Manual of fumigation for insect control — Chemicals used as fumigants (cont

Dichlorvos from wasps: how does a remedy affect insects?

Chloropicrin is a powerful tear gas; it is one of the most toxic to insects of the fumigants commonly used today. It is sometimes added in small proportions to other fumigants, e.g., hydrogen cyanide and methyl bromide, to serve as a warning agent (see Chapter 3).

Although the tear gas effect of Chloropicrin is helpful in preventing persons from staying in dangerous concentrations during the fumigation process, it is also a handicap because fumigated commodities are unpleasant to handle for some time after fumigation. Even comparatively small amounts diffusing from the treated material may be extremely irritating. If it were not for this disadvantage, Chloropicrin would be useful for commodity treatments as it penetrates effectively into many materials.

Chloropicrin is also toxic to nematodes and certain fungi and it has found wide application as a soil fumigant.

Chloropicrin is corrosive to metals and care should be taken to protect metal surfaces and equipment during treatment.

In humans, a concentration of 2.4 g/mі can cause death from acute pulmonary oedema in one minute (Hanslian, 1921). concentration us low as I ppm of Chloropicrin in air produces an intense smarting pain in the eyes, and the immediate reaction of any person is to leave the vicinity in haste. If exposure is continued, it may cause serious lung injury.

Alternative names: trichloronitromethane, nitrochloroform
Strongly irritating tear gas

Chemical formula CCl3 NO2
Boiling point 112°C
Freezing point -64°C
Molecular weight 164.39
Specific gravity
gas (air = 1) 5.676
liquid (water at 4°C = 1) 1.651 at 20°C
Flammability limits in air Nonflammable
Solubility in water 0.227 g/100 ml at 0°C
Pertinent chemical properties Nonflammable; relatively inert; corrosive in presence of moisture
Method of evolution as fumigant By evaporation of liquid from pure compound or mixed with carbon tetrachltride. Sometimes dispersed as aerosol with methyl chloride as carrier.
Commercial purity 99%

Natural vapour pressure at different temperatures

0°C (32°F) 5.7 mm Hg
10°C (50°F) 10.37 mm Hg
20°C (68°F) 18.3 mm Hg
25°C (77°F) 23.8 mm Hg
30°C (86°F) 31.1 mm Hg
40°C (104°F) 51.1 mm Hg

Weights and volumes of liquid

1 lb (avdp) at 20°C has volume 274.7 ml
1 U.S. gal weighs 13.76 lb (6.243 kg)
1 Imp gal weighs 16.51 lb (7.489 kg)
1 kg has volume 605.69 ml
1 litre weighs 1.651 kg

Dosages and concentrations of gas in air (25°C and 760 mm pressure)

By volume

Weight per volume

Parts per million Percent №g/mі lb/l 000 ftі 20.1 0.00001 0.00067 20 0.002 0.13 50 0.005 0.34 100 0.01 0.67 149 0.015 1.00 200 0.02 1.34 0.08 500 0.05 3.36 0.21 1 000 0.10 6.72 0.42 2 380 0.24 16.00 1.00 20 000 2.0 134.46 8.40

1 Ounces per 1000 cubic feet or milligrammes per litre

However, it is evident from the preceding statement that an individual will not willingly tolerate concentrations that are actually injurious (Torkelson et al, 1966).


Chloropicrin is extremely phytotoxic and plants exposed to its vapours are often completely destroyed. Even when added in small amounts to other fumigants for warning purposes, it is likely to be toxic. For instance, methyl bromide containing small amounts of chloropicrin should not be used to fumigate plants, fruit or vegetables. When used for soil fumigation, it kills weed seeds and any living plant material present.

Nevertheless, it is possible to fumigate certain seeds with concentrations of chloropicrin toxic to insects without impairing germination (Hsin, 1959; Metzer, 1961; Solodovnik et al, 1963). Metzer recommended that seed treatments with this fumigant be conducted at temperatures lower than 27°C.


Chloropicrin should not be used for the fumigation of fresh fruit or vegetables. It may be used on bagged or packaged plant products at atmospheric pressure if ample time is allowed for postfumigatior aeration. The irritating vapours must be allowed to diffuse before the material is handled or consumed.

If chloropicrin is present in fumigated flour, it may have a bad effect on the baking quality, but this effect disappears once the material is fully aerated.


From available evidence it appears that the residue problem with this fumigant is confined to unchanged chloropicrin persisting in the treated foodstuff. There has been no indication of a significant residue from reaction products under normal conditions of fumigation, but the formation of inorganic nitrites and nitrosamines has been postulated (FAD/WHO, 1965a).

Getzendaner et al (1965) found that in dry beans and field peas fumigated with chloropicrin at 32 to 64 g/mі for 24 hours at 25 to 26°C, residues were not in excess of 2 mg/kg after 4 days of aeration. With the same treatments most of the fumigant disappeared from maize, peas, beans, wheat flour, breakfast food and chicken feed but even after 3() days measurable amounts of chloropicrin, up to 9 mg/kg, persisted in wheat flour and up to 16 mg/kg in chicken feed. In the other materials the residues were less than 2 mg/kg. Flour containing 3.7 my/kg of chloropicrin before baking cuntained no measurable amounts afterwards. These authors point out that the physical state of the material being fumigated may have an important bearing on the amount of initial residue.


Methods for the determination of chloropicrin in air are given by Daecke and Kraul (1961), Berok and Solomon (1962a), and Ioanid et al (1963). The method used by Getzendaner et al (1965) for analysing chloropicrin residues in fumigated foods was sensitive from 0.1 to 100 mg/kg. Kanazawa (1963) and Berck (1965a) described gas chromstographic methods for determining this fumigant.

Chloropicrin is commonly marketed in glass bottles containing 1 pound of the liquid. For safety, the bottles are packed individually in cans, which must be opened with a can opener. The fumigant is also available in steel cylinders containing 25 to 180 lb (11.5 to 81.5 kg) and is said to be noncorrosive to the containers if they are kept tightly closed.

Chloropicrin is difficult to vaporize at ordinary temperatures. In fumigation chambers it may be poured onto a crumpled jute (burlap) sack over which a powerful draught of air may be directed from a fan or blower when the fumigation starts. For volatilizing in flour and mill fumigation, it is sometimes mixed with methyl bromide or methyl chloride in a cylinder and discharged as a mist, from which the chloropicrin is rapidly volatilized.

Its application as a grain fumigant is described in Chapter lO and as a «spot» fumigant in Chapter 8.

As stated above, because of the tear gas effect, a person would be unable to remain in a dangerous concentration of chloropicrin for more than a few seconds. Great care should be taken to prevent unauthorized persons from approaching a fumigation site because the tear gas effect is so powerful that they may become temporarily blinded and panic-stricken, which, in turn, may lead to accidents. If it is necessary for the operator to expose himself to any concentrations of this fumigant, a canister especially designed for protection against «organic vapours and acid gases» should be fitted to the respirator.

For the use of chloropicrin as a warning gas, see Chapter 3.

Dichlorvos (DDVP)

Dichlorvos, sometimes called DDVP, is the common name of dimethyl 2,2dichlorovinyl phosphate. Discussion of this material is pertinent to this manual, despite its high boiling point and low vapour pressure, because for certain usages it is discharged as a true gas to control insects in the open spaces of structures. It is also used as a contact insecticide, but a description of this type of application is outside the scope of this manual (Attfield and Webster, 1966).

Dimethyl 2,2-dichlorovinyl phosphate
Alternative name : DDVP

Chemical formula CCl2 = CHO.P0.(OCH3)2
Boiling point 120°C/14 mm
Freezing point Below -18°C
Molecular weight 221
Specific gravity
gas (air = 1) 7.6
liquid (water at 15.6°C = 1) 1.44 at 15.6°C
Density 0.142 kg per litre at 20°C
Flammability limits in air Nonflammable. Applied as fog or spray, flammability would be governed by solvent used
Solubility in water Slight (about 1%)

Pertinent chemical properties

Stable to heat. Undergoes hydrolysis in presence of water. Corrosive to black iron and mild steel. In absence of moisture, noncorrosive to aluminium, nickel and stainless steel. Nonreactive with Teflon and polyethylene. Stable in presence of hydrocarbon solvents.

Method of evolution as fumigant

(a) By direct evaporation of liquid concentrate by means of heat.
(b) By volatilization from pressurised cylinders with inert Freon-type gases as carriers.
(c) By slow volatilization from resin strips (adult flies and mosquitoes only).
(d) By evaporation from resin cylinders (glasshouse fumigations).

Natural vapour pressure at different temperatures

20°C (68°F) 0.0108 mm Hg
10°C (50°F) 0.0041 mm Hg
30°C (86°F) 0.0272 mm Hg
60°C (140°F) 0.2985 mm Hg

Volatility (saturation — see also Table 2)

At 10°C, 51.5 mg/mі
At 20°C, 131 mg/mі
At 30°C, 318 mg/mі

Dosages and concentrations of gas in air (25°C and 760 mm pressure)

By volume

Weight per volume

Parts per million Percent mg/mі oz/l 000 ftі 20.1 1.00 1.0 10 2 20 5 O.005 50 O.05 10 0.01 100 0.1 15 0.015 150 0.15 20 0.02 200 0.2

1 Ounces per 1000 cubic feet or milligrammes per litre

Because of its low vapour pressure dichlorvos is unable to penetrate into materials. Therefore, it is of no value as a commodity fumigant.

Used as a fumigant, dichlorvos has found effective use at very low concentrations against houseflies, mosquitoes and mushroom flies. At higher concentrations it is effective against cockroaches and a wide range of storedproduct insects. It has been used successfully against moths and the cigarette beetle in tobacco warehouses. Recommendations for free space use are summarized in Schedule Q. Its application as A glasshouse fumigant is discussed in Chapter 12.

An important characteristic of dichlorvos is the fact that it hydrolyses slowly in the presence of water, and this process is accelerated in the presence of alkali and reduced in the presence of acid. An end product of hydrolysis may be dichloroacetic acid. Consequently, formulations and spaces treated with the insecticide may exhibit a vinegar-like odour if hydrolysis has occurred to any extent.

The toxicity of dichlorvos to mammals is moderately high by ingestion, inhalation and absorption through the skin. It is a direct inhibitor of the enzyme cholinesterase but it is detoxified relatively quickly (Hayes, 1963). Although dichlorvos is a potential alkylating agent of DNA and RNA in vitro, this potential is apparently not realized in vivo owing to the rapid degradation in mammals. Investigations for possible carcinogenic effects of dichlorvos have shown no increased incidence of tumours in experimental animals (Blair et al, 1976); this and other studies support the view that any potential for producing cancer in humans by dichlorvos would be extremely low (Anon, 1977; FAD/WHO, 1977). Studies for teratogenic effects have shown no serious changes in the progeny from treated animals (Schwetz et al, 1979; Wrathall et al, 1980).

On the other hand, insect cholinesterase inhibited by dichlorvos is not readily reactivated, and in consequence intoxication is irreversible (O’Brien, 1960). This reaction is typical of the dimethyl phosphate insecticides.


Dichlorvos in the concentrations used for control of glasshouse pests is not generally phytotoxic. Pass and Thurston (1964) listed 42 plants, including 20 flowering plants, which were not injured by exposure for 15 hours at 20°C to dichlorvos vapours generated at the rate of 1.5 fluid ounces (U.S.) of 90 percent concentrate per 10 000 cubic feet (eriaivalent to 204 mg/mі of dichlorvos vapour which in full concentration, would be above the saturation point of 131 mg/mі at this temperature). The only adverse effect noted was a slight discoloration or fading of chrysanthemum blooms. Glancey and Naegele (umpubIished data, 1965) also reported that dichlorvos was safe to use for the insecticidal fumigation of glasshouse plants, except that one variety of chrysanthemum (Shasta) showed severe leaf burning. However, Harnlen and Henley (1979) were unable to obtain satisfactory control of green peach aphid and twospotted spider mite on a number of indoor ornamental plants in continuous room fumigation tests. They found that a multiple fumigation treatment at seven-day intervals with dichlorvos impregnated polyvinyl chloride resin strips in polyethylene hags was more effective, but some injury to the plants did occur.


The amount of dichlorvos absorbed can vary considerably with different food materials. In measurements of residues absorbed in prepared meals during disinfestation of aircraft it was noted that margarine contained three times as much as the cooked meal and the beverages approximately one tenth as much (Dale et al, 1973).

Table 13 summarizes some information on the persistence of dichlorvos in certain foodstuffs exposed to the insecticide when discharged as a vapour. Dichlorvos breaks down rapidly after application, so the residues decline to very low levels during storage and shipment. The higher the temperature and moisture content of the material or its environment, the more rapid is the breakdown. In studies on the use of dichlorvos in mills at temperatures ranging from 18 to 22°C, dichlorvos residues degraded within a period of 2 4 weeks, depending on the kind of product treated (Wirthgen and Raffke, 1979).

It may be concluded that, when dichlorvos vapour is applied to closed spaces containing foodstuffs, rapid hydrolysis leads to the disappearance of significant residues of this chemical in a very short time.

The toxicological evaluation of dichlorvos is that 10 mg/kg in the diet, equivalent to 0.5 mg/kg of body weight per day, is the level that will cause no toxicological effect; the estimated acceptable daily intake for man is 0 to 0.004 mg/kg of body weight (FAD/WHO, 1978b).


Determination of Vapours

Concentrations of dichlorvos in confined spaces can be measured by several different methods including cholinesterase inhibition (Webley and McKone, 1963; Heuser and Scudamore, 1966), calorimetric (Bracha et al, 1963; Mukherjee et al, 1973), titrimetric and bioassay procedures (Muthu et al, 1973). Gas chromatographic methods can provide rapid, specific and sensitive determinations (Bond et al, 1972; Bryant and Minett, 1978). Samples from the atmosphere can be taken directly by gas syringe and injected into a gas chromatograph or they may be collected by passing a known volume of air through glass tubes packed with potassium nitrate and taken to a gas chromatograph, as described by Bryan and Minett (1978).





Insect species Concentration dichlorvos vapour Temperature Duration of exposure Residue dichlorvos observed
Applied Saturation
Cocoa beans Ephestia elutella (Hbn.) 0.04 to 131 18 54 days Whole beans, surface layer near strips, 0.02
in sacks 0.05
in store (moths only) (from resin strips) Whole beans, middle of top sacks, 0.01
Meat *05 84 13-15 24 hours Minced meat 0.33
Bacon 0.32
Steak 0.23
Fat 0.17
Mushrooms (a) Megaselia halterate (Wood) 218 131 17-20 Nil 3 hours after exposure
Mushrooms (b) Sciarid and Phorid flies 2120 131 17-20 ? 4 hours Nil 24 hours after exposure with 1,2 or 3 consecutive applications

* Concentration of 0.5 mg/mі lasted for 30 minutes, no vapour detectable after 3 hours.
SOURCES: Schulten and Kuyken (1966) for cocoa beans: Miller and Aitken (1965) for meat:

Hussey and Huges (1964) for mushrooms (a); Snetsinger and Miner (1964) for mushrooms (b).

Determination of Residues

The dichlorvos remaining in food material can be extracted and coricentrated by using appropriate solvents for the type of food concerned. Details of methods for such extractions with subsequent analysis by gas chromatography are given by various authors (Abbott et al, 1972; Bond et al, 1972; 1 a Hue et al, 1975; Schmidt and Wohlgemuth, 1979).).

A simple method of vaporizing dichlorvos as a fumigant for closed spaces is to place the required amount of liquid concentrate in an open dish or beaker and heat it on a hot plate. An ordinary electric fan or blower placed close to the source may be used to disperse the vapours to effect even distribution.

Jensen et al (1961) described a mechanical system for dispersing known amounts of dichlorvos. This system was originally designed for fly and mosquito control in aircraft, but would also be suitable for use in a wide variety of structures requiring routine applications at regular or intermittent intervals.

Dichlorvos is also available in cylinders mixed, up to 20 percent by weight, with Freon-type inert propellants. This method of vaporization is very convenient but is more expensive.

For the control of flies and mosquitoes attacking humans and animals in houses and buildings, resin strips each containing about 20 percent dichlorvos by weight are extremely effective. By slow vaporization these strips will maintain concentrations up to 0.44 mg/m in a tightly closed room at 23.3°C. Concentrations of 0.15 to 0.25 mg/mі are fully effective against flies and mosquitoes within 30 minutes. Use of these strips is not an effective method for controlling cockroaches and stored-product insects.

The application of this chemical as a fumigant in glasshouses is discussed in Chapter 12.

Dosages and Concentrations

In the literature and in trade publications, dosages of dichlorvos are expressed in different ways. Sometimes both the metric and British systems are used in the same prescription. Equivalents of some of the more common methods of expressing dosage are given in the table of properties of dichlorvos .

Concentrations Toxic to Humans

According to the threshold Iimit set by the American Conference of Governments I Industrial Hyigienists (ACGIH, 1981 ), the maximum perrmissible concentration of daily exposure to humans is 1 microgramme (µg) per litre (1 mg/mі). Hayes (1963) stated that tests have shown that men can withstand brief exposure to air concentrations at least as high as 6.9 µg/l without clinical effect or depression of blood cholinesterase; intermittent exposure totalling 5 hours daily at a concentration of ().5 µg/l produces no clinical effect and no effect on red cell cholinesterase, but does cause a gradual moderate reduction of plasma cholinesterase.

Zavon and Kindel (1966) studied the effect of prolonged exposure, up to six months, on humans exposed to the low concentrations of dichlorvos not exceeding 0.01 µg/l of air generated from resin strips of the type marketed for control of flies and mosquitoes in houses. They concluded that the handling and use of the resin vaporizers, under recommended conditions, would be unlikely to result in adverse effects among persons so exposed.

Durham et al (1959) studied the effects of exposure to dichlorvos in human volunteers, not wearing respirators, who worked in a tobacco warehouse where the insecticide was applied at regular intervals to control insects. They concluded that the conditions would be safe for workers where dichlorvos was applied twice a week or less at 70 mg/mі.

Dichlorvos is easily absorbed through the skin and if even small amounts of formulations containing this insecticide are spilled on the clothes or body, these may produce very serious results requiring medical treatment (Hayes, 1963).

When dichlorvos is being applied as a fumigant indoors or in glasshouses, those applying the insecticide must wear an industrial-type respirator (gas mask) with a filter-type canister which gives full protection against organic vapours and acid gases (see Chapter 3 Table 8). A small cartridge-type respirator of the kind described in Chapter 3 does not give adequate protection when dichlorvos is being used for industrial purposes.

The following is a summary giving the salient information on first aid and subsequent treatment by a physician for poisoning or suspected poisoning by dichlorvos. The manufacturers of this insecticide supply special booklets on first aid and treatment, with detailed information for physicians arid a list of important precautions to be taken during formulation.

Warning symptoms include weakness, headache, tightness, in the c-nest, blurred vision, nonreactive pin-point pupils, salivation, sweating, nausea, vomiting, diarrhoea and abdominal cramps.

In all cases of suspected poisoning, remove the patient from further exposure to the poison, restore breathing anti get medical help immediately. Details of the accident, including the name of the poison, the quantity involved anti how the accident occurred, i.e. by inhalation ingestion or by skin contact, should be supplied to the doctor or hospital emergency centre.

If dichlorvos has been swallowed, induce vomiting (in fully conscious patients only) by stroking or tickling the back of a patients throat with a finger. Do not give salt water as this may involve serious risk.

If the patient has been poisoned by external contact with dichlorvos, remove contaminated clothing immediately and wash the skin thoroughly with soap and water; use plenty of water in rinsing. If dichlorvos gets into the eyes, wash it out immediately using running water for at least 10 minutes.

Emergency treatment personnel should be aware that atropine is the antidote of choice for treatment of dichlorvos poisoning. However, atropine should never be administered unless warning signs of intoxication appear.

Information for Physicians

Regardless of the route of absorption, dichlorvos inactivates the cholinesterase enzymes of both the blood and tissues. Intoxication produces signs and symptoms of excessive cholinergic stimulation. Diagnosis may be substantiated by plasma and red cell cholinesterase analyses using the Michel method (J. Lab. Clin. Med., 34: 1464, 1949) or the Ellman colorimetic method (Biochem. Pharmacol., 7: 88, 1961). Atropine should be given intravenously in doses of I to 2 mg; if cyanosis is present, the atropine should be given intramuscularly while simultaneously initiating measures to improve ventilation. Atropine administration should be repeated at 5- to 10- minute intervals until atropinization is complete. A mild degree of atropinization should be maintained for at least 24 hours and in severe cases for at least 48 hours. Morphine, adrenaline, tranquilizers and similiar substances are contraindicated.

Complete recovery may be anticipated even in those cases where severe poisoning has occured and after many hours of artificial respiration. A patient should be watched continuously for 48 hours when the exposure has been severe enough to produce symptoms. No further exposure to any organophosphorous or carbamate insecticide should be allowed until the blood cholinesterase, as determined by blood tests, has returned to normal.

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