesh netting material.

In areas where bednets are too expensive an alternative may be to use curtains made of locally available fibres (Fig. 1.69) or strings hung around the bed. To offer protection from flying insects these open curtains must be treated with an insecticide. A roof is not essential as mosquitos generally fly low. Curtains offer considerable protection but are not as effective as treated bednets.

Fig. 1.69. A cheap alternative to mosquito nets may be bed curtains made of locally available fibre material treated with an insecticide.

Suitable materials include fibres from polypropylene or jute sacks. Sacks should be cut open and unpicked to obtain a loose arrangement of fibres. Flammable material (e.g. sisal) should not be used.

Treating fabrics with an insecticide

The impregnation of fabrics with an insecticide is simple: an emulsion is made in water, and the material to be impregnated is soaked in it and allowed to dry. After drying, the insecticide remains attached to the fibres.

Insecticides 1

1 Further information on insecticides is available on request from Division of Control of Tropical Diseases, World Health Organization, 1211 Geneva 27, Switzerland.

Many well known insecticides, e.g. DDT, are not suitable for the treatment of fabrics because they act too slowly and allow insects to make contact and escape before they pick up a lethal dose. Moreover, many insects have developed resistance to a number of insecticides. The synthetic pyrethroids do not have these disadvantages. They are quick-acting and highly toxic to insects. In addition, they are considered to be generally safe to humans at the recommended dosages. They are also relatively safe for the environment because of their quick breakdown in the soil.

Pyrethroid insecticides are available as solutions, usually called emulsifiable concentrates. These can be mixed with water, producing a milky liquid. Oil-in-water emulsion formulations have been made specially for the treatment of fabrics; they give good adherence of the insecticide to the fabric material and do not produce an unpleasant odour during treatment. Pyrethroids can also be obtained as wettable powders or suspension concentrates, also known as flowable concentrates, but these formulations are less suitable for treating fabrics since they are more easily dislodged. This reduces the period of effectiveness and the dislodged particles may cause irritation of the skin. A number of photostable pyrethroids are available, of which only permethrin, cyfluthrin, deltamethrin and lambdacy-halothrin have been tested for their efficacy and safety in the treatment of mosquito nets. Permethrin and flumethrin have been tested for the treatment of clothing.

Pyrethroids for treatment of fabrics

Not all pyrethroid insecticides are suitable for the treatment of fabrics. To be suitable, a pyrethroid must remain effective in the fabric for at least several weeks, be resistant to sunlight and safe. The first-generation pyrethroids, such as the natural pyrethrins (pyrethrum), the allethrins and phenothrin, are unsuitable because they decompose rapidly when exposed to daylight. The second- and third-generation pyrethroids are much more stable and are therefore suitable (104).

Permethrin is commonly used for agricultural and public health purposes and is widely available. It is the most commonly recommended insecticide for the impregnation of bednets and clothing. It has proved to be highly effective in pest control, and there are no reports of adverse side-effects.

Deltamethrin is commonly used in agriculture and public health and is widely available. This pyrethroid is used extensively in China for the impregnation of bednets. It is more than 30 times as powerful as permethrin. Recommended dosages are much lower than for permethrin but this is compensated by a higher price per unit weight. The toxicity to domestic animals and humans is higher than that of permethrin, but formulations contain less active ingredient. There have been complaints about an irritant effect during the impregnation procedure. People sleeping under dry nets do not usually experience any side-effects but this may depend on the fibre material of the net and the insecticide formulation. A burning sensation of the face has been reported by people sleeping under polyethylene nets treated with a flowable formulation of deltamethrin (C.F. Curtis, unpublished observations, 1990) and by people sleeping under cotton nets treated with deltamethrin wettable powder (105).

This insecticide has been developed recently and is increasingly used for public health purposes. It is widely available for agricultural use. In general its properties are similar to those of deltamethrin. It is reported that lambdacyhalothrin causes nasal irritation to some people sleeping under freshly treated nets, even when these are dry. Reports demonstrating its prolonged insecticidal effectiveness have recently become available (93, 106) and more trials are under way.

This product is used in agriculture and public health and is widely available. It is more toxic to insects than permethrin but less toxic than lambdacyhalothrin and deltamethrin. No side-effects have been reported but testing has been very limited so far (G. Hesse, personal communication). A special oil-in-water emulsion is available which gives better adherence to fibres than the emulsifiable concentrate formulation and does not produce an odour or irritant effect during treatment.

The toxicities of cypermethrin, flumethrin and alphacypermethrin range between those of permethrin and deltamethrin. However, these insecticides have not yet been fully tested for efficacy and safety in the treatment of mosquito nets or clothing.

Optimal combination of mosquito net materials and pyrethroids

For the same insecticidal effect, nets made of cotton fibre need to be impregnated with 3 — 5 times as much permethrin or lambdacyhalothrin as those made of nylon fibre (107, 108). This may be because in cotton fibres much of the insecticide is contained in the hollow spaces inside the fibres where it is unavailable to mosquitos. Nylon fibres are not hollow and most of the insecticide remains on the outer surface, where landing mosquitos pick it up on their legs. However, with deltamethrin there seems to be no difference in effectiveness between cotton and nylon (77, 108).

Before a choice of material is made the local availability of materials should be investigated. The choice will then depend on a comparison of costs and technical considerations.

Recommended dosages

The recommended dosages of insecticide per quantity of fabric are usually given in grams of active ingredient per square metre (g/m 2 ) or in milligrams of active ingredient per square centimetre (mg/cm 2 ) (1 g/m 2 = 0.1 mg/cm 2 ).

If impregnated with the same mixture of insecticide, a square metre of thick fabric absorbs much more insecticide than a similar area of thin open-weave fabric. However, some of the insecticide is not available on the surface, having penetrated inwards. Higher dosages of insecticide per unit area of thick fabric are presumably needed to provide a toxic resting surface equivalent to that obtained on the thinner fabrics (Table 1.3).

The duration of activity of an insecticide would normally be expected to be longer with a higher dosage. However, if a fabric is washed regularly it may be advisable to treat it with a lower dosage after each wash.

The more potent pyrethroids may be more economical because of the lower dosages needed. However, prices per unit weight of these insecticides are higher than that of permethrin and the choice will depend on local availability and prices.

Table 1.3. Dosages of insecticide needed to impregnate different types of fabric

Wide-mesh netting (more than 2 mm)

Standard mosquito mesh (1.5 mm)

Cotton cloth (sheeting, shirts)

Thick fabrics, jackets, trousers

Safety measures

Pyrethroid insecticides recommended for treatment of mosquito nets are relatively non-toxic to humans, mammals and birds. A distinction should be made between safety for people using treated fabric and safety for people who carry out the treatment. Fabric treated at the recommended dosage is not hazardous after drying.

During treatment the insecticide mixture should not come into contact with the skin, particularly the lips, mouth, eyes and any open wounds. Rubber gloves should be used during the treatment process, and care should be taken to avoid splashing solution into the eyes and inhaling fumes. When many treatments are to be done it is better to work outdoors or in a well ventilated space and use open, shallow containers.

People who inhale the fumes of the insecticide mixture may develop a headache or irritation of the nose or eyes. This occurs more frequently with deltamethrin or lambdacyhalothrin than with permethrin or cyfluthrin. A tingling sensation in the skin of the hands may be felt when treatment is carried out without gloves. These side-effects disappear within a few hours. If the eyes are contaminated or the skin shows an irritant reaction, the affected part should be rinsed thoroughly with clean water. Medical advice must be sought if pyrethroids are swallowed.

Safety of treated nets

After drying, care should be taken to prevent small children who sleep under a net from putting part of it in their mouths. Synthetic nets (nylon, polyester) freshly treated with a relatively high dose (0.030 g/m 2 ) of lambdacyhalothrin may cause cold-like symptoms, such as sneezing and a runny nose during the first 1 — 2 weeks of use. At the lower dosage of 0.010 g/m 2 , which still has a prolonged insecticidal effect, the side-effects last only a day (93). No side-effects have been reported with synthetic or cotton nets treated with permethrin or with cyfluthrin as an oil-in-water emulsion.

How to prepare the appropriate solution and treat and dry the fabric

The fabric to be treated should first be thoroughly cleaned or washed if it is dirty; it should be completely dry by the day of treatment. This is of special importance when bednets belonging to different people are impregnated in the same mixture. When several nets are treated at the same time, they should be marked with a waterproof marker to allow each to be returned to its owner.

1. Calculate the surface area of the fabric to be treated (Fig. 1.70).

2. Determine the amount of water needed to completely soak the fabric (Table 1.4):

· Partially fill a bowl or bucket with a known quantity of water (Fig. 1.71).

· Soak the fabric in the water.

· Wring it out gently and/or allow it to finish dripping, collecting the run-off in the container.

· Measure the difference between the initial and the remaining amount of water. You can do this with a large measuring cylinder, or by finding the difference in weight of fabric before and after soaking and dripping. The difference in grams is equal to the number of millilitres (ml) of water absorbed by the fabric. This gives the amount of water needed to prepare the solution.

Fig. 1.70. The surface area of a rectangular bednet is calculated using the formula: S = 2(a × c) + 2(b × c) + (a × b).

3. Prepare the solution for treatment:

· Calculate the total weight of insecticide needed (T) as:

D = the chosen dosage (g/m 2 )
S = the total surface area of fabric (m 2 ).

The amount of insecticide concentrate needed to prepare the solution (I) can be calculated as follows:

C = the amount of active ingredient in the insecticide concentrate (g/ml).

For example, 25% emulsifiable concentrate contains 25 g per 100 ml, thus 1ml contains 0.25 g of active ingredient. Measure out the volume needed with a small measuring cylinder or pipette (Fig. 1.72).

· Measure out the required quantity of diluting water with a large measuring cylinder (or measuring can), as calculated in step 2.

· Mix the emulsifiable concentrate with water in a suitable container.

Table 1.4. The quantity of water absorbed by different types of netting material and the amount of permethrin required to treat netting at a rate of 0.5 g/m 2a

Quantity of netting material

Amount of water (ml)

Amount of 10% permethrin
(ml)

Amount of 25% permethrin
(ml)

Amount of 55% permethrin
(ml)

Nylon/ polyester netting
(den. 100/ mesh 156)

a This table is based on field data from Dr R. Montanari, WHO, Papua New Guinea and from Dr C. Curtis, London School of Hygiene and Tropical Medicine, London, England.

Fig. 1.71. Utensils and equipment needed for treating fabrics with insecticide.

Fig. 1.72. Pouring emulsion concentrate into a measuring cylinder.

To make sure the fabric is completely impregnated, it may be advisable to prepare some excess mixture. The excess, as well as the run-off liquid after wringing out, can be reused to impregnate other fabrics.

4. After calculating the amount of water absorbed (step 2), dry the fabric. Submerge the clean dry fabric in the mixture, pressing and squeezing thoroughly to remove air and make sure the fabric is completely soaked (Fig. 1.73). Large articles such as bednets should be folded into a neat package to facilitate removal of air and penetration of the mixture. This is especially important with stiff, non-elastic bednets made of polyethylene.

The container in which the articles are treated should be large enough to allow them to be handled and turned over without spilling the mixture. Buckets, dustbins, washing-up bowls or plastic bags may be suitable, depending on the number and size of the items to be treated. If a plastic bag is used, it should be filled with the amount of mixture needed to saturate the fabric without leaving any excess. After putting the item into the bag, seal the top by tying or twisting. Shake and knead the bag vigorously for 10 minutes (Fig. 1.74). Then remove the fabric and allow to dry without wringing it out.

5. Take the wet item out of the treatment container, gently wring out any excess liquid or allow it to drip back into the container. Place the fabric on a plastic sheet or other clean, non-absorbent surface to dry, e.g. banana leaves (Fig. 1.75). If a plastic bag is used for treatment it can be cut open to make a drying sheet. During the drying process the fabric should be turned over from time to time. The drying time depends, among other factors, on the thickness of the fabric, the quantity of water absorbed and the surface area exposed to sun and wind. A cotton mosquito net takes several hours to dry. Exposure to bright sunlight may partially destroy pyrethroid insecticides, so it is preferable to keep wet fabrics away from sunshine.

Generally, items should not be hung up to dry immediately because insecticide will be lost as a result of dripping and will spread unevenly in the fabric. When they have been drying for some time on the ground they may then be hung up to speed up the process (Fig. 1.76). Cotton is less likely than synthetic materials to drip when hung up to dry after being wrung out.

Fig. 1.73. Treatment of a fabric by pressing and soaking. Rubber gloves should be used to protect the hands.

Fig. 1.74. Impregnation of fabric in a plastic bag.

Fig. 1.75. Dry the treated material on a plastic sheet or other clean nonabsorbent surface, avoiding direct sunshine.

Fig. 1.76. When a freshly impregnated fabric has been drying for some time on a horizontal surface, it can be hung up to speed up the process.

In some instances a gradient of insecticide in a fabric may be useful, for example in hammock nets because the lower part is where the net comes into contact with the body (see p. 72). Such a gradient can be achieved by hanging up the item at an early stage in the drying process (Fig. 1.77).

Fig. 1.77. Devices made of poles and plastic sheeting for large-scale treatment of mosquito nets. Bowls collect the excess liquid running down from a plastic sheet (design: S. Meek).

To treat a single (rectangular) net

To treat a single family-size (12.5 m 2 ) mosquito net made of nylon/polyester material at 0.5 g/m 2 , mix 375 ml of water with 25 ml of permethrin 25% emulsifiable concentrate. Stir the mixture and pour it into a plastic bag. Put the net into the bag and seal it by tying or twisting. Shake and knead the bag vigorously for 10 minutes (see Fig. 1.74). Then remove the net and place it on the bag (cut open to make a sheet) to dry.

To treat 20 nets

To treat 20 standard family-size (12.5 m 2 ) mosquito nets made of nylon/polyester material at 0.5 g/m 2 , fill a plastic dustbin with 7.5 litres of water and 500 ml of permethrin 25% emulsifiable concentrate. Stir the mixture and add the nets one by one, immersing and pressing them until completely saturated. Make sure you wear gloves. Take the nets out of the solution, allow to drip, and dry flat, preferably in the shade, on a plastic sheet.

Spray-on application

Spray-on application of insecticide (Fig. 1.78) may be preferred for the large-scale treatment of fabrics (74).

The advantages of such application are:

— quick application and quick drying;

— for thick fabrics, application on only the outer surface may reduce losses of insecticide by inward penetration;

— less insecticide is used if it is applied only to the parts where contact with insects is likely to occur;

— suitability for quick mass treatment of nets in villages, where some people may object to their nets being washed or impregnated together with other nets in one container.

The disadvantages are:

— some training is needed to make sure the correct dosage is applied;

— a considerable quantity of insecticide may be lost to the atmosphere.

Pressurized spray cans

Spray cans containing permethrin (0.5%) or flumethrin are available; they are convenient but expensive. One can containing 85 g of permethrin is sufficient for the treatment of only 3.5 — 4.5 m 2 of fabric, because much insecticide is lost to the atmosphere during spraying.

Several models exist which are widely used in malaria control programmes. These sprayers are suitable for the application of a mixture of insecticide in water.

These sprayers were developed for spraying specially prepared solutions of pyre-throids without pressure. No water has to be added and the fabrics dry quickly. Because the droplets are electrically charged they are attracted to electrically grounded material; the spraying of wide-mesh netting is therefore possible.

How to spray a fabric

Method 1. The procedure for the dilution of insecticide is as follows. Samples of fabric with a known surface area are sprayed with water by moving the spray nozzle at constant speed and distance from the fabric (see Chapter 9).

Fig. 1.78. Applying insecticide to a mosquito net with a hand-compression sprayer.

By adjusting the speed of movement of the nozzle it is possible to avoid dripping and ensure quick drying. The water consumption is measured from the reservoir of the pump. The solution can then be prepared as explained on p. 88. It is important to spray the side that will be exposed to insects because the other side may acquire less insecticide. Following this procedure, fabrics for spraying can be suspended from a line to which they can remain attached for drying.

Method 2. Spray thin fabrics and netting material to the point of run-off (full saturation) with an insecticide solution made up as described on p. 88. A plastic sheet should be hung behind the fabric to be sprayed to collect excess solution. It is also possible to hang the nets, one by one, on a dripping device for spraying (see Fig. 1.77).

When to re-treat

Fabrics must be re-treated when the insecticide has lost its strength. Loss in effectiveness occurs for the following reasons.

· The insecticide slowly degrades or evaporates, processes that are accelerated by exposure to direct sunlight.

· Insecticide leaches out on exposure to rain.

· Washing causes loss of insecticide.

· Frequent handling and daily folding up of nets causes loss of insecticide.

To extend the period between treatments, it is important to:

— avoid unnecessary handling of treated fabrics;

— treat fabrics soon after washing, so that they will not need to be washed again for some time after treatment;

— store the fabric in a plastic bag or box (this avoids both deterioration of the insecticide and the accumulation of dust);

— use alternative methods of cleaning, e.g. shaking or brushing with a soft brush; if washing cannot be postponed, the fabric should be washed in cold water without using soap;

— use coloured nets that do not show dirt and dust;

— time treatments in accordance with the seasonal patterns of biting and disease transmission.

Approximate duration of residual efficacy of permethrin (74)

Unused mosquito net

>6 months
(1-2 years in airtight bag)

Mosquito net used daily

Net used daily and washed after 1 month in

Net used daily and washed weekly in cold water

Clothing worn daily and washed weekly

Measuring residual efficacy

Reduced effectiveness can sometimes be observed by an increase in numbers of biting insects and the survival of insects that make contact with treated fabrics or bednets. If impregnation is primarily aimed at controlling certain species of mosquito, it is their survival that should be observed.

Re-treatment is needed when:

— mosquitos manage to enter a bednet and stay alive;
— bloodsucking insects manage to feed through treated material and stay alive after walking, crawling or resting on it.

In many cases the loss in effectiveness is difficult to observe. However, if transmission is seasonal it is usually sufficient to treat the net once a year at the beginning of the transmission season. Where there is insect-borne disease it is very important not to wait until the treated fabric loses its protective action before retreating. Several simple methods are available for measuring residual effectiveness (Fig. 1.79). Tests should be conducted on freshly impregnated fabrics, to obtain baseline results for comparison with later tests. Each test should be repeated several times.

· Release in mosquito net. This is a simple but inaccurate test that does not require any special materials other than those needed to collect live mosquitos. Hang the net in such a way that it makes contact with the floor and put a white sheet underneath. Collect 50 mosquitos and release them in the net. After 15 minutes, enter the net and collect the mosquitos; record the number dead and alive. Re-treatment is needed when fewer than 16 of the mosquitos are killed, and no mosquitos have been observed resting on the sheet. If mosquitos do rest on the sheet, the test is invalid and must be repeated. Resting on the sheet can be avoided by closing the net from below for the duration of the test.

· Petri dish method. This method is suitable for all kinds of fabric and for many types of biting insect. Invert a flat transparent container, such as a Petri dish without a lid, over the treated fabric (Fig. 1.79a). Collect live insects of the species against which protection is sought and confine them in the space between the fabric and the container. Measure the time required to knock down or kill at least 80% of the insects. Thus if five successive batches of 10 mosquitos are exposed and the average time for the eighth mosquito to fall down is six minutes, this is the knock-down period for 80% of the mosquitos. If the test is repeated several months later and the time required to obtain 80% knock-down is much longer, say 60 minutes or more, then it can be concluded that the fabric should be re-treated.

· Bioassay cone method. This method, recommended by WHO (78), requires special equipment and training. The cone can be attached to the fabric with a rubber band (Fig. 1.79b) or by pinning it to a piece of wood or cardboard held under the fabric. If unwoven fibres or wide-mesh material are used, the cone can be applied to a pad of the material made by folding it several times. Expose 10 mosquitos at a time for about three minutes. Then, remove the mosquitos and transfer by means of an aspirator (Fig. 1.79c) to a clean paper or plastic cup which is screened and contains a piece of cotton soaked in 10% sugar water. Record the numbers knocked down at 1 hour and 24 hours after exposure.

Repeat the test five times, so that 50 mosquitos are tested. The insecticide is considered to be still effective if at least 40 of the exposed mosquitos (80%) have been knocked down. Because mosquitos might die as a result of rough handling or poor condition, a control test should be carried out in a similar way, but with the mosquitos exposed to untreated netting. If the mortality in the control group is over 20%, the test should be repeated.

Determination of the residual effectiveness of a treated bednet:

Fig. 1.79.(a) Petri dish method;

Fig. 1.79.(b) bioassay cone method;

Fig. 1.79.(c) removing mosquitos from a bioassay cone by means of an aspirator.

Fig. 1.80. Surplus insecticide can be disposed of safely by pouring it into a pit latrine or a specially dug hole in the ground.

Disposal of insecticide 1

Insecticide solution can be used for reimpregnation for a few days after preparation. Any solution remaining after this time should be disposed of carefully. It should not be disposed of where it may enter drinking-water, washing-water, fish ponds or rivers, as pyrethroids are very toxic to fish. It should be poured into a specially dug hole in dry ground where it will be absorbed quickly, degraded and will not cause any environmental problems (Fig. 1.80). The solution may also be used to treat sleeping-mats or string mattresses to prevent mosquitos from biting from below. Where bedbugs are a problem, mattresses can be treated. Surplus solution can be used for killing insect pests such as ants and cockroaches; it should be poured or sponged on to infested places (under kitchen sinks, in corners). Insect breeding can be temporarily reduced by pouring solution in and around latrines or similar places.

Making houses and shelters insect-proof

Many mosquitos attack people at night inside houses. To a lesser extent, biting midges and, in some dry areas, sandflies also enter houses to bite. Methods that restrict or prevent the entry of mosquitos into houses offer significant protection to the inhabitants.

Methods that prevent entry or kill insects that have entered include the use of aerosols, mosquito coils, vaporizing mats and repellent smoke. With all of these methods there is the disadvantage that there is no residual effect. In addition to bednets, more permanent solutions that are more effective, convenient and longer-lasting are needed.

House design

Relatively few mosquitos enter houses built on poles, or apartments above the ground floor, because many species prefer to fly close to the ground (109). However, mosquitos have sometimes been found high in apartment blocks, e.g. in Calcutta (110).

Fewer and smaller openings in a house also mean that fewer mosquitos enter. In tropical areas, ventilation openings such as windows and eaves provide easy access to flying insects, although some mosquito species are less likely to find the openings and enter the house than others (111). Openings not needed for ventilation should be closed when possible (112). Blocking the eaves may be unacceptable because of the restriction on ventilation. However, screening the eaves is a good idea (see below). Doors and windows should fit and close properly. Only modern air-conditioned houses can be kept completely closed at night in hot areas.

If eaves cannot easily be blocked or screened, a ceiling may be constructed to stop mosquitos entering the living quarters. If a solid ceiling is too heavy for the house structure, a lightweight ceiling of hessian-type cloth, woven (matting) material or mosquito netting can be constructed (Fig. 1.81). In houses with corrugated iron roofs, ceilings offer the added benefit of partial insulation from heat radiated from the roof. However, a disadvantage of a solid construction is that it may provide a habitat for small mammals, birds and snakes and, in South America, for triatomine bugs, vectors of Chagas disease (113,114).

Fig. 1.81. Mosquitos can enter houses in the tropics through the eaves. A lightweight ceiling not only prevents this but also offers some insulation from heat radiated from the roof (© WHO).

Anti-mosquito screening

Screening of doors, windows and other openings in houses prevents insects from entering, while maintaining some ventilation. To stop most mosquito species, the openings in the netting should be 1.5 mm or less. To stop sandflies or biting midges the openings must be much smaller. Screening is often unacceptable because of the restriction on ventilation. However, it is commonly used in areas where mosquitos and mosquito-borne diseases are a problem throughout the year and where artificial ventilation is available.

Screening can be fitted permanently to the openings of a house or put on frames to make it removable (Fig. 1.82). The latter is more expensive and requires skilful fitting.

Screening should be regularly inspected for tears and holes.

Fig. 1.82. Properly constructed screen door (left) and improvised hinges for a screen door or window (right) (© WHO).

Cotton netting: efficient but easily damaged; ventilation is reduced by up to 70%.

Metal screens: ventilation is reduced by 30 — 50%; rodents are prevented from entering. Many metals corrode rapidly in humid areas; stainless steel or copper screens avoid this problem but are expensive.

Plastic screens: cheap and easily fitted; ventilation is reduced by up to 35%. Nylon screening is not durable when exposed to direct sunlight; fibreglass coated in PVC is more durable.

Insecticide-treated screening and curtains

The treatment of screening with insecticide may provide a cheap and practical solution to some of the above problems. Treated screening or curtains provide a toxic barrier to mosquitos and other biting flies that try to enter houses (115119). Because the treated surface of the screening irritates or kills mosquitos on contact they are not able to find openings in it. The disturbed behaviour of surviving mosquitos after such contact ends their attack. Mosquitos entering a house through unscreened openings may be killed later when attempting to leave through a treated screened opening.

In some houses, treated screening and curtains can be as effective as mosquito nets. In addition, they require considerably less netting and insecticide, and are thus cheaper, and unlike mosquito nets, once installed, little or no action is needed on a daily basis on the part of members of the household.

Problems with screening (Fig. 1.83)

Holes: mosquitos are persistent and often find openings.
Ventilation: fine-mesh screening obstructs flow of fresh air.
Windows and doors: movable screens are needed.
Eaves: often difficult to attach screening without leaving openings.

Treatment method

The instructions given for the treatment of fabrics with insecticide (see p. 85) can be applied. Recommended dosages per square metre are 0.75 — 1.00 g of permethrin, 0.05g of cyfluthrin, or 0.025 — 0.035 g of deltamethrin or lambda-cyhalothrin.

Fig. 1.84. Treated screening in the eaves need not fit exactly because mosquitos are unlikely to find openings (© WHO).

Practical advantages over untreated screening:

· Treated screening is easier to install. Because of the toxic effect, mosquitos are unable to search for holes or other small openings and so there is no need for the screening to fit perfectly (Fig. 1.84).

· A wider mesh size can be used (102,103) (see p. 84), allowing better ventilation, an important advantage in hot climates.

Alternative materials for screening

Instead of gauze it is also possible to use fibres and strips or loose hanging curtains for treatment (Fig. 1.85).

Materials that can be used:

— fibres obtained by unpicking polyethylene or jute sacks;
— string;
— bead curtains;
— plastic strips.

Fig. 1.85. Fibres or strips treated with insecticide can be used to protect doorways.

Protection measures for tents

Campers sleeping in tents are often attacked by biting insects. Many tents have screening of net or gauze but mosquitos may enter through small spaces when the screening is being opened or closed. Moreover, the mesh size of the screening is usually too large to stop biting midges that occur near swampy areas. To stop these insects the mesh size must be smaller than the usual 1.2 — 1.5 mm. However, a smaller mesh size could significantly reduce ventilation.

· Pressurized insecticide sprays and vaporizers could be used inside tents after closing. In small tents this could be unpleasant for the occupants because of the confined space. In bigger tents, mosquito coils offer protection throughout the night, but should be used with care because of the limited space and the flammability of tents and sleeping bags. If coils are used in a tent, they should be placed in coil holders (p. 65). Alternatively, the coil can be placed just outside the tent, in a coil holder to protect it from humidity and wind (Fig. 1.86).

· Screening can be treated with a repellent or a pyrethroid insecticide to deter flying insects such as midges which would otherwise pass through the mesh.

· A repellent for skin application (e.g. deet), sprayed on screens, may stop insects from passing through for several days. Treatment by spraying with or dipping in a long-lasting pyrethroid insecticide is preferable because it is cheaper and the effect lasts several months longer. Of these pyrethroids, only permethrin and flumethrin are available in spray cans (see p. 94). Alternatively, screening can be soaked in an emulsion of pyrethroid. Dosages are the same as for bednets (see Tables 1.3 and 1.4). Another method of application is to wet the screening with a sponge. The widely available pressurized spray cans containing knock-down insecticides are not suitable for treatment of screens because the insecticidal effect does not persist. Soaking the tent material itself with emulsifiable concentrate is not advisable (see box).

Fig. 1.86. Additional protection measures for campers: mosquito coils placed just outside the tent.

Spraying the interior surface of a tent

Nomadic people, refugees, soldiers and others living in tents in areas with endemic vector-borne diseases or insect nuisance may, under certain conditions, obtain protection by spraying the interior surfaces of their tents. As with the indoor spraying of houses, this kills indoor-resting mosquitos and sandflies and reduces other pests.

Because of the close contact with the tent material it is recommended to spray only with residual insecticides of low toxicity to humans, such as the pyrethroids. With permethrin, a dosage of 0.5-1.0 g/m 2 on the inside surface is recommended. On thick tenting material the spraying procedure is the same as for the spraying of house walls. Wettable powders are not suitable for this purpose, and emulsifiable concentrates should be used (120). However, emulsifiable concentrate formulations should not be used on waterproofed tent material, because they may affect the water-proofing. Oil-in-water emulsion formulations are suitable for such material.

Treated sheeting for temporary shelters

Temporary shelters are used by people who are on the move, among them gold-miners, hunters, loggers, rubber-tappers and semi-nomadic forest people. In addition, new settlers may live for some time in unfinished buildings. Such shelters offer little protection from biting insects, and consequently bednets and repellents are often used to reduce biting.

In addition to the use of treated mosquito nets, insecticide-treated sheeting (121) offers a more lasting solution. This material is attached to the poles of the shelter which support the roof, and can also be used to cover door and window openings (Fig. 1.87); it can be rolled up during the day. Some mosquitos that rest outside or inside on the sheeting are killed, and others are repelled after brief contact. Additional advantages offered by treated sheeting are those of privacy and protection from the wind. When the shelter is abandoned the sheeting can be removed and reused elsewhere.

Fig. 1.87. Insecticide-treated sheeting of woven polypropylene can be attached to the poles of temporary houses.

The material must be strong, cheap and suitable for treatment. Woven polypropylene meets these requirements and is widely available. The pyrethroid insecticides appear to adhere well and show good resistance to being washed off by rain (G.B. White, personal communication).

The sheeting can be soaked or sprayed with pyrethroid insecticides, following the instructions given on p. 85. For speed and convenience, spraying may be preferred where spray pumps are available (see p. 93). Recommended dosages per square metre are 0.75 g of permethrin, 0.05 g of cyfluthrin, or 0.025 g of deltamethrin or lambdacyhalothrin.

Avoidance and diversion of biting Diptera

Avoidance

Personal protection is sometimes possible by avoiding places where mosquitos and biting flies are known to rest or breed, and by not visiting risky places during peak biting hours. For many species, these are the hours immediately after sunset and before sunrise.

Many mosquitos and biting flies prefer to fly against a slight wind, as it carries odours to them. Thus, when a new house or village is to be built or a tent or temporary structure erected, mosquitos can be avoided to some extent by choosing a site downwind of the nearest mosquito breeding sites (assuming that there is a prevailing wind direction). New settlements in forests could be surrounded by a forest-free belt between 1 and 2 km wide in order to gain protection from forest-dwelling mosquitos; a belt about 300 m wide is appropriate if protection is sought from phlebotomine sandflies. Sometimes it is possible to eliminate potential mosquito breeding or resting places outdoors by environmental measures such as drainage, levelling, and cutting bushes (see p. 114).

Many people prefer to place their houses close to rivers, creeks or ponds so as to be close to a supply of water. Depending on the breeding and resting habitats of the local vector species, this may increase the risk of being bitten. One solution could be to provide piped water or to collect rainwater in a mosquito-proof collection system.

Diversion to animals

In some areas, zooprophylaxis could be an effective way for communities and individuals to reduce their exposure to biting insects and the transmission of disease. Many mosquito and fly species prefer to feed on animals rather than on humans. Relocation or the introduction of cattle or other domestic animals may divert many mosquitos from humans to animals. Differences between villages in the same area in the mosquito biting rates or in the numbers of malaria cases can sometimes be explained by the presence or absence of domestic animals (122124). Cattle placed between settlements and mosquito breeding and daytime resting sites, for instance on the outskirts of villages, attract mosquitos and thus provide some protection for humans. In Japan, the siting of animal shelters far away from rice fields proved effective against the Culex vector of Japanese encephalitis (14).

However, the local situation needs to be studied by experts before this method can be recommended. Pigs, for example, may serve as a reservoir of Japanese encephalitis in rice-growing areas in parts of south-east Asia. If they are kept near human habitations in an attempt to attract mosquitos away from people, some mosquitos may carry the disease from animals to humans, making the situation worse rather than better.

In practice, there are few known instances of people using this method successfully to reduce biting nuisance or disease transmission. However, there are examples of people suffering from an increase in bites and disease transmission because cattle and other animals were removed and bloodsucking insects were left with only people to feed on. This has happened where draught oxen have been replaced by tractors, where cattle farming has been abandoned (125) and in settlements in forest areas where wild animals have disappeared as a result of hunting. Malaria epidemics in India have been explained by a decrease in the number of cattle linked to severe drought in one year followed by heavy rains in the next, creating abundant mosquito breeding sites (126).

The presence or absence of animals in a village may have an impact on the effectiveness of vector control measures. For example, domestic animals may enhance the effectiveness of mosquito nets by providing easily available alternative blood-meals to mosquitos that have failed to feed on people sleeping under bed-nets. Without attractive animals in a village to feed on, hungry mosquitos are likely to persist until they manage to feed on a person not protected by a mosquito net (see box, p. 82).

Insecticide spraying

Insecticide spraying of walls

Mosquitos and biting flies seek shaded undisturbed resting sites for part of their life. In drier regions, houses are an important resting place for mosquitos and phlebotomine sandflies. In humid forested areas the insects are less dependent on houses and often rest in vegetation outdoors. However, even species that usually rest outdoors may enter houses to feed and may then spend some time resting indoors before and after feeding.

When mosquitos and other insects rest in houses it is possible to kill them by spraying the walls with a residual (long-lasting) insecticide. Mosquitos resting on sprayed walls come into contact with insecticide through their feet and are killed. Some insecticides irritate mosquitos and cause them to leave houses. In dry or windy areas, this may also result in death due to lack of suitable outdoor resting places. Wall-spraying may not prevent biting. Hungry mosquitos entering a house may bite first and then be killed when resting on a treated wall.

As most anopheline vectors of malaria enter houses to bite and rest, malaria control programmes have focused primarily on the indoor application of residual insecticides to the walls and ceilings of houses. House-spraying is still an important malaria control method in some tropical countries while in others its importance is diminishing because of various problems that have arisen. Methods that are less costly and easier to organize, such as community use of impregnated bednets, and that produce long-lasting improvement, such as elimination of breeding sites, are now being increasingly considered.

Indoor residual spraying is generally not very effective against Aedes aegypti, the vector of dengue, or against Culex quinquefasciatus, the vector of lymphatic filariasis, at least partly because of their habit of resting on unsprayed objects, such as clothes, curtains and other hanging fabrics rather than on walls and ceilings (127). Moreover, Culex quinquefasciatus is resistant to DDT and other chlorinated hydrocarbon insecticides. Other insecticides, with the exception of the residual pyrethroids, would be too expensive for sustained control over many years. A practical problem in urban areas is the large number of rooms that would have to be sprayed.

The spraying of houses and animal shelters in rural areas to control the Culex vectors of Japanese encephalitis is also generally ineffective because of the outdoor biting and resting habits of the vector species (5).

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Sandflies that rest indoors can be effectively controlled by spraying the inside surfaces of walls and the interiors and exteriors of doorways, windows and other openings with residual insecticides. The insecticides, dosages and application techniques are similar to those used against anopheline mosquitos for malaria control. Only in a few areas have insecticides been sprayed against leishmaniasis alone. In most cases the control of malaria mosquitos has been the main priority, that of sandflies being coincidental.

Before spraying is undertaken, detailed studies should be conducted to obtain data on the localities where disease transmission occurs, the season of transmission, the vector, its resting and biting behaviour, and its susceptibility to insecticides.

Proper insecticide spraying also requires trained personnel; these may be professionals employed by a government programme or community members employed by a local health organization to carry out spraying duties seasonally. Spraying equipment needs maintenance, and spare parts must be available.

How to spray

The insecticide is normally sprayed on to a surface with a hand-compression sprayer. For a discussion of suitable insecticide formulations, spray pumps, spraying techniques and the maintenance of equipment, see Chapter 9. The insecticide should be selected for its effectiveness against the target mosquito species, its price and its availability. A decision should be taken only after consulting the health authorities.

The entire inner surfaces of roofs and walls and the lower surfaces of large items of furniture are usually sprayed (Fig. 1.88). In some areas, vector species rest only on the lower wall surfaces, for example below 1.5 m, so substantial savings can be made by spraying wall surfaces only up to that height. Conversely, some mosquito species prefer to rest in the upper parts of houses, close to the roof.

In areas where mosquitos transmit malaria or other diseases seasonally, insecticides should be applied just prior to the onset of the period of transmission. This is particularly important when short-lasting insecticides are used which offer protection only for a few months. Large programmes may have timing difficulties because of the need to spread the spraying operations over the year; priority should be given to optimal timing of spraying in localities known to have most cases of malaria.

Special precautions to be taken before a house is sprayed

Furniture and food must be removed from the house or placed in the centre of a room and covered with a plastic sheet to stop insecticide particles settling on them (Fig. 1.89). The person carrying out the spraying must wear a hat and clothing that covers as much of the body as possible, including arms and legs. For indoor spraying it is recommended that the nose and mouth be covered with a simple disposable or washable mask (see Chapter 10).

Fig. 1.88. Wall and roof surfaces can be sprayed with a residual insecticide against indoor-resting mosquitos.

Fig. 1.89. Furniture and food must be removed or covered with a plastic sheet before a house is sprayed.

Some problems related to house-spraying

· In some areas vector insects may be resistant to the commonly used insecticides.

· The spraying of walls often leaves a visible deposit of insecticide, especially when a wettable powder suspension is used. The use of the emulsifiable concentrate formulation of the same insecticide or a more potent compound requiring a lower dosage (e.g., one of the pyrethroid insecticides) may partly solve this problem; however, some of the alternative formulations and insecticides may be too costly for use on a large scale.

· Some people may object to wall-spraying on religious grounds.

· The washing or replastering of walls, as may be done for religious or cultural reasons, reduces or eliminates the efficacy of insecticides.

· The community may be reluctant to allow strangers into their houses, for fear that they will interfere with women or steal.

· Some domestic pests, such as bedbugs, have become resistant to DDT and certain other insecticides. As a result, house-spraying no longer offers the incidental benefit of their control; furthermore, it is widely believed that spraying makes these pests more aggressive.

Alternative methods for applying insecticides to walls

Insecticidal paint can be applied to suitable surfaces, such as timber or plaster (Fig. 1.90). This method requires more time but can be done without spray pumps. If a wall surface is to be painted anyway, the only extra cost is that of the insecticidal ingredient.

Insecticidal paints are commercially available but can also be made by mixing insecticide with ordinary paint. The following factors have to be taken into account:

— the paint must have a neutral pH because most insecticides degrade rapidly when mixed with an alkaline paint emulsion;

— the insecticide must have a high vapour pressure to ensure movement of the insecticide particles to the surface of the paint (e.g., propoxur, pirimiphos methyl, fenitrothion).

Where it is common practice to replaster interior wall surfaces of mud or cement, attempts have been made to mix insecticide with the plaster before its application. This is not recommended because most of the insecticide is wasted, being unavailable for contact with insects at the surface.

Fig. 1.90. Insecticidal paint can be applied with a brush.

Space-spraying with insecticides

Insecticidal aerosols are sometimes used for the killing of flying and resting insects in situations where immediate results are needed, for example during outbreaks of disease or when high densities of nuisance insects are a public health problem (Fig. 1.91). Because the insecticidal action does not last long it is usually necessary to repeat the procedure several times. Space sprays are usually applied in and around houses in cities or villages and sometimes on outdoor resting places in dense vegetation or salt marshes. Special equipment is needed, such as motorized knapsack mist-blowers or shoulder-carried thermal foggers. Vehicle- or aircraft-mounted aerosol generators are also available. Space-spraying involves the use of thermal or cold fogs and ultra-low-volume sprays (128, 129).

Fig. 1.91. During epidemics and when the density of nuisance mosquitos reaches an unacceptable level, space-spraying can be carried out for immediate short-term results.

Acceptance of space-spraying by communities

In many communities there is growing concern over the use of insecticides and their impact on health and the environment. The extent to which this affects public cooperation varies widely between countries and localities. However, with appropriate educational messages, especially during vector-borne disease outbreaks or when the nuisance problem is severe, compliance with official requests to open doors and windows to allow better penetration into houses of aerosols and fogs is likely to be increased. The spraying of insecticides from motorized truck-mounted machines is a high-profile activity, often regarded as a means by which political leaders can be seen to be taking action to address the problems of nuisance or disease transmission.

Space-spraying has a number of advantages and disadvantages in comparison with residual wall-spraying:

— it has an immediate effect on adult populations of insects and is therefore suitable for the control of disease outbreaks;

— for a single application, it is less labour-intensive and large areas can be treated fairly quickly;

— less insecticide is required for one application in urban areas;

— it kills mosquitos that do not rest in houses.

— it has a high recurrent cost: the effect of the spray does not last and spraying may have to be repeated;

— the cost of equipment is high, as are operational and maintenance costs;

— there is a need for specially trained staff for maintenance and repair;

— its high cost makes it unsuitable for multiple applications in rural areas;

— it may cause pollution and contamination of non-target areas and organisms;

— there are problems with acceptability among inhabitants of some areas because of the odour and the belief that the spraying is unhealthy.

Prevention of breeding

This section provides practical information on methods for preventing breeding by mosquitos. The other groups of biting Diptera are not included here because the available methods are generally not suitable for use by non-professionals.

Mosquito species differ in their preferences for breeding habitats. Thus, some species breed in clean water containers in and near houses, whereas others prefer polluted water in sanitation systems, or man-made and natural habitats in rural areas. In order to gain knowledge of the exact type and location of the breeding habitats of a target species, careful study by an expert is generally required; once the breeding sites are known, appropriate control measures may be simple and inexpensive.

In the domestic environment, such studies are less important: most breeding sites in and near houses are easy to identify and simple methods are available to eliminate them. Community members can and should take action against any breeding by mosquitos observed on their premises, irrespective of the importance of the species as a nuisance or vector of disease.

Larval control may be the only effective approach when mosquitos bite outdoors and do not enter houses to feed or rest, or when the mosquitos are not susceptible to the available insecticides. An important advantage of larval control is that some of the measures provide permanent protection. Permanent control of mosquitos can be obtained by altering or eliminating the breeding places; this is called source reduction. Such measures include covering or screening water containers, draining ponds and marshes, and filling in ditches, pools, etc. Semipermanent measures that have to be repeated include cleaning up refuse and containers serving as breeding sites, clearing vegetation from the shores of ponds and creeks, changing water levels in lakes and reservoirs, flushing streams and repairing drains.

Many breeding sites in both urban and rural areas are man-made and their creation should be avoided as much as possible. Examples of such breeding places are: used tins and bottles, leaks from taps and water pipes, badly designed drainage and sewage disposal systems, faulty irrigation systems, borrow-pits and reservoirs. Good planning, design and maintenance can prevent much mosquito breeding.

Larval control is also possible without changing breeding sites. Fish that eat mosquito larvae can be released into breeding sites, and substances that kill the larvae, such as chemicals, bacterial larvicides, oils and polystyrene beads can be applied to the water surface.

The control of larvae does not have an immediate effect on the numbers of biting mosquitos, and it may be several days or weeks before a reduction in their numbers can be achieved. Larval control provides protection for a community or a few neighbouring households rather than strictly personal protection: all people living close to the former mosquito breeding places will benefit. On the other hand, mosquitos will continue to fly in and bite if breeding continues nearby.

Methods to control larvae include the following:

— eliminating or changing the breeding place to make it unsuitable for development of larvae;
— making the breeding place inaccessible to adult mosquitos;
— releasing fish or other predators that feed on larvae;
— applying larvicides.

How appropriate is larval control?

The control of breeding places must be carried out around human settlements in an area with a radius greater than the flight range of the target mosquito species. For many species this is about 1.5 — 2 km. Control measures that are not permanently effective have to be maintained throughout the period when the mosquito acts as a disease vector. The effort and expense needed to obtain effective control in such a large area for the necessary time vary little with the size of the settlement. Larval control is therefore more costly per person in sparsely populated areas than in densely populated ones. In contrast, the cost per person of measures to control adult mosquitos, such as the use of insecticide-treated bednets or indoor residual spraying, is similar in rural and urban areas. In urban areas, larval control is often more cost-effective than the control of adult mosquitos.

In places with intense transmission of malaria, almost all anopheline breeding sites need to be eliminated in order to achieve a reduction in the prevalence of malaria. Even a much reduced population density of anopheline mosquitos may be able to maintain a high prevalence of the disease.

Effective larval control is most feasible where breeding places are:

— limited in number;
— easily recognizable;
— easily accessible.

It is also preferred where:

— the mosquito breeds only during a short period;
— measures to control adult mosquitos are ineffective or culturally unacceptable;
— permanent source reduction measures are more cost-effective than repetitive control measures.

Source reduction

The term source reduction refers to any measure that prevents the breeding of mosquitos or eliminates their breeding sites. If such measures are long-lasting or permanent changes in land, water or vegetation, they are often referred to as environmental modification. When such measures have a temporary effect and need to be repeated, they are known as environmental manipulation. The drainage of swampy areas, land reclamation and other permanent methods were already being implemented early in the twentieth century. In many areas they have played an important role in the elimination or reduction of a number of vector-borne diseases.

Removal or destruction of breeding sites

Small containers, such as used cans, bottles, tyres and coconut husks used as breeding sites can be removed or destroyed. This method is commonly used to control the breeding of Aedes aegypti and A. albopictus.

The filling of mosquito breeding sites with soil, stones, rubble, ash or rubbish is the most permanent control measure available. It is most suitable for reducing breeding in small depressions, water holes, borrow-pits, abandoned ditches or pools, which do not require much filling material. On a small scale, no special expertise is needed and communities can carry out the work with shovels, picks, wheelbarrows, carts and other simple equipment. For larger landfills, tractors or other motorized equipment may be preferable. The filling material should be obtained without creating new breeding sites. Waste materials can be used for most filling.

If refuse is used it should be compacted and covered with earth to prevent breeding by flies. All fills should be topped with clean earth and graded to make the areas attractive and suitable for use as building sites, playgrounds, etc. It may be possible to collaborate with industrial or agricultural firms or public works departments, so that trucks transporting waste materials are diverted at no extra cost to places that need to be filled.

Very large areas can sometimes be filled at little cost by making use of the spoil from mining, harbour dredging, building demolition and other operations.

The drainage of water can be accomplished by constructing open ditches and dykes with tidal gates, subsoil drainage and pumping. Proper drainage reduces mosquito breeding; however, the drainage systems used in agriculture or for the transportation of sewage and rainwater in cities are often an important source of breeding because of poor design and maintenance. Leakages, obstructions, and small pools or puddles of residual water in drainage ditches often afford suitable breeding sites for mosquitos. The planning and construction of drainage systems are complicated and require the expertise of engineers. However, some small-scale drainage works intended to control mosquitos can be carried out by less experienced people using simple equipment (130).

Open earth drains are the simplest to construct. They are used to prevent the accumulation of excess rainwater in depressions in the ground and to dry out marshy areas, borrow-pits, ground pools and other accumulations of surface water.

Layout The ditches carry the water away to an appropriate, lower-lying outlet, such as a river, creek, pond, soakaway pit or main drainage ditch. They should follow the natural flow of water along the surface. To prevent erosion of the lining of the ditch they should be as straight and short as possible. Sharp bends should be avoided wherever possible (Fig. 1.92).

A main ditch may have several lateral or secondary ditches to collect water that does not readily drain into the main ditch. However, the number of such lateral ditches should be kept to a minimum to reduce maintenance.

Where lateral ditches enter the main ditch they should be brought together at an angle of about 30 degrees with the direction of the flow. If the angle is larger, the flow of water from the lateral ditch may erode the opposite bank of the main ditch. The lateral ditch should preferably enter the main ditch slightly above the normal water level in the main ditch.

Gradient To give the water enough velocity the gradient should be between 1 and 5 cm per 10 m. If the gradient and velocity are too high, this will cause erosion of the bottom and sides of the ditch.

Shape (cross-section) The optimum shape depends on the soil texture, among other factors. In stiff clay and other types of stable soil the sides may be vertical, but in sandy soils the slope may need to be 4:1, that is 40 cm horizontally for every 10 cm vertically. In most soils the slope should be about 1:1 to 2:1.

Depth This depends on the elevation of the area to be drained and on the outfall. The bottom of the ditch must be 15 cm lower than the bottom of the pool, marsh or other area to be drained.

Excavated soil or spoil Excavation of the ditch should start at the outfall end and proceed up to the area to be drained. The excavated soil is used to fill in depressions. If left alongside the ditch it should be spread or piled up evenly on each side at some distance from the edge so that it cannot be washed into the drain. A spoil bank should be perforated at frequent intervals to permit drainage into the ditch (Fig. 1.93).

Fig. 1.92.(a) Correct drainage of pools. (131).

Fig. 1.92.(b) Incorrect drainage of pools. (131).

Fig. 1.93. Location of a spoil bank at some distance from the edge of a ditch. The spoil bank is perforated to permit drainage into the ditch (132).

Fig. 1.94. A precast concrete slab used for lining ditches (131).

Lining of banks Where necessary the banks can be stabilized with masonry, bricks, poles or turf. The banks usually need to be stabilized in areas where water is turbulent, for instance near bends or where a lateral ditch enters a main ditch. By lining earth drains their performance can be improved and the cost of maintenance can be reduced. The drains last longer, are more easily cleaned, require less inspection and may ultimately be less costly than open earth drains. Open earth drains are of no use in areas with very heavy rainfall.

A drain may be roughly lined with flat stones and the spaces filled in with small stones and sealed with cement. Alternatively, a layer of concrete, 4 — 5 cm thick and reinforced with wire mesh may be used. Connecting precast slabs are also commonly used. They are usually made of concrete in sections of 60 — 70 cm with a rounded bottom and a joint to facilitate laying them in a prepared ditch (Fig. 1.94). In larger ditches, side-slabs of turf or concrete may be laid above the slabs (Fig. 1.95). In small ditches it may not be necessary to line the entire drain; lining the bottom and the sides up to 8 cm above the normal water line is usually sufficient. The banks should be kept clear of vegetation.

Culverts In places where the drain has to pass beneath a road or embankment by means of a culvert or pipe the gradient should be increased to prevent the accumulation of debris and silt (Fig. 1.96). At the entrance, a screen of vertical rods may be provided to prevent debris from entering. Culverts can be made of wood, concrete, corrugated iron, or plastic. The last two materials are preferable since they withstand stresses better than the others. Pipes can be made cheaply from used oil drums by cutting out the bottoms.

Fig. 1.95. A ditch lined with concrete and turf (131).

Subsoil drainage is more expensive than open drainage and therefore of limited value in the control of mosquitos. It is used where the ground surface has to remain unbroken by ditches to allow free movement and use of the land, and where the carth is so unstable that open ditches cannot be maintained. The advantage of this system is that the drains do not become choked with vegetation or blocked by refuse. It usually requires little inspection and additional larval control measures, such as the spraying of larvicides or oils, are unnecessary.

Subsurface drains are often used in irrigated areas for draining fields and improving agricultural production. They have been constructed specifically for mosquito control in Malaysia with the intention of lowering the groundwater level so that pools of surface water are more readily absorbed. They are also used to intercept seepage water from hills and to deal with hill streams in ravines.

Fig. 1.97. Cross-section of a simple subsoil drain: a ditch filled with a layer of stones covered with coarse sand (130).

Fig. 1.98. A subsoil drain made of unsealed tile pipes. The joints are covered with a collar of clay, roofing paper or other resistant material (133).

The simplest such drain is made by filling a deep ditch with large stones that offer little resistance to the flow of water. Cover the stones with leaves, pine needles, palm leaves or coarse sand to serve as a filter (Fig. 1.97). This prevents silt and clay from clogging the lower section of the drain. Other simple filling materials that can be used at the bottom of the drain are thick wooden or bamboo poles and inverted halves of coconut husks. This layer can be covered with coarse grass or litter and topped with soil.

An effective type of drain consists of ceramic tile pipes. The pipes are laid end to end at the bottom of a narrow ditch about 0.5 — 2 metres below ground level. The joints are left unsealed so that water can enter. On the upper half the joints are covered with garbage, leaves, strips of roofing paper, plastic or other resistant materials to reduce silting (Fig. 1.98). The pipes should be laid in an absolutely straight line with a gradient of between 1:200 and 1:400, depending on the quantity of water to be transported. Greasy water and domestic waste must not be allowed to discharge into any part of the system. Where pipes come close to the surface they may have to be protected from being crushed by vehicles by the construction of small bridges.

Fig. 1.99. Fast-growing eucalyptus trees dry low-lying marshy areas and prevent breeding by mosquitos.

Eucalyptus trees can be used for drying marshy areas and other plots of land with a high water table (Fig. 1.99). Species that grow rapidly and use a lot of water are particularly suitable. The trees dry the land by allowing water to evaporate through their leaves. For optimum evaporation they should be planted with adequate spaces between them. An additional advantage of the trees is their commercial value.

Closing, screening or covering breeding sites

Potential breeding sites in relatively small enclosed habitats, such as drinking-water storage containers and wells, should be made inaccessible to adult mosquitos. Removable covers, such as mosquito-proof lids or wire mesh screening, can be fitted in some cases (see p. 144). Wells can be made mosquito-proof by closing them with cement slabs and installing hand pumps. Latrines can be made insect-proof by improving their design (p. 149). A less conventional approach is to cover the water surface completely with a material that is impenetrable to mosquitos. Examples are expanded polystyrene beads and fast-growing plants that float on the surface, such as the aquatic fern Azolla (p. 163).

Expanded polystyrene beads

Expanded polystyrene beads can be spread on water to form a floating layer (Fig. 1.100). A layer 1 — 2 cm thick is sufficient to prevent mosquito breeding if it covers the surface completely. The mosquito larvae die because they cannot reach the water surface to breathe. The beads do not decay and remain floating for years. Because they are easily blown or washed away, the beads can only be used in sites where the water remains confined between surrounding walls. Polystyrene beads are not toxic to humans, animals or fish and are safe for use in drinking-water (134140).

Fig. 1.100. A layer of polystyrene beads prevents mosquito larvae from reaching the water surface to breathe.

Production and application Small balls (or beads) of expanded polystyrene pressed together in blocks are often used as packing material. Small quantities of beads can be obtained by breaking up and crumbling such blocks. Larger quantities can be obtained from the petrochemical industry, which produces unexpanded polystyrene beads with pentane in solid solution in each bead. The beads are expanded by heating them to 100 °C in steam or boiling water. The plastic softens and the pentane, which becomes gaseous, expands the beads to 30 times their initial volume. The beads can be expanded in a suitably equipped factory and then transported in sacks or drums to the sites of application.

Transport of the beads is easier if they are in unexpanded form, particularly if long distances are involved. The beads can then simply be expanded near the site of application in boiling water in a cooking pot (Fig. 1.101). The unexpanded beads are added to the water a cupful at a time, stirred, and then removed from the water surface with a sieve. With this method the beads expand to only 15 — 20 times their volume. It is important to keep the unexpanded beads sealed until the expansion process is undertaken, in order to avoid leakage of the pentane, since this would reduce their final size.

Beads with a diameter of 2 mm when expanded are the best for controlling mosquito larvae. Thirty litres of beads (about three bucketfuls) weigh about 1.25 kg and are sufficient to cover 3 m 2 of water (as in a typical pit latrine) with a layer 1 cm thick.

In contrast to the environmental modification methods, methods of environmental manipulation have to be repeated to remain effective. They are usually directed against one particular mosquito species and depend largely on its behaviour. While such measures may be very simple and cheap they should be applied only after careful study of the vector. Mosquito control experts may be needed to advise communities and health organizations on the method that is most appropriate locally.

Fig. 1.101. Unexpanded polystyrene beads can be expanded near the site of application in boiling water in a cooking pot.

Fluctuation of the water level in large reservoirs of drinking-water or irrigation water reduces mosquito breeding by:

— stranding the larvae at the margins;

— dislodging larvae from vegetation along the shoreline so that they are more exposed to wave action and fish;

— reducing the growth of plants along the margins between which larvae could find shelter.

The interval between the fluctuations must be less than the life of the larvae, i.e. about 7 — 10 days. The difference in water levels should usually be 30 — 40 cm (see p. 156).

Intermittent irrigation is used to control mosquitos in rice-growing areas (see p. 163).

Flushing (stream sluicing)

The principle of flushing is similar to that of water-level fluctuation. It is employed in small streams where there is a continuous and abundant supply of water flowing slowly enough to permit mosquitos to breed in quiet places along the margins. A periodic discharge of a large volume of water washes away the eggs, larvae and pupae from the edges or strands them on the banks.

In order to collect the water needed for flushing, a small dam is constructed upstream of the area where breeding occurs. The dam site should be at a point where the stream or channel is narrow and the banks are high. The dam should have a hand- or machine-operated sluice gate or an automatic siphon, to release the water at least once a week. The method requires high initial investment but is long-lasting and requires little maintenance. It has been used in tea and rubber plantations in south-east Asia to control Anopheles minimus and A. maculatus.

Changes in water salinity

Mosquitos that breed in lagoons and coastal marshes can be controlled by letting in additional seawater. Most species will not be able to tolerate the increase in salt concentration. The connection between sea and lagoons can be made with tide-gates (see box, p. 157) or simple drains or culverts.

Shading of stream banks

Where mosquitos prefer breeding sites that are partly or fully exposed to sunlight, they can be controlled by planting shrubs and trees along the banks of streams to provide dense shade. The method has been used successfully in tea gardens in Assam, India, to control Anopheles maculatus and A. minimus.

Clearing of vegetation

Clearing of vegetation may result in increased breeding by mosquito species that prefer sunlit water. However, some species need shaded water and may be effectively controlled, as is the case with Anopheles balabacensis in Sabah, Malaysia. This method may also be effective in removing resting places for adult mosquitos. In addition, it promotes evaporation and the drying up of small accumulations of water and makes breeding sites more visible for control purposes.

Removal of water plants

The larvae and pupae of Mansonia attach themselves to the submerged parts of water plants on which they depend for breathing. In ponds and swamps where Mansonia is a problem it can be controlled by periodically removing or destroying the vegetation (see p. 159). Other mosquito species can be controlled by removing vegetation which provides larvae with a safe hiding place from larvivorous fish as well as protection from wave movement and currents. In small breeding sites, such as borrow-pits and ponds, the vegetation can be removed manually, for example by the members of nearby communities, using rakes and other simple equipment. For somewhat larger sites, the vegetation can be removed by the application of herbicides or the introduction of herbivorous fish, e.g. the grass carp (see p. 159).

Sometimes, as for instance in swamp forests in parts of Indonesia and Malaysia, the removal or destruction of vegetation is impracticable because of the large size of the breeding area.

Straightening and steepening shorelines

Shorelines of streams, ditches and ponds can be modified to reduce the availability of shallow places suitable for breeding of mosquitos and to increase the flow of the water.

Biological control

The biological control of mosquitos and other pests involves introducing into the environment their natural enemies, such as parasites, disease organisms and predatory animals. They may include insects, viruses, bacteria, protozoa, fungi, plants, nematode worms and fish. The effective use of these agents requires a good understanding of the biology and behaviour of the insects to be controlled as well as of local environmental conditions. Such methods may be most effective when used in combination with others, such as environmental manipulation or the application of larvicides that do not harm the biological control agents.

Several organisms have proved effective against mosquito larvae. The most important are:

— fish that eat mosquito larvae (larvivorous fish);

— predatory mosquitos of the genus Toxorhynchites, the larvae of which feed on other mosquito larvae;

— dragonflies, the larvae of which feed on mosquito larvae;

— cyclopoid copepods, small crustaceans that attack first- and second-instar larvae of mosquitos;

— nematode worms that are parasites of mosquito larvae;

— fungi that grow in the bodies of mosquito larvae;

— bacterial larvicides, the toxic products of the bacteria Bacillus thuringiensis H-14 and B. sphaericus;

— neem, an oil extract of seeds of the neem tree, Azadirachta indica, which has larvicidal properties;

Azolla, a free-floating fern that can completely cover water surfaces and prevent breeding by mosquitos.

Of these methods only two have become widely employed: the use of larvivorous fish and the use of bacterial larvicides; the latter are discussed in the section on larvicides.

Larvivorous fish feed on mosquito larvae. They have been widely used around the world in attempts to control malaria, other mosquito-borne diseases and mosquito nuisance.

Suitable species of fish usually have the following characteristics:

— preference for mosquito larvae over other types of food located at the water surface;
— small size to allow access to shallow water and penetration into vegetation;
— high reproduction rate in small bodies of water;
— tolerance to pollution, salinity, temperature fluctuations and transportation;
— they should preferably originate from the region where control is to be effected.

Locally collected fish have been evaluated for their efficacy in controlling mosquitos and a number of species have proved useful. Most of them are tooth carp (Poeciliidae and Cyprinodontidae), small fish including many popular aquarium species. The juvenile stages, but not the adults, of some larger species may also eat mosquito larvae. Some of the most successful species to have been introduced into different countries are the top minnow or mosquito fish (Gambusia affinis) and the guppy (Poecilia reticulata). Gambusia is most efficient in clean water, while Poecilia can be used successfully in organically polluted water (141, 142). Poecilia tolerates higher temperatures than Gambusia and may therefore be more effective in rice fields in hot areas. However, unlike Gambusia, it cannot survive temperatures below 10 °C. The annual killifishes, Cynolebias,Nothobranchius and Aphyosemion, have drought-resistant eggs and could be used in breeding sites that temporarily dry out, such as borrow-pits and irrigated rice fields (143).

The original geographical distribution of some larvivorous fish belonging to the tooth carp family (Cyprinodontidae)

Tropical and subtropical Africa

India and south-east Asia

Central and South America

The importation of exotic fish species should be avoided and an evaluation should be made of the suitability of local species. When released in the natural environment, imported species may cause unwanted side-effects by replacing local species or affecting other aquatic animals. However, such fish can be freely used in man-made breeding habitats giving no access to the natural environment. Examples of such places are: water tanks and cisterns for the storage of drinking-water, swimming pools, garden ponds and water reservoirs in desert locations. These places can be stocked with Gambusia without risk of escape into nature.

Advantages and disadvantages of the use of larvivorous fish

· In a suitable environment the larvivorous fish may establish themselves and provide a self-perpetuating larval control method.

· The cost of introducing and maintaining the fish is generally low and no complicated or expensive equipment is needed.

· The fish are environmentally clean and do not render water unsuitable for drinking.

· They are only effective when large numbers eventually establish themselves and even then they do not always provide total control. Mosquitos may continue to breed at low densities. For complete control other measures have to be added, such as the use of larvicides that do not harm the fish.

· Larval control with fish may take 1 — 2 months; the method is therefore not suitable when quick results are needed.

· The fish are less effective in waters with much vegetation or floating garbage; when these are present, they must be removed.

· The fish have to be reared in special ponds; transportation and stocking require special care.

In ponds and marshes with dense aquatic vegetation, larvivorous fish are not very effective because of the difficulty of finding mosquito larvae. Bigger fish such as the carp (Cyprinus carpio) (144), the giant gourami (Osphronemus goramy) (145) and the tilapia (Tilapia or Oreochromis mossambicus) (146) can enable the larvivorous fish to reach the larvae by uprooting and eating vegetation. The bigger fish can also serve as an additional food supply for local populations (147). In some countries, fish are being reared both for consumption and as predators of mosquito larvae in various types of habitats. Cichlid fish, such as Oreochromis mossambicus, O. niloticus and O. spiluris have proved suitable for this purpose in Indonesia, Malaysia, Somalia and Sudan (148,149). The common carp, Cyprinus carpio, and the grass carp, Ctenopharyngodon idella, have been used with success in south India (144) and China. Larvivorous fish can also be used as food for the bigger fish that serve as food for the human population.

Rearing larvivorous fish

In regions where larvivorous fish frequently occur in particular habitats, these can be used as a source of fish for introduction into mosquito breeding places. This is the usual method in relatively dry areas where water is limited to canals, ditches, wells and so on. Although large numbers of fish may not always be available, this system should lead to widespread colonization with fish (150, 151).

To guarantee a regular supply of fish it is necessary to rear them in large quantities in special breeding ponds. Fish ponds are already widely used for the cultivation of fish for food and they can be simultaneously used for rearing larvivorous fish. Dykes can be built up from the soil excavated to make the pond. The dykes are built in layers of about 20 cm and each layer should be dampened and rammed down before a new layer is added. Grass and other vegetation on the dykes can serve as protection against erosion. The top of the dyke should be at least 30 — 50 cm above the water level. Large cement tanks have also been used successfully for rearing fish. Adequate space and aquatic vegetation are needed to protect the young fish from older fish. Communities can rear their own stock of larvivorous fish and distribute them to farmers and householders. Judicious artifi-cial feeding with organic waste material, animal manure and so forth can increase production. The proliferation of algae, which consume much oxygen, should be avoided, possibly by the use of a herbicide.

Transportation and distribution

The fish are best transported in small containers of up to 40 litres, such as plastic buckets and jerry cans, or in strong plastic bags, half-filled with water from the rearing pond (Fig. 1.102). Water from the new location should be added before the fish are released, to avoid the shock of a sudden change in water temperature or quality. If transportation lasts several hours or more, special care should be taken to maintain the oxygen supply in the water and to prevent major changes in temperature. Containers should be closed with about a third of their volume occupied by air. The number of fish per bag or container should be kept low and the bags or containers should be wrapped in wet cloth or placed in cardboard or wooden boxes or in polystyrene boxes with some temperature control. To supply fish to small mosquito breeding places in a community, buckets can be used containing, for example, 50 Gambusia in 8 litres of water. Six Gambusia are sufficient for a pool of 5 — 10 m 2 which has few aquatic plants (151).

Fig. 1.102. Fish can be carried from a breeding pond to where they are needed in a plastic bag half-filled with water.

Effective larvivorous fish species

The mosquito fish or top minnow, Gambusia affinis

This species is the most widely used against mosquito larvae. Together with the guppy it belongs to the live-bearing tooth carp family, Poeciliidae. Their mouths are adapted to feeding from the surface. It originates from Central America but, because of its success in controlling mosquitos, has been introduced into many parts of the world. These fish can withstand large fluctuations in temperature as well as pollution of the water, but they are most productive in relatively clean water of moderate temperature (152).

actual size (reproduced from 152)

The guppy, Poecilia reticulata

Similar to the mosquito fish, this is a live-bearing tooth carp that is adapted to taking food from the surface. It originates from South America and has become very popular as an aquarium fish. It has been introduced for mosquito control in many countries, especially in South America and Asia. The species prefers higher temperatures than the mosquito fish and can withstand highly polluted water. It has therefore been most successful against Culex mosquitos which breed in organically polluted water (153).

actual size (reproduced from 153)

The panchax, Aplocheilus panchax

This egg-laying tooth carp is found in the Indian subcontinent, Indonesia, Malaysia and Sri Lanka, where it commonly occurs in paddy fields and ditches and is important in the control of mosquitos (154). The fish can withstand pollution and water temperatures between 20 °C and 45 °C.

actual size (reproduced from 154)

The Argentine pearlfish, Cynolebias bellotii

This is one of the annual fishes that occur in South America and Africa, known as instant fish. They cannot reproduce in permanent water bodies and occur only in habitats where the water disappears every 2 — 3 months or at least once a year. The eggs, which survive the dry period buried in the soil, may be concentrated, transported and dispersed in slightly damp material. They hatch within a few hours after flooding. Although not extensively evaluated, these fish may be useful in borrow-pits and temporary dry pools as well as in rice fields and irrigated pastures where other fish cannot survive (153).

actual size (reproduced from 153)

The Mozambique mouthbrooder, Oreochromis (Tilapia) mossambicus

This cichlid fish occurs in East Africa. It has been reared successfully in irrigated rice fields where it was used both to control mosquitos and as a source of food. With an optimal temperature of 22 °C it reproduces very rapidly. The species can live and reproduce in fresh and brackish water (146).

(shown at 40%, actual size is 20 cm; © WHO)

This edible fish can be reared in irrigated rice fields, ditches and ponds; it is hardy and prefers rich, shallow waters with muddy bottoms and good aquatic weed growth. The species multiplies when the water temperature is over 18 °C. The fingerlings feed on mosquito larvae, the adults on aquatic vegetation, weeds and algae but not on rice plants. The carp can be used to control both mosquitos and weeds (153).

(shown at 25%, actual size is 32 cm; reproduced from 153).

Larvicides

Larvicides are applied to mosquito breeding sites to kill larvae. By the end of the nineteenth century, petroleum oils were being used to control mosquitos even before their role in disease transmission was discovered; the arsenical compound Paris green was also blown as a powder over water to kill surface-feeding anopheline larvae. These larvicides have largely been replaced by newer products, although oils are still being used on a small scale. The advantages and disadvantages of larvicides are given on p. 130. Larvicides may act as stomach poisons, which must be ingested by the larvae while feeding, or as contact poisons, which penetrate the body wall or the respiratory tract. Larvicides are used on breeding sites that cannot be drained or filled and where other source reduction methods or the use of larvivorous fish would be too expensive or impossible.

Paris green (copper acetoarsenite) is an arsenical compound that was used extensively from 1921 until the 1940s to control anopheline larvae. A green powder, it is practically insoluble in water. The particles float on the surface where they poison the surface-feeding anopheline larvae. Other mosquito species are usually unaffected. Its advantages were low cost, high effectiveness against anopheline larvae, portability and ease of distribution. No ill effects were recorded in animals, fish and insects, and treated water remained suitable for domestic use. It was an important tool for many malaria control programmes but its use diminished after the introduction of the relatively safe and highly effective organophosphorus compounds.

Petroleum oils

The application of oil to water surfaces in order to kill larvae was one of the earliest mosquito control methods (155, 156). The larvae are killed in two ways when they rise to the surface to breathe: by suffocation and by poisoning with toxic vapour. Larvicidal oils are not effective against Mansonia mosquitos because their larvae and pupae do not come to the surface. The oil should be applied in a thin film completely covering the surface. Many different grades of oil may be suitable for mosquito control, depending on local conditions. At higher temperatures a thicker oil is required, e.g. crude or fuel oil, while in the presence of vegetation a lighter oil with greater spreading power, e.g. kerosene or diesel oil, is necessary. The oils kill larvae very quickly but last only between a few hours and several days. Because of their relatively high cost compared with some other larvicides and because of their limited persistence, their use for mosquito control has decreased. They are of special interest in situations where mosquitos have developed resistance to insecticides. For small-scale applications by individual households or communities, they offer the advantage of wide availability.

Locally available oils

For the treatment of small water surfaces, as in wet pit latrines, a small quantity of fuel oil or waste oil from a garage may be appropriate. Detailed specifications are available on many different larvicidal oils suitable for large-scale applications (151) but in practice the user is often limited to materials that are obtainable locally in large volumes and at moderate prices. Diesel oil and kerosene (paraffin) are generally available and equally effective. About 140 — 190 litres of diesel oil have to be applied per hectare, making the method rather expensive. The cost can be reduced by 20 — 75% if a spreading agent (detergent) is added to diesel oils, fuel oils or kerosene so as to improve the penetration in vegetation and polluted water. Octoxinol is such an agent and is effective at 0.5% in oil. Alternatively, the addition of 1 — 2.5% vegetable oil, such as castor oil or coconut oil, can be used to increase spreading power. Between 18 and 50 litres of such oils per hectare may be sufficient. The exact quantity required for control depends primarily on the amount of vegetation and debris on the surface and on the degree of pollution of the water.

Specially prepared commercial oils have been developed which contain surface-active agents that increase spreading power and toxic action. These oils may be effective at 9 — 27 litres/ha. The addition of temephos may increase effectiveness. Properly used, the lighter oils are non-toxic to fish, birds and mammals.

The oils can be applied simply by dripping from a can or bucket or pouring from a watering-can. For large-scale applications it is better to use hand-compression sprayers. Very large areas may be sprayed from the air.

Advantages and disadvantages of the use of larvicidal oils

· The oil is visible on the water surface and so it is possible to see whether it has been applied properly.

· For small surfaces such as borrow-pits, pools, latrines, drains and soakaway pits, it is a relatively cheap method and easy to apply.

· Mosquitos cannot develop resistance against this method.

· At recommended dosages there is no toxicity to mammals, fish and most other non-target organisms.

· For large surfaces the method is costly.

· It is not very effective in the presence of vegetation and floating debris, which therefore has to be removed before the oil is applied.

· The effect usually lasts only a few days.

· The oil coats vegetation, tree trunks and so on.

· Under windy conditions the oil will be dispersed.

Synthetic organic larvicides

The discovery in the 1940s of the organochlorine insecticides led to the abandonment in most places of traditional mosquito control methods and the adoption of the spraying of breeding sites with the new compounds. In the course of the 1950s the organochlorine insecticides lost much of their effectiveness in many places as a result of the development of resistance by some mosquito species. It also emerged that the organochlorines were very persistent in the soil and in tissues of plants and animals. These insecticides are no longer recommended by WHO for the control of mosquito larvae, although with the exception of dieldrin they can still be used safely for spraying walls in houses. The organophosphorus compounds, the carbamates and the pyrethroids are less persistent, breaking down quickly in the environment, and they are therefore recommended as larvicides. However, the pyrethroids are very toxic to fish and should not be used where there are fish or crustaceans. Water contamination with these larvicides is temporary and most of the chemicals disappear from water within a day, although the organo-phosphorus compounds may persist much longer.

In situations where mosquitos have developed resistance to all the conventional larvicides, consideration may be given to using larvicidal oils, the more expensive insect growth regulators, or bacterial larvicides as alternatives. The last two groups are non-toxic to fish, mammals and most other non-target organisms in the environment. Formulated as slow-release briquettes they show better residual effectiveness in stagnant bodies of water of relatively small volume than any of the other available larvicides.

Among the most commonly used larvicides are the organophosphorus compounds, such as temephos, fenthion and chlorpyrifos (Table 1.5).

Table 1.5. Compounds suitable as larvicides in mosquito control

Dosage of active ingredient
(g/ha) b

Duration of effective action
(weeks)

Toxicity/ hazard of active ingredient c

Insect growth regulators

Bacillus thuringiensis H-14

a BR = briquettes; EC = emulsifiable concentrate; GR = granules; S = suspension; SC = suspension concentrate; SRS = slow-release suspension; WP = wettable powder.

b The highest dosages are for use in polluted water and for residual effect.

c U = unlikely to present acute hazard in normal use.

d Litres per hectare.

Advantages and disadvantages of larvicide application

· Mosquitos are destroyed before they disperse to human habitations.

· The operations can be carried out in a very short time.

· Many effective larvicides are widely available.

· For small-scale treatments, larvicides can be applied by hand; for larger-scale treatments use can be made of agricultural sprayers or the hand-spray pumps widely used in antimalarial house-spraying programmes.

· Control is temporary and frequent repetition could be costly in areas with many or extensive breeding sites.

· Some larvicides may harm other organisms in the environment, including the natural enemies of mosquito larvae.

· The larvicides may be toxic to humans; consequently, training in technique and safety precautions is necessary for those who apply them.

Larvicide formulations

Most larvicides are available in the following formulations:

· Wettable powder. A dry powder of the insecticide treated with a wetting (dispersing) agent to permit quick mixing with water to form a suspension that can be sprayed; easy to store and transport.

· Suspension concentrate. A liquid containing finely divided insecticide particles, a wetting agent and water; it is mixed with water to make a water-based suspension for spraying.

· Emulsifiable concentrate. A solution of insecticide in a special solvent; the addition of emulsifiers enables it to be easily diluted with water. Application involves pouring out over the water surface or spraying.

· Granules and pellets. Inert materials, such as grains of sand or absorptive materials, coated or impregnated with insecticide. Granules and pellets are relatively heavy and penetrate dense growth of water plants better than liquid formulations. Some types sink to the bottom of the breeding site, while others float on the water surface where they are more effective against surface-feeding Anopheles larvae. Some allow rapid release of the active ingredient, others permit slow release. Application is by hand or with portable blowers. Granules are heavy and may pose transportation problems for large-scale applications. They are often made locally by mixing sand or other carrier materials with the insecticide solution.

· Briquettes. This is a block of an inert matrix material impregnated with insecticide; it floats on the water surface and slowly releases the active ingredient. Briquettes are applied by hand.

The most commonly used formulations are the emulsifiable concentrates, which are usually applied with a portable sprayer; wettable powders and suspension concentrates can be applied in the same way. In large-scale programmes, spraying is often carried out with machines mounted on vehicles. Aircraft are sometimes used to spray very large or inaccessible areas. For small-scale operations the material can be distributed by hand. Liquid can be poured from a bottle, can or bucket over the water surface. Granules can be spread by hand. Direct contact between skin and insecticide should be avoided by using gloves. Because most products have a very limited residual effect when used as larvicides they have to be reapplied every 1 — 2 weeks in most tropical areas.

Temephos, an organophosphorus compound, is highly active against mosquito larvae and other aquatic insects, while its toxicity to fish, birds, mammals and humans is very low. Its low toxicity to non-target organisms and low effective dosage make temephos the most appropriate larvicide in many situations. It is recommended for the control of mosquito larvae in drinking-water and in areas where vertebrates may come into contact with it (155, 158, 159) and has been widely used in rivers in West Africa for the control of blackfly larvae. It is also effective in polluted waters. It is commonly available as emulsifiable concentrate (46% and 20% (w/v) active ingredient) and granules (1% active ingredient).

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Large surfaces: granules and water suspensions of the emulsifiable concentrate are applied by spraying. The target dosage should be 55 g of active ingredient per ha on relatively clean open water and 110 g of active ingredient per ha where there is dense aquatic vegetation. The granules are more effective where there is dense vegetation and should be applied at intervals of 1 — 3 months.

Small surfaces: small quantities of granules can be added to drinking-water containers, where they remain effective for about five weeks. The recommended dosage in drinking-water is 0.5 — 1 mg of active ingredient per litre, which corresponds to 20 g (two teaspoonfuls) of a 1% sand granule formulation in a 200-litre drum. Liquid formulations can be applied by pouring the appropriate quantity from a can or bucket on to the water surface.

Floating plastic pellets impregnated with temephos are currently being tested. This formulation remains effective for up to six weeks and is of particular relevance to the control of Anopheles larvae, which feed on the surface.

Fenthion is an organophosphorus compound with a quick killing action on larvae and a long residual effect. The compound has a relatively high toxicity to humans, mammals and birds, and precautions should be taken (see Chapter 10). At normal dosages for larval control, fish are not affected. It is mainly applicable to polluted water in ditches, ponds, swamps, septic tanks and other mosquito breeding sites that are not used as drinking-water supplies by humans or domestic animals. In polluted water, fenthion is more effective than freshwater larvicides such as temephos and methoprene. Frequent applications in certain areas have caused resistance to develop in some species of target insects, especially Culex quinque-fasciatus. Fenthion is commonly available as emulsifiable concentrates (46% and 84.5% (w/v) active ingredient) and sand granules (2% active ingredient).

Large surfaces: spray is applied at a dosage of not more than 112 g of active ingredient per ha. The final concentration in treated water should not exceed 0.1 mg/litre. Emulsifiable concentrates can be applied directly or after mixing with water. The 2% sand granules are applied using a portable blower at 5.5 kg/ha and are preferred for treating areas with dense vegetation or with a layer of floating debris. Granules are used in shallow water and slow-moving streams that are not more than 30 cm deep.

Small surfaces: the emulsifiable concentrate formulation can be poured directly into ponds, small ditches, septic tanks, etc. With the 46% emulsifiable concentrate, 0.2 ml should be used per cubic metre of water, corresponding to 0.1 mg of fenthion per litre. For application with a hand-compression sprayer, 10 ml of the 46% emulsifiable concentrate should be mixed with 10 litres of water. Two litres of this mixture should be sprayed per 100 m 2 of water with a depth of about 10 cm. The granules should be applied by hand, using gloves, at 1.25 g/m 2 of water surface not deeper than 50 cm.

Malathion, an organophosphorus compound, is effective against a great variety of insects. Although primarily used as a residual spray against adult mosquitos, it also kills mosquito larvae in breeding sites in sprayed areas. It is particularly effective against Aedes aegypti in urban areas. At the usual dosages (224 — 1000 g/ha) it is considered safe for humans and domestic animals in the treated areas, but it may cause harm to fish.

Various formulations are available but they are not routinely used for the control of larvae and are applied only by specialized mosquito control agencies.

This organophosphorus compound is commonly used as a larvicide in moderately to highly polluted water, where it has a residual effect lasting up to several weeks (157). It is used successfully in catch basins, ditches containing sewage, pit latrines, cesspits and sewage collection and treatment sites. It is highly toxic to fish and moderately toxic to mammals and birds. It should therefore never be used in water used for drinking or containing fish, and should be handled only by people trained in the safe use of insecticides (see Chapter 10).

Chlorpyrifos is commonly available as emulsifiable concentrate (48% (w/v) active ingredient), granules and wettable powder.

Hand-compression sprayers are used to apply 11 — 25 g of active ingredient/ha (157).

Pirimiphos methyl is an organophosphorus compound, effective against a large variety of insects, including mosquito adults and larvae. It has levels of activity similar to those of fenthion but is much less toxic to humans. However, it cannot be used for the treatment of drinking-water. It is relatively unstable in polluted water. It is commonly available as a 50% emulsifiable concentrate.

Hand-compression sprayers are used to apply 100 g of active ingredient per hectare. The treatment remains effective for 1 — 11 weeks, depending on water quality.

Pyrethroids such as deltamethrin and permethrin can be used as mosquito larvi-cides. However, because they have serious effects on all insects, fish, crustaceans and aquatic animals in general, their use should be limited to special cases only, under the close supervision of specialized mosquito control agencies.

Insect growth regulators

Insect growth regulators are chemical compounds that are highly toxic to insect larvae or pupae, interfering with their development into adults. They have a very low toxicity to mammals, birds, fish and adult insects, but are highly toxic to crustaceans and immature stages of aquatic insects. Their use is limited by their high cost and restricted availability, but they may be of particular interest where target insects have developed resistance to the organophosphorus larvicides or where these compounds cannot be used because of their effect on the environment. Insect growth regulators may not be acceptable where immediate killing of larvae is required, for instance where householders are legally obliged to control mosquito larvae on their premises. They break down rapidly in the environment but they may last between several weeks and several months when applied as granules, microcapsules or briquettes. They can be divided into the two following groups.

· Juvenile hormone analogues, e.g. methoprene, prevent the development of larvae into pupae or of pupae into adults; they do not kill larvae.

· Chitin synthesis inhibitors interfere with the moulting process, killing the larvae when they moult. They thus act more rapidly than the juvenile hormone analogues. Examples are diflubenzuron and triflumuron.

Although insect growth regulators are unlikely to pose a threat to humans or domestic animals, they can disturb the development of various species of arthropod living on the breeding sites where they are used. Most manufacturers therefore advise their use only on aquatic sites where there is a low risk to populations of crabs, shrimps and other non-target arthropods through direct application, run-off or drift.

Methoprene is considered by WHO to be safe for use in drinking-water (160). The active ingredient is fairly rapidly decomposed in water. Briquettes containing 1.8-8% methoprene and granules of various concentrations have been devised to obtain a longer residual effectiveness. The briquettes release methoprene slowly over a period of up to four months in stagnant water in containers but for considerably shorter periods in flowing water. If a breeding site dries up, the briquettes may remain effective until it is flooded again (161). In anticipation of flooding or rains they can be applied to dry places known to be potential breeding sites. The treatment of ground pools in Kenya five weeks before flooding with rainwater effectively controlled mosquito breeding in the month following the rains (162, 163). The main advantage of such pretreatment of breeding sites is that it can be done in areas that become inaccessible during the wet season. Target areas are ditches, drains, catch basins, pools, tidal marshes, freshwater swamps and borrow-pits. The briquettes are unlikely to be effective in sites where they can be removed by flushing. In muddy areas the briquettes may become clogged and this reduces their effectiveness.

The briquettes are applied by hand; no equipment is needed. For this reason they are particularly suitable for application in remote areas, in breeding sites where a long residual effectiveness is required. They should be placed in the deepest part of a breeding site so as to maintain control during the dry season. For Aedes, 4 — 6 kg of active ingredient should be applied per ha. One briquette should be placed per 20 m 2 in shallow pools (less than 60 cm deep) where the water is stagnant. For the other mosquitos, such as Anopheles, Culex and Mansonia, the dosage should be doubled.

Diflubenzuron is used mainly for spray-on application to mosquito breeding sites in open water, whether clear or polluted. It is suitable for use in irrigated fields with food crops. It may last for 1 — 2 weeks but in closed systems, such as cesspits or latrines, the effect may persist for up to a month. It has also been used to control the breeding of biting midges in swampy areas. Diflubenzuron is commonly available as wettable powder (25% active ingredient) or as granules (0.5% diflubenzuron).

The wettable powder is mixed with water and applied with spray equipment at a rate of 25 — 50 g/ha on clear water surfaces and 50 — 100 g/ha on polluted water surfaces. The granular formulation is used on breeding sites with heavy vegetation or flowing water. It is applied by hand or with portable blowers.

Bacillus thuringiensis H-14

The bacterium Bacillus thuringiensis serotype H-14 (B.t. H-14) produces toxins which are very effective in killing mosquito and blackfly larvae after ingestion. At normal dosages it is harmless to other insects, fish, higher animals and humans and is suitable for use in water used for drinking or for the irrigation of food crops. It is effective against insects that have developed resistance to chemical larvicides (148, 164). It breaks down quickly in the environment and has to be reapplied periodically. The product is more expensive than most conventional larvicides but cheaper than the insect growth regulators.

B.t. H-14 is commonly available as wettable powder and granules. A briquette formulation has recently been developed which floats on the surface and releases B.t. H-14 for about 30 days. The effectiveness of the briquettes is not affected by alternate wetting and drying and they are therefore suitable for both permanent and temporary habitats. The briquettes, which are ring-shaped and have a diameter of about 5 cm, are intended for the treatment of small breeding sites in the domestic environment (165), such as ponds, basins and tanks, and for areas that are difficult to reach. On open water surfaces the briquettes are not very effective because winds may blow them to one side (166). Briquettes that have an insoluble matrix become ineffective in slightly polluted water because the matrix material becomes clogged; they can therefore only be used in clean water.

B.t. H-14 is generally referred to as providing biological control. However, the product contains mainly dead bacteria, living spores and toxic crystals in the spores which do not multiply, and it could therefore also be considered as a biologically produced insecticide.

The wettable powder formulation is mixed with water and sprayed with hand-compression pumps or other spray equipment. Granules are applied by hand or with portable blowers to breeding sites covered with vegetation. Briquettes are applied by hand, using up to 4 per 10 m 2 of water surface. Where they are likely to be blown by the wind, they should be attached by string to plants, poles or other fixed objects (167). The briquettes should be stored in sealed packages in a cool place so as to protect them from humidity.

Another bacterium, Bacillus sphaericus, also produces a toxin. It has characteristics similar to those of B.t. H-14 but is more effective in polluted water while B.t. H-14 is more effective in clean water. It is not effective against blackflies or Aedes aegypti (148, 164). Unlike B.t. H-14 it is produced as a formulation containing living bacteria that multiply even in polluted water. B. sphaericus usually has a longer action than B.t. H-14. It is considered very suitable for the treatment of breeding sites of Culex mosquitos in polluted water (165). It has a higher residual effectiveness in such habitats than most other larvicides and offers the added advantages of safety to non-target organisms and lack of resistance (168). This method is still being developed but some products have already reached the market. In field tests, slow-release B. sphaericus pellets and briquettes have been found to be effective against mosquito larvae for over eight weeks (165). Granule, wettable powder and flowable or soluble concentrate formulations also exist.

The soluble concentrate is thoroughly mixed with water and applied with a hand-compression sprayer. During spraying the tank has to be shaken from time to time. Dosages are dependent on the target species and type of water. To control Culex in small accumulations of stagnant water, the suspension should be applied at 0.1 — 10 ml/m 2 . Residual activity may continue for 1 — 2 weeks at the lowest rate and for 2 — 3 months at the highest rate. Larger surfaces with polluted water are sprayed at 1 — 4 litres/ha.

Habitats in and around houses

Mosquito breeding places in and around houses can be divided into two main types:

· Breeding sites with clean water: mainly rain-filled receptacles in humid tropical areas which are suitable breeding sites for some Aedes species.

· Breeding sites with polluted water: mainly in on-site sanitation systems and bodies of stagnant and polluted water favoured by Culex species.

Measures to prevent breeding in and around houses are usually simple and based on source reduction. They can be implemented by householders on their own premises without expert advice.

Breeding sites with clean water

Most accumulations of clean water are only temporary. Rain-filled receptacles in gardens may dry out in a few days or weeks. These habitats are favoured by Aedes aegypti, which can act as a vector of dengue and yellow fever, and by Aedes albopictus, also a dengue vector and known in the Americas as the Asian tiger mosquito. These species also breed in containers that are used to store water for drinking or washing. While Aedes aegypti commonly breeds and feeds inside houses, Aedes albopictus is more common outside, in open spaces with shaded vegetation and suitable breeding sites such as car tyres and garbage dumps. Anopheles stephensi, a vector of malaria in some urban areas in south Asia, often breeds in wells, ponds, cisterns and containers used for the storage of drinking-water.

The breeding sites with clean water can be divided into two groups for which different control measures are needed:

— temporary breeding sites indoors and outdoors;
— permanent breeding sites in water storage containers, wells and pools.

Temporary breeding sites indoors

Breeding sites for Aedes mosquitos can be found in and around houses in flower vases, pot plants, pot-plant saucers and neglected ant traps (containers filled with water and placed under the legs of food cupboards) (Fig. 1.103). In vacant houses, breeding may occur in toilet bowls, toilet-flushing cisterns, and drains in bathrooms and kitchens.

· Avoid excessive watering of pot plants.

· Change water in flower vases weekly and scrub to remove adhering mosquito eggs before refilling with fresh water. Temephos or fenthion sand granules can be added to flower vases and other temporary breeding sites (Fig. 1.104).

· Salt, temephos sand granules (p. 132) (Fig. 1.105) or a floating layer of oil (p. 128) can be added to the water in ant traps; alternatively the water may be replaced by grease.

· In vacant houses, toilet bowls and gully and floor traps should be covered and the overflow pipe of the flushing cistern should be made mosquito-proof with a piece of netting or cloth (Fig. 1.106). For shorter periods the use of a disinfectant or a larvicide might be advantageous (p. 150).

Fig. 1.103. Aedes can breed indoors in (1) pot plants, (2) pot-plant saucers and (3) ant traps.

Fig. 1.104. Temephos or fenthion sand granules can be added to flower vases.

Fig. 1.105. Add salt or temephos sand granules to the water in ant traps.

Temporary breeding sites outdoors

Breeding sites can be found outdoors in rubbish, discarded tyres, discarded household and garden utensils, construction materials, roof gutters, water storage containers, drinking-water tanks, plants and various other objects (Fig. 1.107). If villages are located near a beach or river bank, breeding also occurs in water in the bottom of boats.

· Small pools should be filled up with earth, stones or sand and levelled. Deeper rain-filled pools can be filled with rubble and covered with a layer of soil. Where there are many pools during the rainy season, rapid treatment with a suitable larvicide (p. 128) by spraying or hand application may be more practicable.

· Rubbish should be cleared up and disposed of through the local refuse collection system if one exists (Fig. 1.108). Communities may use refuse to fill borrow-pits, pools and other low-lying areas. Refuse should be covered regularly with a layer of soil to prevent flies, mosquitos and rodents from breeding. The final cover of compacted earth should be at least 50 cm deep and should have a slope of 1 — 5 cm per 10 m for drainage. Such sanitary landfills (p. 114) eliminate mosquito breeding, permit refuse disposal and improve land values. Landfill areas have been used for house construction, children’s playgrounds and so on.

Fig. 1.106. Toilet bowls, floor traps and overflow pipes should be made mosquito-proof.

· Old tyres can be stored under a roof or cover to prevent the collection of rainwater (Fig. 1.109). Piercing a hole will also prevent the collection of water. Tyres can also be filled with soil and used as plant pots. The application of larvicides or oil (p. 128) to accumulations of rainwater in tyres kills larvae.

· Large objects such as old cars, refrigerators and washing machines are important breeding places and should not be left in the open where they can collect rainwater.

· Buckets, bowls and watering cans should be stored in a dry place, covered, or turned upside down.

· Construction materials should be covered with a plastic sheet or stored under a roof. Holes in construction blocks used as wall material should be filled with sand or cement (Fig. 1.110).

· Gutters should be inspected periodically. If necessary they should be cleaned (Fig. 1.111) or repaired with a suitable gradient (an inclination of about 1 cm over 10 m length) to avoid standing water.

· Tree holes can be filled with sand or concrete (Fig. 1.112). The leaf axils of banana trees and bromeliads often contain rainwater to which temephos (p. 132) can be applied.

· The open-ended stumps of bamboo fences should be cut down to the nodes (Fig. 1.113) or filled with sand to prevent the accumulation of rainwater.

Fig. 1.107. Some examples of outdoor breeding places of Aedes. Breeding occurs in (1) discarded cans and plastic containers, (2) bottles, (3) coconut husks, (4) old tyres, (5) drums and barrels, (6) water storage tanks, (7) bromeliads and axils of banana trees, (8) obstructed roof gutters, (9) plant pot saucers, (10) broken bottles fixed on walls as a precaution against burglars, (11) holes in unused construction blocks, and (12) the upper edge of block walls.

Permanent breeding sites

Water storage containers

Jars, cisterns and water storage tanks provide suitable breeding places for Aedes species and Anopheles stephensi. The introduction of a reliable and properly designed piped water supply reduces dependence on water storage containers and should lead to a reduction in breeding sites. Measures to prevent breeding in water containers must not adversely affect water quality or interfere with the addition or removal of water.

Fig. 1.108. Dispose of rubbish safely through the organized collection system or by burying it.

Fig. 1.109. Store tyres under a cover.

Fig. 1.110. Fill holes in construction blocks with cement or sand.

Fig. 1.111. Clean gutters regularly.

Fig. 1.112. Fill tree holes with sand or concrete.

Fig. 1.113. Cut down bamboo stumps to the node.

Fig. 1.114. Cover open jars with a tight-fitting lid.

Fig. 1.115. Cover barrels and drums with cloth or netting.

· Small water jars should be completely emptied about once a week and the inner surfaces scrubbed to remove mosquito eggs.

· Open jars can be covered with rigid lids (Fig. 1.114). This only stops mosquitos if the jars have smooth edges and the lids fit tightly.

· Jars, drums, barrels and other containers can be covered with cloth or netting (Fig. 1.115). Flexible lids can be made by fitting durable netting on a frame. This allows rainwater to enter.

· Water tanks can be made mosquito-proof with a fixed cover incorporating a sieve to allow rainwater to enter (Fig. 1.116). If a tap is attached to the bottom of the tank, the cover can be left permanently in place. Regular cleaning or changing of the sieve is necessary.

Large water tanks can be equipped with a self-cleaning wire-mesh screen cover. One model consists of stainless steel screening embedded in a cement mould (Fig. 1.117). Rainwater runs through the screening into the tank, leaving behind dirt, most of which is automatically washed off because of the slope of the screening.

This strainer has been developed in Tonga. It is not commercially available, but could be made on a large scale using a metal mould. If small numbers are needed, it would be more practical to attach the wire mesh to a base made of metal, timber or other water-resistant material.

Fig. 1.116. A fixed cover incorporating a sieve.

Fig. 1.117. A self-cleaning wire-mesh cover for a water tank.

Fig. 1.118. Elevated storage tanks should have a tight-fitting lid.

Elevated or roof-top water storage tanks, to which water is pumped from below, do not have an inlet for rainwater, but often have an opening to allow for cleaning and inspection. This opening should have a tight-fitting lid (Fig. 1.118).

· A layer of polystyrene beads (p. 119), completely covering the water surface, stops mosquitos from breeding and reduces evaporation. The beads can be applied to the water in tanks that have an outlet at the bottom. If the water level in the tank falls to the level of the outlet, there is a risk of the beads blocking the pipe; to prevent this, the outlet should be screened with durable mesh or fitted with a downward bent pipe (Fig. 1.119). The latter device is commonly used to prevent floating dirt from being drawn from a tank.

Fig. 1.120. Temephos sand granules can be wrapped in permeable cloth for use in water containers.

Tanks should be covered to prevent birds, squirrels or lizards from trying to walk on the layer of beads.

· Temephos (p. 132) is a relatively safe and effective insecticide that can be used in drinking-water at a dosage rate of not more than 1 mg/litre. At the recommended dosage it does not give a detectable taste to drinking-water and is harmless to humans, other mammals and fish. It is available as sand granules impregnated with 1% insecticide. In a water tank, the sand granules slowly give off the insecticide for about 4 — 6 weeks. Temephos granules are cheaper and more widely available than methoprene or B.t. H-14.

Granules can be wrapped in permeable cloth so that the packets can be removed easily when a domestic water pot is emptied for cleaning and then replaced when it is refilled (Fig. 1.120).

· Methoprene (p. 135) is safe for use in drinking-water, but is more expensive than temephos. Slow-release briquettes may last up to five months.

· Bacillus thuringiensis H-14 (B.t. H-14) (p. 136) is tasteless and safe for use in drinking-water. It is available in granules and tablets (briquettes) which float on the surface and slowly release the larvicide. Briquettes may last about four weeks in a drinking-water tank.

· Larvivorous fish: some fish species that feed on mosquito larvae can be used in large water storage containers that are located in shade, so that there are no large fluctuations in temperature. Some light and a minimum amount of food are required.

Suitable fish species need to be able to survive long periods with little food and to tolerate temperature fluctuations. Fish should be available to re-stock water tanks. The mosquito fish (Gambusia affinis) and the guppy (Poecilia reticulata) are suitable because they are easy to rear in large quantities.

In China, good results have been obtained with the Chinese catfish (Clarias fuscus), one of which is sufficient in each domestic water pot (20 — 100 litres). The species survives for long periods. Measures may have to be taken to prevent them from jumping out of the container. In Somalia, a tilapia species (Oreochromis spiluris spiluris) has been used with success in underground water tanks; one fish is sufficient for 3 m 3 (133, 148).

Wells and pools

In many tropical countries, wells and pools in rural and suburban areas with relatively clean water are used as breeding sites by anopheline mosquitos. Larvivorous fish (p. 123) or larvicides (p. 128) such as temephos, methoprene and Bacillus thuringiensis H-14 (p. 136) can be applied in wells used for drinking-water.

Many wells have been abandoned following the installation of piped water supplies. Filling them would be an effective solution. If wells have to remain available for possible reuse the introduction of larvivorous fish could also be considered. In places not subject to flooding, an effective and long-lasting solution is the application of a layer of polystyrene beads (p. 146).

Small pools of clean water are often found on construction sites and in the basements of buildings. They should be filled up or drained. In some cases larvivorous fish (p. 123) or larvicides (p. 128) such as oil are appropriate. Pools of relatively clean water are often formed near standpipes that do not have a proper drainage system, such as a gutter or a soakaway pit.

Breeding sites with polluted water

Culex quinquefasciatus, the vector of urban filariasis in some areas, breeds in on-site sanitation systems such as wet pit latrines, septic tanks, cesspits, cesspools, drains and canals containing stagnant water polluted with organic waste. They also breed in polluted water associated with home industries, for example in coconut husk pits. Other breeding sites are pools and disused wells used for dumping garbage. Culex gelidus, an important vector of Japanese encephalitis in south-east Asia, also breeds in polluted water. Pit latrines are the main breeding sites of blowflies (Chrysomyia), which are sometimes present in large numbers and may carry disease agents from faeces to food.

Pit latrines, used for the disposal of human excreta, basically consist of a hole in the ground covered with a floor with a hole and surrounded by walls to provide privacy. The pits are normally dry in areas with dry porous soil. Only flies breed in dry pits. However, in areas with a high water table the pits contain water and often produce thousands of Culex quinquefasciatus. The improvement of sanitation leads ultimately to the development of water-borne systems whereby excreta are flushed through a short pipe to a septic tank or sewerage system.

Fig. 1.121. Mosquito-proof lid for a pit latrine.

Fig. 1.122. Mosquito-proof lid for a pit latrine: (a) plan and (b) section with dimensions in millimetres. Reproduced from reference 169 with permission. Copyright John Wiley & Sons Limited.

Insect-proof covers prevent insects from entering or leaving pits (135). Lids of wood or metal do not fit tightly enough to be mosquito-proof. However, lids made of concrete can be cast in the holes in which they are to fit (Figs. 1.121 and 1.122).

· The lid is relatively heavy, so it is difficult for children to lift.
· The edges of the lid are easily damaged, leading to loss of insect- and odour-proofing.
· Insects may enter and leave and odours may escape when the lid is lifted.
· The cost is high.

Pour-flush latrine with water seal (135). As for flush toilets, pit latrines can be provided with an S-bend water seal to prevent entry or exit of insects and the escape of odours (Fig. 1.123). Latrine slabs incorporating a water seal are widely available in Asian countries. The system has to be flushed with at least one litre of water and works best where people are accustomed to taking water with them to the toilet for washing. To avoid blockages and damage to the seal, solid objects should not be deposited in the latrine.

Ventilated improved pit latrine (135, 170175). A ventilated pit latrine has a ventilation pipe fitted which draws away odours when an air current blows across the top of the pipe. Fresh air is sucked into the pit through the squat hole in the slab covering the pit.

The inside of a latrine building with a roof is relatively dark. Blowflies emerging in the pit are attracted up the vent pipe because it is better illuminated than the squat hole. This method is partly effective against Culex. Because the top of the pipe is covered with flyproof netting, insects are unable to escape and eventually die.

Fig. 1.123. A pit latrine incorporating a pourflush system for insect- and odour-proofing.

For proper functioning it is essential that it is dark inside the latrine building. In rectangular structures, which have a door in one wall, care has to be taken to keep it properly closed, but there should be a gap, usually above the door, to allow air to enter. An open door reduces the effectiveness of the system by letting in daylight. Permanently dark conditions are ensured by building a doorless structure with a spiral ground plan (Figs. 1.124 and 1.125).

Application of expanded polystyrene beads

Polystyrene beads poured on to the water in a pit form a floating layer on the water surface. A complete layer 1 — 2 cm thick is sufficient to prevent mosquito breeding (p. 119). An additional advantage is the suppression of the odour that emerges from the pit. After faeces have dropped through it, the layer of beads reforms immediately. If a pit dries out the beads are buried under faeces. However, because of their buoyancy, the beads return to the surface when water enters the pit. The beads will last for several years provided they are not swept away by flooding.

Application of larvicides

Oil (156), chemical larvicides such as fenthion and chlorpyrifos, and the bacterial larvicide B. sphaericus can be used to control mosquito larvae in pit latrines(p. 128). In liquid form they can simply be poured into the pits, but a better distribution over the surface is obtained by spraying. Larvicides can be applied quickly and have an immediate effect. Their main disadvantage is the need for repeated applications, which in the long term may make this method costly. Oil and most chemical larvicides remain effective for only a few weeks at most. B. sphaericus may remain active for up to eight weeks after application, especially at high dosages.

Fig. 1.124. Flow of air in a ventilated improved pit latrine (© WHO).

Fig. 1.125. Components of a ventilated improved pit latrine with mud-and-wattle walls and a thatched roof (176).

Oil has the advantage that it is widely available. Small quantities of waste oil may be obtained free of charge and are sufficient for the treatment of a latrine. Oil kills mosquito larvae only if the surface area is completely covered. On heavily polluted surfaces, however, oil does not always spread well and may be quickly destroyed.

The chemical larvicides have to be applied in higher dosages than are needed for the treatment of unpolluted water. Compounds other than fenthion and chlorpyrifos may be used if available. Larvicides should remain effective for at least a few weeks.

In areas without a piped water-borne sewerage system, septic tanks are commonly used for the disposal of sewage. The watertight settling tanks receive wastes carried by water flushing down short sewers. Inside the tanks the waste separates into liquid and solid matter, the latter having to be removed at intervals. The liquid effluent may flow out of the tank through an outlet and is usually disposed of in a soakaway pit or led into a drain. The overflow sometimes forms a puddle in which mosquitos can breed. The tanks are inconspicuous but very important breeding places for Culex mosquitos. Aedes mosquitos may also be found in septic tanks. The mosquitos enter the tanks through the ventilation pipes and the water outlets (Fig. 1.126). Cracks or other openings in covers are commonly formed when tanks are opened for desludging and periodical inspections; these should be sealed immediately.

· Cover the ventilation pipe with aluminium or stainless steel mesh screening.

· Ensure that the cover is effectively sealed; a practical solution is to cover it with sand; large gaps can be filled with foam rubber.

· A soakaway pit (see below) should be installed if excess water is periodically discharged from the tank.

· Close the outlet with material that can easily be removed.

· Apply oil, chemical larvicide or polystyrene beads if the above measures are not possible (see section on wet pit latrines). If polystyrene beads are used the outlet should be screened to prevent them from being flushed out.

In many rapidly expanding urban areas, there are few facilities for the disposal of wastewater. Under such conditions, householders may dig pits on or near their premises for the disposal of effluents. Water in soakaway pits tends to stagnate and can become a favourable breeding place for Culex and, less commonly, Aedes.

· Fill the pit with small stones (Fig. 1.127).

· If the pit does not overflow regularly apply polystyrene beads (p. 119).

· Apply oil or chemical larvicides (p. 128) to the surface to obtain immediate short-term protection.

Fig. 1.126. Septic tanks often constitute important breeding places for Culex, which enter and leave through the ventilation pipe, the overflow outlet and improperly closed openings.

Fig. 1.127. A soakaway pit that collects run-off water used for washing and bathing can be filled with small stones to prevent mosquitos from breeding.

Urban areas generally have two drainage systems, one for the disposal of sullage (washing) water and sewage and the other for the drainage of rainwater. Rainwater is often drained through a surface drainage system while sewage is disposed of via either an underground or a surface drainage system, the latter often merging with the rainwater drainage system. Underground systems may seem the best because they are not easily accessible to mosquitos. However, lids on the inspection sites are not always properly closed and when problems arise they are difficult to solve. The underground system may become blocked, overflow and form puddles in which mosquitos can breed. The surface drains often replace the underground system when it becomes clogged.

The surface drains should have a gradual slope along their course to enable a sufficiently rapid flow of water to prevent mosquito breeding and blockages. Once blockages start to form, the water flow is retarded and complete blockage of the drains becomes likely. Together with the dumping of garbage in drains this creates favourable conditions for breeding by Culex and other mosquitos. If surface drains are partly covered, inspection, cleaning and maintenance are made more difficult.

· Make sure that inspection openings of underground drains are properly closed.

· Ensure that the system is properly maintained and repairs are carried out promptly.

· Remove dirt, debris and other obstacles in the system to allow the water to flow freely (Fig. 1.128).

· Flush the drains periodically with clean water to remove dirt and debris and destroy mosquito breeding sites; this may be practicable in areas where water is abundant (e.g. by the sea).

· For an immediate short-term solution, apply oil or chemical larvicide (p. 128) to the places where larvae are observed.

Fig. 1.128. Dirt, debris and other obstacles in drainage systems should be removed to allow the water to flow freely.

Habitats in the field

Several species of disease-carrying mosquito breed away from the domestic environment, both in naturally occurring habitats, such as swamps, rivers, creeks, lakes and ponds, and in man-made habitats, such as reservoirs, irrigation systems, irrigated fields and borrow-pits. Breeding in man-made habitats can sometimes be prevented by proper planning and design to make them unsuitable as breeding sites or to facilitate the implementation of control measures.

Effective larval control in rural areas requires a thorough understanding of the behaviour and breeding sites of the target species. Control activities should be planned, designed and supervised by experts in vector control so as to avoid mistakes and the waste of valuable resources. On some sites, such as swamps, rivers and lakes, these activities may have to be carried out by specialized teams. However, for the smaller breeding sites the involvement of local health services, communities, farmers and others is often essential to secure control.

The most important targets of control methods are the malaria-carrying Anoph-eles mosquitos, which breed in a wide variety of habitats. Other targets are the Mansonia vectors of brugian filariasis which breed in swamps and pools, and the Culex vectors of Japanese encephalitis which breed mainly in rice fields and adjacent ditches.

Larval control measures usually have to be carried out over an area with a radius of 1.5 — 2 km from human habitations, the maximum flight range for most species of mosquito. In some areas the transmission of disease and the breeding of mosquitos are largely limited to well-defined periods of the year, during which control efforts are particularly important.

Swamps and marshes

In many countries, swamps have become less important as breeding sites for mosquito vectors of disease as a result of land reclamation associated with urban and rural development.

If swamps prove to be breeding sites they may be filled if relatively small and close to towns. Filling is usually uneconomic if swamps are extensive and there are no large settlements within flight range. Breeding sites can be drained (p. 114) and dried out by reducing the amount of water going into swamps or by increasing the rate of drainage. The input of water can in some cases be reduced by digging interceptor drains around the outer edges of swamps, usually at the foot of a hill (Fig. 1.129). In other cases drainage ditches may be made across swamps. Sometimes a dam can be constructed at the lower end of a swamp, causing conversion of the swamp into a deep lake in which larvivorous fish can be kept. Shallow marshlands and plots of land with a high water table can be dried out by planting eucalyptus trees (p. 119).

Larvicides are sometimes used in emergency situations. Granular formulations are most suitable, because they are easy to disperse and fall through vegetation into the water. Ultra-low-volume sprays (p. 132) may be applied by hand- or vehicle-carried mistblowers or by aircraft, the latter being used for quick action in extensive areas that are difficult to enter by other means. If fogging is conducted at the upwind side of a swamp, the wind will carry the particles into it over a considerable distance. Seasonal marshes that become submerged during and after the wet season may be treated with slow-release formulations (briquettes) of insect growth regulators (p. 134).

Lakes and reservoirs

If breeding occurs in lakes and reservoirs it normally takes place along the margins in shallow places protected by vegetation from fish, waves and excessive sunlight.

Sometimes it is possible to construct a dam with a sluice at the outlet of a lake, as is usual in man-made reservoirs. This allows the water level to be raised and lowered (p. 121) at intervals. Keeping the water at a high level kills terrestrial vegetation on the shore. The level is kept as low as possible during the mosquito breeding season so as to strand mosquito larvae and floating objects along the margins and to inhibit the growth of aquatic plants. This leaves a clean margin around the lake which does not offer any suitable breeding sites. The water level is raised and lowered at intervals of 7 — 10 days, less than the time needed for the development of the aquatic stages of the mosquito. Where needed this measure can be supplemented by the removal of accumulations of floating debris and vegetation.

Coastal swamps and lagoons

Swamps and pools often occur behind coastlines, which are filled with seawater during spring tides but which are not subject to daily tidal action. As with freshwater swamps they can be filled or drained if this is economically feasible. Alternatively, automatic sluice gates can be constructed which let water out at low tide but prevent the influx of seawater at high tide. A simpler solution is to connect swamps and pools to the sea by ditches or culverts. This provides tidal action and the alternating presence of fresh and salt water, which prevents the development of suitable breeding habitats for most mosquito species (other than Anopheles melas in West Africa) (Fig. 1.130).

To control pest mosquitos (especially Aedes species) breeding in tidal salt marshes, granular insecticides are sometimes used; they release insecticide only after flooding, which coincides with the hatching of the eggs.

Larvicides

Larvicides can be applied from a boat with a spraying machine.

Rivers and creeks

Mosquitos breed in quiet places close to the banks of rivers and creeks where there is protection from currents by obstacles, protruding roots, plants and so on. Effective control of larvae is generally difficult because of the large areas to be covered. Careful study is required to find out the exact location of the breeding sites.

Fig. 1.131. During the dry season mosquitos may breed in stagnant pools in river beds.

Breeding sites may be reduced in some cases by removing obstructions and vegetation from the edges of the rivers and smoothing and increasing the gradients of the banks to increase the water velocity. In the dry season, pools may form in river beds (Fig. 1.131). If breeding occurs in such pools they can be drained into the main stream. Some smaller pools may be filled up.

Pools in river beds may be treated with larvicides (p. 128) that are not toxic to fish, or to other animals or humans using the water for drinking. In pools that dry out quickly, one application is sufficient. In pools that are likely to exist for a month or longer it is advisable to apply a slow-release formulation, e.g. temephos sand granules, to avoid the need for frequent reapplication.

Some success has been obtained in the control of Anopheles maculatus, a vector of malaria in parts of south-east Asia, which breeds in forest creeks in hilly areas. The breeding of this mosquito can sometimes be prevented by spraying quiet places and pools in the creeks with larvicidal oil or a chemical larvicide that does not harm fish (p. 128). In a few places small dams have been built upstream of breeding sites. The water collected behind the dams is released at intervals to flush out larvae (p. 121) and destroy breeding habitats. The dams, which are either hand-operated or automatic, provide a long-lasting solution without high recurrent costs.

Ponds

Permanent ponds containing unpolluted water are commonly used for breeding by Anopheles and Mansonia. Mansonia occurs only in the presence of aquatic vegetation to which the larvae and pupae can attach.

Earth for filling can sometimes be obtained from a deep pond to be used for the cultivation of fish (p. 114). In most cases, however, filling is too costly and may even be unacceptable where ponds are used as water supplies.

Larvivorous fish are effective in long-lasting control of mosquito larvae in ponds (Fig. 1.132; p. 123). The shorelines of ponds should be made steep and water plants along the edge should be removed to enable the fish to reach the larvae (p. 122). In addition, carp or gouramis can be introduced to eat the vegetation. It may be possible to deepen one end of the pond so that the fish can survive if the water level falls in the dry season.

Removal of water plants

The removal of water plants renders ponds temporarily unsuitable for breeding by Mansonia (Fig. 1.132; p. 122) (177, 178).

Fig. 1.132. Ponds can be rendered unsuitable for mosquito larvae by removing water plants.

Removal of vegetation along margins and steepening shorelines

These measures reduce the breeding of most mosquito species temporarily by taking away protective cover and removing shallows (Fig. 1.132; p. 122).

Oil and chemical larvicides

These can be applied to the water surface for a quick effect of short duration (p. 128). Slow-release formulations, such as briquettes of methoprene (p. 135) or B.t. H-14 (p. 136), last a month or longer. Care should be taken not to use a compound that could kill naturally occurring predator insects and fish. Herbicides are sometimes used to destroy the plants to which Mansonia larvae attach themselves.

Soil is used in rural areas of many countries for the construction of houses and roads. It is collected from pits that are usually located outside villages. The pits may collect rainwater or seepage water and provide breeding places for a number of mosquito species. Older pits containing vegetation are usually better breeding sites than freshly dug pits.

Soil for filling up borrow-pits can sometimes be obtained from village fish ponds that are being deepened or expanded (p. 114) (177,179 — 181). Alternatively, borrow-pits can be used for the disposal of household rubbish or industrial waste, such as sawdust and cinders (179). The rubbish should be covered with a layer of soil to prevent exposure of potential water containers, such as bottles and cans, or access of flies and rats to the rubbish.

A row of borrow-pits may be drained with ditches into one single pit, so that only one wet pit remains to be dealt with.

Application of oil and larvicides

This provides a quick solution of short duration. However, slow-release briquettes of methoprene (p. 135) or B.t. H-14 (p. 136) last 1 — 4 months and may be sufficient to cover most of the breeding season. Once the pits dry out the briquettes stop releasing larvicide, but under favourable circumstances may be reactivated when the pits are again filled with water. If the water is used for drinking by animals or humans the larvicides should be safe for use in drinking-water; temephos, B.t. H-14 and methoprene could be considered.

Because the pits are likely to dry out from time to time, larvivorous fish are usually not appropriate for the control of larvae. However, the so-called instant fish (p. 127), whose eggs can survive dry seasons and which mature in one wet season, may be suitable under such conditions (181).

Fig. 1.133. Road construction often causes blocking of streams, resulting in the formation of bodies of stagnant water alongside the road.

Accumulations of water near roads

The construction of roads on causeways often leads to the blocking of transverse streams. This prevents natural drainage of the land and may result in the formation of large ponds alongside the roads (Fig. 1.133). Water also accumulates in borrow-pits along roads that were used for the construction of the causeway. For example, the construction of highways through the Amazon forest created numerous suitable breeding sites for the malaria vector Anopheles darlingi.

· Construct culverts through the causeway allowing streams to continue on their natural courses.

· Use larvivorous fish and larvicides.

Irrigation systems and irrigated fields

Wet rice cultivation (paddy rice) and other activities involving irrigation may create suitable breeding places for some Anopheles mosquitos that transmit malaria and for some Culex mosquitos, such as Culex tritaeniorhynchus, which transmit Japanese encephalitis in Asia.

Mosquitos often find breeding sites in or near irrigation systems that have been poorly constructed, managed and maintained (Fig. 1.134). Breeding may occur in irrigation channels and ditches between vegetation growing along the margins. Holes in the beds of channels provide breeding places when there is no water flowing through them. Leaks may provide breeding sites in puddles outside the channels. The prevention of breeding in irrigated fields is difficult because of the very large surfaces of standing water often available to mosquitos.

Construction and maintenance

The bottoms of channels and ditches should be smooth with a gentle slope so that no stagnant water remains when they dry out. Proper maintenance should prevent leakages from sluices and the dykes and linings of fields and channels. Breeding in ditches and irrigation channels filled with water can be reduced by making the banks steeper and removing water plants. This helps to speed up the flow of water and expose the larvae to larvivorous fish and other predators.

During certain periods of the cropping cycle, rice plants do not have to be submerged continuously. It is therefore possible to dry the fields once a week for 2 — 3 days in order to kill larvae (152, 178). This is only feasible if the farmer has sufficient water to irrigate the field again after having drained it. The fields have to be level and very well drained so that they dry out completely after the inflow of water has been stopped. In some places the rice crop may improve with this system of irrigation. The most effective periodicity of the alternating wet and dry cycles has to be determined by an expert since it depends on the type of irrigation, soil texture, variety of rice plant and other factors. Experts would also have to determine the possibility of any negative side-effects, such as an increase in floodwater mosquitos.

A problem in the application of intermittent irrigation is the need to keep fields flooded during the first 2 — 3 weeks after transplanting the rice seedlings to permit them to establish themselves. During this period, mosquitos can be controlled with one of the other methods. If larvivorous fish are used, deep pools are needed in the fields or ditches to allow the fish to survive during dry periods. Intermittent irrigation has to be used simultaneously for all rice fields over a large area throughout the entire cropping season (152). In some countries, the drying out of fields at intervals has been legally imposed on farmers (154).

Free-floating water ferns (Azolla) can form thick layers of vegetation which completely cover large areas of water. In China and India, Azolla is cultivated as a fertilizer and animal feed. If a high degree of coverage of a rice field is achieved it offers the extra benefit of mosquito control (183185). It is important to obtain good coverage before the usual peak in mosquito breeding. In many situations this is difficult to achieve because of fluctuating water levels.

Paddy fields can be made suitable for rearing fish for food and for larval control if the dykes are made stronger and higher than usual, allowing water to be retained to the desired depth. Inlets and outlets of water should be screened with a wire grille to prevent fish from escaping. A deeper pond or trench is needed near the outlet to provide shelter for the fish when the field dries out or when the fish are not foraging in the shallow water among the rice plants. A problem that may occur is the predation of fish by birds such as herons. If insecticides of the organochlorine group, such as DDT, dieldrin or lindane are applied in rice fields to control agricultural pests, they may accumulate in the fish, rendering them unfit for human consumption. Some insecticides, especially those of the pyrethroid group, are very toxic to fish.

Keeping fish in rice fields may have a positive effect on rice production: several fish species eat the weeds that compete with rice plants; fish excrement fertilizes the soil; fish stir up the soil and improve the access of oxygen to the roots of the plants (148, 152,186188).

Some suitable fish species are:

— tilapia (Oreochromis mossambicus); the fingerlings (young stages) feed on mosquito larvae and grow rapidly;

— carp (Ctenopharyngodon idella, Cyprinus carpio); these eat weeds and, when small, mosquito larvae;

— guppy (Poecilia reticulata);

— mosquito fish (Gambusia affinis);

— panchax (Aplocheilus panchax); these are commonly found in paddy fields and ditches in south-east Asia.

The use of larvicides is costly, and requires special equipment and trained staff because of the large surfaces to be covered. Larvicides have been applied from fixed-wing aircraft, helicopters, and by hand with the aid of portable knapsack sprayers (p. 128). The resistance of mosquitos to insecticides is a problem in some areas. Another disadvantage is the potential harm to other organisms such as larvivorous fish and predator insects (dragonfly larvae). The application of Bacillus thuringiensis H-14 (p. 136) or insect growth regulators (p. 134) would avoid the problems of resistance and harmful side-effects. They have to be applied in a special formulation to prevent the particles from sinking to the bottom. The surface-feeding Anopheles larvae are killed only if the particles remain at the surface, where dispersion is improved by adding a spreading agent. Floating granular formulations remain on the surface and release toxic particles over a period of several weeks, but their effectiveness may be reduced if the wind blows them to one side of the field. The use of larvicidal oil also avoids the problem of resistance and usually does not harm fish, although some aquatic predator insects may be affected.

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