Aphids — Everything you need to know, CANNA UK

Aphids — in detail

By Iñaki García

When we refer to aphids, or plant lice, we usually mean a super family of insects which includes over 4,000 species of plant-specific parasites. They are not longer than about 4 millimetre, have a bulbous abdomen and can be many different colours. Many species bear a common or scientific name that indicates their favourite host plant, either for food or for raising their offspring, or to some of their distinctive characteristics. So, for example, Hyadaphis coriandri prefers coriander, the cotton aphid (Aphis gossypii) prefers cotton, the black peach aphid (Brachycaudus persicae) prefers peach and so on.

Aphids and their syringe needle

All aphids are characterized by a stylus (a kind of syringe needle) that is used to pierce and suck the sap from the plant. Also, they have a couple of tubes in the back called cornicles or siphunculi through which the animals excrete a kind of honeydew called cornicle wax. For nutrition, aphids usually feed on the plant’s phloem sap, which is rich in sugars, minerals and other elements. The phloem is responsible for distributing this kind of sap throughout the plant. For water, aphids draw fluid from xylem, where raw sap runs directly from the roots. This allows them to stay hydrated during hot or dry periods.


Ladybird feeding on an aphid.

Life cycle of aphids

A generation of aphids survives the winter as eggs, which allows them to withstand extreme environmental conditions of temperature and moisture. In spring the eggs on the plant (primary host) hatch, leading to the first generation of aphids. All the aphids born from the winter eggs are females. Several more generations of female aphids are born during the spring and summer. A female can live for 25 days, during which time she can produce up to 80 new aphids. Spring and summer reproduction occurs asexually – without males. In these cases, the resulting aphids are basically clones of the mother. In addition, the young are born live rather than as eggs. When the fall approaches, there is a generation that grow into both male and female individuals. Females fertilized by the males lay winter eggs on the plant where they are, closing the cycle.

Winged and wingless aphids

Aphids can be winged or wingless. Usually the first generation to emerge from the winter egg are wingless. However, after several generations there can be a lack of space on the host plant. This triggers the birth of a generation of winged aphids, which can migrate to other hosts.

Some species of aphid develop only on plants of a particular species. These types of aphids are called monoecius. The most common species that attack crops are heteroecious aphids. Heteroecious means that they feed on different plant species. Heteroecious aphids that reproduce sexually at least once in their lives start their cycle when the winter eggs hatch on the primary host. The primary hosts are usually annual weeds, shrubs or trees.

A couple of parthenocarpic generations (reproduced without fertilization) then give rise to a generation of winged females that migrate to the secondary host. This is usually a cultivated plant. In this new environment the aphids reproduce asexually for several generations of females until the arrival of autumn, when there is a generation of winged males and females which return to the initial host plant and lay fertilized winter eggs, closing the cycle again.

Aphids, Damage and Control

Aphids are among the most feared pests among growers in temperate regions. What kind of damage can they do?

Damage caused by aphids

Direct damage

The removal of phloem sap for food weakens the plant and causes a metabolic imbalance, twisting of the leaves and, in extreme cases, leaf loss. Leaf loss affects the quantity and quality of the final harvest. They also introduce toxins into the plant, systemically altering its development.

Indirect damage

The honeydew secreted by the aphids is an ideal culture medium for various fungi which form a barrier on the leaf, stopping it from taking in all the light that hits it.

Most harmful damage

But the most harmful consequence for the crop is the transmission of viruses. Aphids can transmit dozens of viruses from a diseased plant to healthy in few seconds, especially through the winged generation. The biggest problem with viruses is that there is no remedy for them, so that the infection of a plant that is not tolerant or resistant to the virus leads inevitably to a decline in the final production.

The way to understand the degree of danger that virus transmission by aphids may represent and to choose an appropriate method of prevention is to understand the mechanisms by which these viruses are transmitted.


Potato plant damaged by the Potato Virus Y which is transmissible by aphids. The Potato Virus Y(PVY) is one of the best known viruses affecting Solanaceae (tomato, potato, sweet pepper etc.)

How do aphids transmit viruses?

When an aphid inserts its stylus to feed on a plant infected with a virus, some viral particles attach to the mouth parts or are ingested with the sap. This is what is called the acquisition phase and it lasts a few seconds to several days depending on the type of virus. When the aphid migrates to another healthy plant and inserts its stylus to feed, the inoculation phase begins. Depending on the mechanism of transmission, viruses are classified as non-persistent and persistent.

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Non-persistent viruses

Non-persistent viruses can infect a healthy plant immediately. This type of virus transmission by aphids is the most difficult to avoid because once the aphids carrying virus have attacked the host plant, infection occurs immediately, with consequent loss of production. Fortunately, however, the number of plants that can be infected is very low. The time within which an aphid with the virus can transmit the disease to other healthy plants is called the retention period, and for non-persistent viruses the retention time is a few minutes. If the acquisition phase and retention time are longer, the virus is known as semi-persistent.

For this type of virus, the solution is to prevent access of aphids carrying the virus to the crop and prevent the aphids that feed on infected plants from dispersing.

Persistent viruses

Persistent viruses are rather different. When an aphid feeds on a plant infected with a persistent virus, a viral load enters and remains in the cells of the digestive system and is excreted through the insect’s saliva. Some viruses can replicate in the aphid body and are called a persistent-propagative virus. These persistent viruses require a period of time inside the aphid — which can range from days to months — before they can be transmitted. This period is known as latency. The retention period of these viruses – the time a virus can live without a host — also ranges from days to months.

Control mechanisms


2 aphids feeding on a plant.
This photograph was made by
Luc Viatour / www.lucnix.be

Applying pesticide

Usually, chemical pesticides are absorbed by the leaves and transported to the tips of the leaves, the place where aphids prefer to be.

However, due to leaf deformation in infested leaves, insecticides cannot always come into contact with all the aphids because the leaves form a barrier.

To provide continuous protection, the pesticide should also be long-lasting. Systemic pesticides (chemicals that are absorbed into the system of a plant) circulate through the vascular system of the plant.

These pesticides tend to leave beneficial fauna — which do not feed on sap — unaffected.

Biological control

Biological control methods have been used successfully against parasitoids (organisms that, unlike parasites, kill their hosts in the end) and insect-eating predators and entomopathogenic fungi (fungi that can kill or seriously disable insects).

The predators of aphids include ladybird beetles (or ladybugs) and lacewings. Green lacewing larvae (Chrysoperla sp.) are voracious predators of aphids.

There are several cultivation techniques that we can use to prevent or minimize an attack of aphids. These include:

  • eliminating weeds that can serve as a reservoir of eggs and adults
  • using insect nets (sometimes insecticide-impregnated) to cover crops
  • avoiding the excessive use of nitrogenous fertilizer
  • removing crop residues
  • establishing plant species that can serve as a reservoir for predators (banker plants)

How to combat winged aphids

Winged aphids are specially dangerous for your crops, as they destroy plants much faster than regular aphids. To know when winged aphids are flying over your crops, you can use sticky yellow traps in the air and place water traps on the ground. The water traps consist of a yellow plate containing two fingers of soapy water. In addition to monitoring insect numbers, they can keep a large proportion of aphids from ever reaching our crop.

Barrier crops can also be used. This method involves using plants that are not susceptible to viruses transmitted by aphids to protect the primary crop from these vectors. Its mode of action is twofold: First it forms a physical barrier, preventing the winged aphids from reaching the primary crop. Second it cleans the non-persistent virus, which carries the aphid.

When the aphid reaches the barrier plant and tests its sap, many viral particles attached to the stylus and mouth parts are detached. When the same aphid goes from the plant barrier to the primary crop it will be virus-free. In this way, the barrier crops have a cleansing effect on the non-persistent viruses that aphids can carry.

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Spotting and Eliminating Aphid Eggs In Soil

An aphid infestation can literally suck the life out of your plants if they are not dealt with as a matter of urgency. The unfortunate thing about aphids is that they breed quickly and become highly concentrated in a short space of time. There are various methods to get rid of them but one of the most important aspects of killing them off is to act quickly. It is important to know how to spot and eliminate aphid eggs in soil.

Step 1 – Act Early

Aphids on their own are quite tiny and can be difficult to spot. By the time you do see them, they are likely to be quite concentrated. This means that spotting the eggs of the aphids is even more difficult. Aphids tend to lay their eggs near browning leaves and rotting foliage. The best defense against these eggs is to clean out your flower beds and dispose of rubbish. Be sure to rake leaves and dig out any plants that may not have survived the frost. Don’t leave them in the ground or the aphid eggs will prosper.

Step 2 — Spray Spices

There are several preventative sprays that can be used in the early spring to keep the aphids out and stop them from laying their eggs. Many of these sprays are natural and free of chemicals. Items such as sea weed, hot pepper and coriander spray are detested by aphids and many of their unwelcome friends. They are also free of toxins and will not harm other animals or birds that visit the garden.

Step 3 — Hit Back with Herbs

By planting herbs such as Basil, Mint and chives, you will keep aphids and their eggs out of your soil. Aphids do not like the smells from these spices and will stay away. As a reward, the garden will be full of fresh herbs in the spring time for general kitchen use.

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Step 4 — Go Bananas!

Save your banana peels and start mulching them into your garden or flower beds. Aphids dislike bananas and will most likely stay away from them. The banana peels will also provide your soil with plenty of key nutrients that will be highly beneficial.

Step 5 — Foil Them!

Take some aluminum foil and line the bottom of your garden with it. Make sure that the shiny side is facing up so that it reflects the sun. This will create a large amount of light that will scare the aphids away. No aphids, no eggs.

Step 6 — Ladies First!

Introducing some insects that destroy aphids is an excellent and safe way to rid your garden of the adults and their eggs. The best known of these is the ladybug. Ladybugs can be found in abundance and will be more than happy to feast on the aphids and their offspring. Other alternatives include certain breeds of wasp and lace wings. Introducing these insects offers a natural way of chasing the aphids away.

www.doityourself.com

Aphids, Greenfly, Blackfly (Aphidoidea)

Introduction

Aphids are related to Adelgids (Conifer Aphids)and Phylloxerids all of which probably evolved about 280 million years ago (MYA) in the Carboniferous, they probably bred on non-flowering plants such as Cordaitales and Cycadophyta (flowering plants i.e. Angiosperms, did not evolve until about 160 MYA). The oldest known fossil aphid is Triassoaphis cubitus from the Triassic about 220 MYA. Aphids did not always look like they do now, the cauda and the siphunculi did not evolve until the Cretaceous about 55MYA.

There are about 4 000 species of aphids in the world of which about 250 are serious pests. Scientists believe that the number of species remained relatively small until the angiosperms (flowering plants) became more common, then as the aphids adapted to these new and rapidly speciating plants so their species numbers increased also. Twenty-five percent of all plant species are infested with aphids, and though it is believed that the speciation of aphids has followed that of plants not all groups of plants are equally parasitised. The Compositae, the 3rd most specious plant family, supports the most aphid species (605 species), but the Orchidacea, the 2nd most specious plant family, only support 9 species of aphids while the Rosaceae which is only the 22nd most specious plant family supports the 3rd highest number of aphid species (293 species). The plant family which supports the 2nd highest number of aphids is Coniferae (363 species) but these are non-flowering plants.

Aphids have a world-wide distribution but there are far more species in temperate zones, we have more than 500 species in the UK while there are about 1350 species in North America. Most aphids are monophagous, though some feed on more than one species of plant, species like Myzus persicae which feed on a number of different plant species are very rare.

Description

Life Cycles

Taxonomy

Superfamily Aphidoidea
Pemphigidae
Anoeciidae
Hormaphididae
Mindaridae
Thelaxidae
Drepanosiphidae
Phloeomyzidae
Greenideidae
Aphididae
Lachnidae

Feeding Ecology

Some aphids always feed by inserting their stylets through the stomata of the plant leaves they are feeding on,i.e. Ceratovacuna lanigera on Miscanthus spp. and Schizolachnus orientalis on pine needles. Other than this aphids penetrate to the phloem either intracellularly (pushing the stylets between cells), intercellularly (pushing the stylets through cells) or by a bit of both. Plant phloem saps are rich in sugars and poor in amino-acids or nitrogen. This results in aphids excreting large amounts of sugary liquid, called honeydew, because the amount of sap they have to drink in order to get enough nitrogen in their diet means they have far more sugar and liquid than they need. This honeydew can often be seen on the lower leaves of infested trees which it falls on giving them a sticky coating. This is then fed on by other insects like the Common wasp Vespula vulgaris and the Brown Hairstreak Butterfly Thecla betulae and a yeast-like fungus which makes the leaves look all black as if they were covered in soot.

Nearly all aphids contain endosymbionts in special groups of cells called bacteriocytes. The exceptions are some members of the Cerataphidini which have yeast cells in their haemocoel instead. In most aphids the main endosymbiont is Buchnera aphidicola a relative of Escherichia coli which is common in our guts. Buchnera aphidicola is believed to complement the aphids diet by synthesising vitamins, sterols and certain amino acids. Whatever their role they are important for the aphids growth and reproductive potential as they decrease as the aphid gets older and are absent from most soldiers and males. The relationship between Buchnera and the aphids it lives with is obligatory on both sides, i.e. the bacteria can’t live outside of the aphids and the aphids can’t reproduce successfully without it. Buchnera is transmitted maternally via the ovary i.e. new aphids get the symbiont from their mums when they are just an egg.

Most aphids are autoecious (living on one or a few species of closely related plants). About 10% are heteroecious spending Autumn, winter and spring on one plant species (its primary host) and Summer on a different unrelated plant (its secondary host). For example the Rosy Apple Aphid Dysaphis plantaginae has Apple Malus sp. as its primary host and Plantain Plantago lanceolata as its secondary or summer host. Some heteroecious aphids such as Myzus persicae the Peach or Potatoe Aphid and Aphis fabae the Black Bean Aphid have a wide range of secondary hosts, but this is relatively rare. Most heteroecious aphids have just one primary and one secondary host.

Though aphids are very successful at feeding on plants they do not always have things all their own way, and small spines, hairs and sticky latex secretions are among the more successful methods that plants have evolved to protect themselves against these parasites.

See also:  Head lice, Ministry of Health NZ

Relations with Ants

The number of ants associated with a given species of aphid and the number of aphid species associated with a given species of ant varies from place to place, up to 17 different aphid species have been found in Lasius niger nests. Sometimes different ant species which live in similar habitats will foster different aphid species, i.e. Lasius niger with Anoecia corni and Lasius flavus with Anoecia nemoralis. However aphid species that have evolved close relationships with ants may have broader environmental tolerances than their hosts hence, the aphid Forda formicaria is attended by Lasius spp. in the lowlands and by Formica spp. in the highlands. Ants gain much nutrition from their relationships with aphids and the honey-dew excreted by aphids such as Aulacorthrum sp. can in some cases be a complete ant food. Generally the amount of proteins and amino acids in excreted in honey-dew varies between species and plant. However because ant attendance stimulates aphids to feed at a rate 2-3 times their normal rate, and because they do not grow at 2-3 times their normal rate, it follows that the reduction of nutrients resulting from aphid digestion is greatly lessened. Also as adult ants do not need proteins but only sugars honey-dew is a perfect food for them. Ants are not always good guys though, like us with cows, if aphid numbers build up the ants quite happily kill a few off and feed them to their larvae.

The protection that ants give the aphids they attend is not always excellent and varies depending on the type of predator or parasite. Generally speaking the ants are better at dealing with Ladybird larvae and Anthocorid bugs than they are at dealing with Lacewing larvae and Hoverfly larvae. They not only remove the larvae, sometimes killing them but also remove the eggs of hoverflies and Ladybirds. Interestingly by herding aphids onto the tops of the plants ants render them more vulnerable to attack from some parasites. In one experiment in 1958 R.C. Banks found that attended aphids were 10X more likely to be parasitised than unattended ones.

Polyphenism

Aphids also occur in different colour forms or morphs. The Pea Aphid Acyrthosiphum pisum occurs in red or green morphs. Two of the aphids main predators are the Ladybird Coccinella septempunctata and the Parasitc Wasp Aphidus ervi. The Ladybird prefers the red morphs and feeds preferentially on them while the Parasitc Wasp prefers the green morph. Both tend to prefer hunting where there are lots of aphids and to move on when their preferred morph is reduced to low numbers. In this way the morph is maintined in the population which wouldn’t happen if one colour morph was always more attractive to both or all predators.

Migration and Dispersal

On their own aphids are weak fliers and in still air they move at about 1.6 to 3.2 km per hour. Their migrations can be quite extensive and they often take advantage of favourable winds to enhance their flight efficiency. Aphids which fly upwards to get above the planetary boundary layer (PBL) early on in the night can end up more than 1000 metres above sea level and are carried by the low level jet streams that occur at these heights. This can result in them travelling more than 400 km in 9 hours. They descend again in the early dawn as sun warming the surface of the planet causes updrafts which disrupt the PBL and make air travel much more dangerous. When coming into land from these great heights the aphids home in on leaves reflecting long wave light, they are particularly attracted to the yellowish light emitted from young actively growing crops or older senescing ones. This is because the flow of nutrients to and from the leaves is greater for leaves at these stages of growth than for ordinary green leaves. Several pest species are recorded as long distance migrants in North America these include, the English Grain Aphid Macrosiphum avenae, the Corn Leaf Aphid Rhopalosiphum maidis, the Oat Bird-Cherry Aphid Rhopalosiphum padi and the Greenbug Schizaphis graminum.

Defence

Predators and Parasites

Obligate Aphid predators and parasites.

Facultative Predators and Parasites

Aphids as Pests

Some serious aphid pests are:-

The Pea Aphid Acyrthosiphum pisum, is a large green aphid with long antennae and legs. The Pea Aphid is found on many leguminous plants and transmits Lucerne mosaic virus, pea leaf roll virus, pea Enation mosaic virus and pea mosaic virus in the UK and pea Enation mosaic virus in the USA. The Pea Aphid has only one host, in colder areas eggs are laid by sexually mated females in overwintering forage crops and hatch early in the spring to found new colonies which as they grow in size produce winged individuals which disperse to other crops. In warmer climes overwintering is by asexual females and no sexuals occur.

Cabbage Aphid Brevicoryne brassicae. The Cabbage Aphid is a serious pest of the major cabbage crops, Cabbages, Cauliflowers and Brussel Sprouts, like the Pea Aphid the Cabbage Aphid is a one host species spending all its life on Brassicas, the main overwintering form is eggs from sexual matings but in warmer years asexual females may successfully overwinter as well. The eggs hatch in February or March so the species gets off to an early start. The main cause of its pest status is the transmission of the viruses Cauliflower Mosaic and Turnip Mosaic.

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