Important natural enemies or 'farmers' friends'
There are many other natural enemies, such as spiders, bugs, predatory wasps and mantids which play a vital role in regulating pest numbers.
The adults and larvae of ladybird beetles are important predators of aphids. A single ladybird can eat 200-300 aphids over its lifetime. Adults ladybirds are 7-10 mm long, oval to nearly spherical in shape with short antennae.They are often brightly coloured with black margins, black with bright spots or shiny black. Eggs are elongated, usually yellow to orange in colour and are laid in clusters of 10-50 eggs near the prey (e.g. aphid colonies).
The larvae are soft bodied, wingless, and vary considerably in appearance. They look quite different and are usually longer and thinner (a little like tiny spotted crocodiles), coloured black or dark brown with various types of light markings. Their colour varies from black to dark brown with various types of markings or spots. Some larvae are covered with white wax resembling mealybugs. Larvae are usually very active moving around in search of prey. The pupae are oval-shaped and are glued to the leaf surface.
Ladybird beetles are generalist predators. Both adults and larvae prey on aphids, leafhoppers, mealybugs, mites, scales and whiteflies. The larvae eat more pests (particularly aphids) than adults. A single ladybird can consume approximately 200 - 300 aphids over its lifetime of 1-3 months.
Note that not all ladybird beetles are predators. There are several plant-feeding species such as Epilachna beetles also commonly known as melon bugs. Both the larvae and the adults feed on leaves and fruits. Epilachna beetles are pests of tomato, potato and cucurbits. The pest ladybird beetles can be distinguished from the beneficial ladybird beetles by their appearance. The wing cases of the adults of pest ladybird beetles are covered with short hairs giving them a non-glossy matted appearance. The larvae are pale yellowish in colour and covered with long spines.
Conservation: Ladybirds can be encouraged by growing non-crop plants which support aphid species which do not attack crops. Grow strips or groups of non-crop flowering plants such as fennel, thistles, coriander, carrots and/or milkweed. They can also be brought in by hand from outside the crop. Spraying with pesticides should be avoided wherever possible in order not to kill the ladybird adults and larvae but if it is necessary, selective pesticides and methods should be used.
|Ladybird beetle larva|
(c) Clemson University, insectimages.org
Lacewing adults have net-like semitransparent wings, large eyes and rather soft, fragile-looking bodies. They are greenish (green lacewings) or brownish (brown lacewings). Brown lacewings are smaller than green lacewings, measuring about 6-12 mm. The eggs of green lacewings are laid on characteristic fine silk stalks. Eggs of brown lacewings are laid singly on the side on plants without any attaching stalk. Lacewing larvae are greyish-brown, wingless and crocodile-like. They have long sickle-shaped mouth parts and some carry the skins of their prey on their backs. Larvae are very active. The pupae are whitish and spherical, resembling spider egg sacks.
Main prey: Larvae and adults of brown lacewings are predacious. They feed on mites and soft-bodied insects, especially aphids, mealybugs, scales whiteflies, small caterpillars, and insect eggs. Some green lacewing adults are predaceous, while others only feed on nectar from flower or honeydew from aphids. Lacewings are among the fiercest predators in the insect world. They can eat more than 200 pests or pests' eggs a week. Older larvae can consume 50 aphids a day. However, if no other prey is available, they become cannibalistic.
Conservation: Make sure flowers (e.g. dill, sunflower, carrots) are present in the field or close to the crop to ensure food supply for adult lacewings. Populations can be maintained on a non-pest aphid species and other hosts outside crop area (in weedy areas on field margins).
Description: Hover-fly adults are usually brightly coloured with yellow-brown or black stripes. They are sometimes confused with wasp and bees, but flies have only one pair of wings, while wasps have to pairs. They owe their name to their ability to float or "hover" in the air, like a helicopter. They can often be seen on or close to flowers, where they feed on nectar and pollen. Although adults help to pollinate plants, it is the larvae which are most useful since they are natural enemies of aphids and small caterpillars.
The eggs, despite their small size, are easily seen on leaves, especially where aphids are present. They are white, cylindrical and 1-2 mm in size.
|Hover fly adult|
(c) Jonny N.Dell, retired, Courtesy of Bugwood.org
The larvae are usually greenish or brown with one to three white stripes along the body. They are 1-13 mm long, depending on the larval stage. They are often mistaken for caterpillars, although they do not have a distinctive head or legs as do caterpillars. They can also be distinguished by their slug-like appearance. The pupae are pear-shaped and maybe green or brown.
Main prey: Hover-flies mainly prey on aphids and small caterpillars.
Conservation. Hover flies can be encouraged by allowing non-crop plants to grow around fields - these support non-pest species of aphids that hover fly larvae can feed on. They are attracted to all flowering plants but especially to fennel, milkweed, sun hemp, flowering brassicas or wild mustard, thistles, sunflower, coriander and dill. In rape and kale, some of the previous crop can be left to flower (provided the pest and disease levels are low) or a small number of plants from the current crop can be encouraged to bolt and flower by stopping watering. Avoid spraying with pesticides whenever possible but if it is necessary, use selective pesticides and methods
|Different types of assasin bugs present on crops. |
(c) A. M. Varela, icipe.
The most common predatory bugs include anthocorid (pirate) bugs, nabid bugs, and assassin bugs. The nymphs of bugs are similar to the adults in shape, but smaller and may vary in colour. Young nymphs are wingless, but wings develop gradually and wing pads can be seen as the nymphs develop.
Predatory bugs are variable in size. Anthocorid bugs or minute pirate bugs are tiny (2-3 mm long). The nymphs are brown, black or orange, while the adults are black with white patches on their wings. Assassin bugs are large insects (10-40 mm long). They are generally oval in shape, but some are elongated and thin, resembling stick insects. They can be distinguished from plant feeding bugs by their curved mouth parts.
Main prey: Both adults and nymphs of pirate bugs are important predators of thrips, mites, aphids, insects eggs and small caterpillars. Assassin bugs prey on other insects including pests, spiders and other small animals such as snails.
Commercial availability: The minute pirate bug Orius jeanneli is commercially available in Kenya under the trade name (Oritech(r)) from Dudutech (K).
Praying mantid adults are usually large with well-developed wings and characteristic forelegs that assume a posture similar to praying when resting, hence their common name. The forelegs are used for grasping the prey.
Females lay their eggs in hardened foam case attached to weeds or twigs. Nymphs are similar to adults, although smaller in size and initially have no wings. The wings develop gradually as the nymphs get older. Mantids wait in the foliage well camouflaged, until prey comes within their reach.
Main prey: Both nymphs and adults are indiscriminate eaters; they feed on pests such as moths, flies, crickets, bugs, etc. , but also on natural enemies such as beneficial bugs, flies and spiders. Their presence however indicates that there is no indiscriminate use of broad-spectrum pesticides.
Conservation: Avoid spraying broad spectrum pesticides. Maintain vegetation as natural habitats for mantids.
(c) A. M. Varela
Rove beetles (Staphylinidae beetles)
Description: The adults resemble earwigs, although they do not have the well-developed pincers at the end of the abdomen. They are very variable in size, ranging from about 1 mm to 20 mm in length. The wing cases of most rove beetles are short and do not cover the segments of the abdomen. The abdomen may be raised in a threatening posture (like scorpions) when disturbed. Rove beetles are often reddish-brown, brown or black. The wing cases may have a different colour from the rest of the body.
(c) A. M. Varela, icipe
Main prey: Some rove beetles are generalist predators and eat all stages of a wide range of insects in the soil or in the foliage. They may be important for control of bean flies, cutworms, scale insects and spider mites. The tiny rove beetles Oligota spp, (about 1 mm in length) are common predators of mites. Some rove beetles are parasitic; young larvae search the soil for cocoons of flies and after eating the pupa emerge sometime later as adults.
Conservation: Conserve or develop refuges for soil dwelling rove beetles. This could be done by promoting plant diversity, using mulches and keeping stones, logs or other items that provide protection.
Description: These are large wasps, often 12-30 mm long, usually black, which lay eggs inside or on other insects such as aphids and caterpillars, or even in the eggs of other insects. When the egg hatches the larva of the parasitoid feeds on its host and kills it. They may be solitary or may live in groups. Some form papery and honeycombed nests often found suspended by a stalk from leaves of plants or sides of buildings. Others have nests in the ground, in natural cavities or within tree stumps.
Main prey: Predatory wasps attack caterpillars, sawfly larvae or other prey; they sting and paralyse the prey and take it back to the nest as food for their brood.
Conservation: Avoid destroying nesting sites, unless they are a threat to humans. Avoid spraying pesticides unless absolutely necessary since adult parasitoid wasps walk around a lot on the leaves while searching for hosts so can quickly pick up a harmful quantity of pesticide. Adult parasitoid wasps can be attracted and sustained by ensuring that there are flowering plants nearby to provide nectar for them to feed on. They can also be encouraged by providing non-crop plants such as milkweed and thistles which encourage non-pest species of aphids.
Description: Ants are social insects and have a division of labour between different groups (workers, soldiers and reproductive forms) within each colony. Worker ants are responsible for food collection and are active foraging for food. Ants are generally not specific. They may feed on both pests and beneficial insects. They can be very effective at removing caterpillars and other pests from crops - they are thought to be one of the most important natural enemies of Helicoverpa armigera - the African bollworm. Some ants are important predators of eggs, larvae and adults of pests.
|Ants attacking a cutworm caterpillar|
(c) A.M. Varel
However, some species of ants also protect pests as they feed on honeydew excreted by some sucking insects such as soft scales, aphids, whiteflies, and mealybugs. In particular they stop natural enemies attacking aphids in order to maintain their supply of honeydew - the sugary liquid excreted by aphids as they suck the plant sap, and which the ants use as a food source.
Conservation: Excessive hoeing or ploughing will destroy ant nests so farming systems which use minimum tillage (very little hoeing or ploughing) are more likely to encourage beneficial ants.
|Weaver ant nest on a citrus tree|
(c) A.A. Seif, icipe
Red fire ants or weaver ants (Oecophylla longinoda)
Description: Red fire ants nest on citrus and other fruit trees (guava, soursop, cashewnuts, coconut palms among others). These ants are present in many countries in Africa. They are common in the coastal regions in East Africa. They built nests on trees by joining leaves with silk produced by the larvae. These ants are very active moving on the trees and on the ground in search of food. They are highly voracious feeding on a large range of insects visiting the trees, and are important in controlling many insect pests in fruit trees and coconut palms. In spite of these benefits, weaver ants are considered by some as a pest due to their aggressiveness combined with painful bites, which makes fruit picking difficult, and to their association with honeydew-producing insects.
They can foster the build-up of honeydew producing, but it has been observed that they do kill some of them when the amount of honeydew produced by these insects is bigger than the amount required by the colony of weaver ants. The benefits provided by predatory ants feeding or deterring insect pests must be outweighed against the damage they may cost indirectly. Weaver ants are considered beneficial. They have been used actively in China for the control of citrus pests for centuries (Way and Khoo, 1992). Experienced farmers in Asia and Africa have developed their own methods to deal with the inconvenience of weaver ants during harvesting.
How to harvest if weaver ants are present:
A common pratice among farmers is to throw wood ash on the branches of the tree they want to climb. The ants fall down of the branches and have difficulties to return giving time to farmers to harvest. Other farmers rub their hands and arms with wood ashes, to prevent the ants from attacking them. Other rub their arms and feet with certain repellant products before climbing the tree, use protective clothing or harvest at times of the day when weaver ants are least active (Van Mele and Cuc, 2007).
Description: Thrips are tiny insects (0.5 to 3 mm in length), they have slender bodies, short antenna, and have piercing-sucking mouth parts, which enable them to cut and remove plant or insect juices. Adult thrips have 4 long narrow wings, fringed with long hairs, giving them a feathery appearance, when viewed under magnification. They vary in colour from pale-green yellow to brown or black; some are banded or have spots markings on the wings. Nymphs resemble adults in both size and colour but are wingless in the first stages. Wings develop gradually as the thrips develop. Most thrips are plant feeders, however some species are predatory on other insects or mites.
Main prey: Predatory thrips feed on mites and other soft-bodied insects such as pest thrips, aphids, maggots of leafminers, whiteflies, etc.
Conservation: Avoid use of broad spectrum pesticides.
Mite Phytoseiulus longipes a predator of the tobacco spider mite (real size-1mm).
Predatory mites (Phytoseiulus persimilis)
Description: Mature adult Phytoseiulus persimilis are shiny, light red to orange-red, and spotless. They eat plant-feeding mites. They are very small (less than 1mm) and difficult to see by eye but if populations of red spider mites are examined closely, any predatory mites present can be identified by their longer legs and much faster movement. The immature stages and young adults are glossy transparent to cream-coloured with a droplet-shaped body. Nymphs are smaller in size and move more slowly than adults. The eggs are glossy transparent to cream-coloured and oval about 0.2 mm long and are deposited on the underside of the leaves.
They can provide effective control of spider mite populations on tomatoes but it can take some time for numbers to build up, especially if the plant variety has very sticky stems and leaves. Most predatory mites cannot survive without live prey to feed on, so hedges and living fences and other non-crop plants can help to provide a refuge and food source for them between crops. Dust tends to kill predatory mites so regular irrigation, which reduces dust, will tend to encourage these natural enemies. Also, colonies of plant-feeding mites can be kept on potted plants and when predatory mite numbers build up they can be transferred to the crop. In some countries the mites are commercially available for releasing onto the crop (a type of inoculation).
Main prey: Phytoseiulus persimilis predates exclusively on spider mites and does not feed on plant material. Phytoseiulus persimilis is very efficient in searching out its prey. An adult can eat up to 5 spider mite adults or 30 eggs and young spider mites per day, whereas an immature can eat 6 eggs and young spider mites in 2 days. Under favourable conditions, 21-27degC and relative humidity beyond 60%, it completes its life cycle within 7 days, which is almost twice as fast as its prey, and is thereby capable of eradicating spider mite populations quickly.
Phytoseiulus persimilis is sensitive to the host crop of its prey. Whereas Phytoseiulus persimilis multiplies very well when feeding on spider mites feeding on beans, it multiplies very poorly when feeding on spider mites feeding on roses. Phytoseiulus persimilis does not feed on the tobacco spider mite (Tetranychus evansi), a very important mite pest in the region. A different species of predatory mite Phytoseiulus longipes, has been introduced from South America for control of the tobacco spider mite, and experimental releases have started (icipe).
Conservation: Phytoseiulus persimilis'
survival is entirely dependent on the maintenance of a low-level spider mite population. Phytoseiulus persimilis
is very sensitive to relative humidity (RH). Its optimum condition is 75% RH and eggs require a minimum of 60% RH to hatch. Phytoseiulus persimilis
is sensitive to most insecticides and many fungi
|A predatory mite (Phytoseuilus persimiles) (red) feeding on spider mites|
Commercial availability: Phytoseiulus persimilis is available from Dudutech (K) Ltd, Koppert Biological Systems (K) Ltd and Real IPM Ltd.(K)
(c) Koppert Biological Systems
Amblyseius spp. (also called Neoseiulus spp.)
Description: Mature adult Amblyseius spp. are shiny, transparent to cream-coloured, and some have a bit of orange markings. They have a more elongated body shape, as compared to Phytoseiulus persimilis, and are about 0.45 mm long. They are less active and slower than Phytoseiulus persimilis. The immature stages look like the adults; they are smaller in size and move more slowly. The eggs are glossy transparent to cream-coloured and oval about 0.15 mm long. Differentiation of the various Amblyseius species is very difficult in the field and requires assistance from a skilled laboratory technician.
Main prey: Amblyseiulus californicus has a preference for spider mites, whereas Amblyseius cucumeris has a preference for thrips and Amblyseius swirskii has a preference for immature whiteflies. Nevertheless, Amblyseius species can feed on preys other than their preferred prey and on mould and nectar. Amblyseius species are not as efficient in searching out their preys as Phytoseiulus persimilis and thus, disperse less widely through the crop. They do not reproduce as much and develop as fast as Phytoseiulus persimilis.
Conservation: Amblyseius spp. can sustain their population at a very low prey density, survive on alternative food sources, even without food supply for some time. Amblyseius spp. can survive higher temperature and the adverse effect of low humidity than for Phytoseiulus persimilis. Population growth under warm dry conditions is therefore better for Amblyseius spp. than Phytoseiulus persimilis. Amblyseius spp. can tolerate more insecticides and fungicides, in general, as compared to Phytoseiulus persimilis. Commercial availability: Amblyseius californicus and Amblyseius swirskii are available from Koppert Biological Systems (K) Ltd, and Amblyseius cucumeris is available from Dudutech (K) Ltd.
Description: Spiders are not insects; they have 8 legs instead of 6. Egg sacs of spiders are usually covered in whitish-silk. The eggs within the sac hatch to release several spider nymphs at once, they are sometimes called 'spiderlings'. Nymphs are similar to adults, but smaller in size.
Main prey: Spiders found on crops can be divided roughly in two groups: webbing spiders and hunting spiders. Webbing spiders catch their prey in nets or webbing, while hunting spiders forage actively or lay in wait for their prey.
Conservation: Spiders can be very important predators in agricultural systems. They prey on moths, flies and other pests they can catch. As generalist feeders, spiders sometimes also prey on beneficial insects as well as pest species.
(c) Edward L. Manigault, Bugwood.org
|Parasitic wasp emerged from caterpillar. Note emergence hole in mummified caterpillar|
(c) A. M. Varela
Description: There is a wide range of parasitic wasps with a wide variation also in size (1-6 mm long). Parasitic wasps attack eggs, larvae, pupae and sometimes adults of many other groups of insects. They usually lay their eggs within or on the body of the host. Eggs hatch into tiny yellow-creamy larvae that feed on the body fluids or the body tissues of their host. The mature larvae of the parasitic wasp pupate in or outside the host body. The number of wasps that can develop within a single host varies with the parasitoid species and the host. In aphids, leafminers, mealybugs, usually one single parasitic wasp develops on a single host. In the case of caterpillars, a single wasp, a small number, or a large number of wasps (sometimes more that 100) may develop within each caterpillar, depending mainly on the species of parasitoid. Larvae of some wasps pupate in the host, while others emerge from the host when feeding is completed and pupate nearby. Adult parasitic wasps feed on pollen, nectar, honeydew and other sugary substances, but some are predacious feeding on the host contributing to mortality caused by their larvae.
There may be few or no symptoms of parasitoid presence within its insect host. In other cases such as aphids, mealybugs and some caterpillars the host insect may become swollen and/or permanently paralysed. When the larvae of the parasitic wasp pupate, the parasitised aphid or mealybug turns brown and hard and remains stuck to the leaf. In this stage they are known as mummies and can be easily recognised.
The parasitic wasps emerge through a round hole in the aphids or mealybug abdomen a few days later. Parasitised whiteflies nymphs can be distinguished from healthy ones. Parasitised nymphs are dark while healthy nymphs are usually yellowish greenish.
Field collection and rearing of immature stages (eggs and larvae) of pests to check if any parasitoid emerges is often a useful method to determine whether parasitism has occurred.
Main prey: Parasitic wasps are important natural enemies of moth and butterfly eggs, bug eggs, caterpillars, leafminers, aphids, mealybugs, and whiteflies. They play an important role in natural control. In cases where the naturally occurring natural enemies do not provide satisfactory control, or in cases where an exotic pest has been introduced without its natural enemies, parasitic wasps have been introduced and released. Some examples of parasitic wasps introduced in the region for control of important pests are: Diadegma semiclausum and Cotesia plutellae for control of the Diamond back moth on crucifers, Cotesia flavipes for control of cereal stemborers, Apoanygyrus diversicornis for control of the cassava mealybug, Gyranusoidea tebygi and Anagyrus mangicola for control of the mango mealybug; Diglyphus isaea for control of leafminers, Cales noacki for control of the woolly whitefly on citrus and Encarsia formosa for control of whiteflies.
|Parasitic wasp Oomyzus laying eggs in diamondback moth caterpillar|
Commercial availability: Parasitic wasps commercially available in Kenya: Aphidius transcapicus (Aphitec(r)) for control of aphids, Trichogrammatoideasp. (Grammatech(r)) for control of moth and butterfly eggs, Diglyphus isaea (Diglytech(r)) for control of leafminers and Encarsia formosa (Encartec(r)) for whitefly control. These parasitic wasps are available from Dudutech (K) Ltd. Eretmocerus eremicus (Ercal(r)) for whitefly control is available from Koppert Biological Systems (K) Ltd.
(c) Scott Bauer, USDA Agricultural Research Service, Bugwood.org
Conservation: Adult wasps feed on nectar, honeydew, or pollen before laying eggs. Keeping flowering plants close to, around and/or within the crop may attract parasitic wasps and provide good habitats for them. Some flowering plants that have been recommended are dill, fennel, coriander, nasturtiums, flowering crucifers, sun hemp, clover, alfalfa, parsley, sunflower, and marigold.
Description: The most important are tachinid flies. The adults look like the common house flies, but are larger (8-12 mm in length) and have more stout, bristly hairs on the abdomen. They are usually grey in colour with some markings. Female flies lay tiny whitish oval eggs on their host, and sometimes on leaves. Upon hatching the fly larvae (maggots) burrow through the body and feed on the prey. With species that lay eggs on leaves, eggs are ingested by caterpillars or larvae of beetles while feeding on foliage. Pupation occurs either within or outside the body of the host. Some species pupate in the soil. Adults feed on nectar and pollen.
Main prey: Tachinid flies are important parasitoids of caterpillars, beetle larvae and adults, and earwigs, bugs and grasshoppers.
|Tachinid fly , a parasitoid of caterpillars. Note brown fly puparium and dead caterpillar. |
(c) A. M. Varela
Conservation: Provide sugar containing food sources for the adult such as flowers or honeydew. Encourage habitat diversity. Avoid spraying pesticides. When necessary select a product and application method that is less harmful to natural enemies.
A large number of naturally occurring, insect-specific bacteria have been isolated from insects, plants, and the soil, but only a few have been studied in detail. Bacillus thuringiensis is the best-known insect pathogen.
Description: Bacillus thuringiensis (Bt) is a naturally occurring bacterium that produces substances toxic to some insects. Bt works by interfering with digestion - therefore, to be effective it must be eaten by the insects. Insects are more sensitive to Bt in the larval stage. After larvae ingest Bt spores, the bacterium produces toxins that paralyse the digestive tract of the larvae causing it to cease eating. Death will follow from hunger or infection; the time of death can range from 12 hours to 5 days after ingestion, but since the insects stop feeding shortly after exposure, damage to the crop is reduced.
Main prey: There are different strains or varieties of Bt that differ in toxicity to insects: The Bt. kurstaki and B. aizawai infect moths (caterpillars), Bt. tenebrionis and B. San Diego infect leaf beetles, and Bt israelensis (BTI) affects mosquitoes, black flies and fungus gnats. The different strains of Bt are specific, this means they do not harm beneficial or non-target insects. Commercial availability: Several microbial pesticides based on different strains of Bt are available in Kenya: Xentari WDG(r) and Florbac(r) (Bt. var. aizawai), Thuricide HP(r) wettable powder and Dipel 2X WP (Bt var. kurstaki). As the larvae must eat Bt treated foliage for these bio-insecticides to work, thorough coverage of the foliage is very important, and use of a sticker is advisable. Bt should be applied when the larvae are small.
For further information on Bt click here
Description: Fungal strands (hyphae) enter the insect body through openings. The fungal hyphae (mycelia) develop and consume the insect from inside. Insects and mites killed by fungi become stiff and, under sustained wet or humid conditions, become covered with whitish, cream, green, red or grey coloured fuzz, the mycelia growth. Spores are transferred on insect bodies, wind, rainwater or wind. Fungi commonly found attacking insects include Beauveria bassiana, Metarhizium anisopliae, Verticillium lecanii, Zoophthora spp. and Entomophtora spp.
Some fungi are natural enemies of plant pathogens. They are antagonistic, that is, they compete with and /or suppress the development of pathogens which cause diseases to plants, and some are used in the biological control of plant diseases.
Commercial availability: Some fungal-based commercial products for control of pests and diseases are available in Kenya: e.g. Beauveria bassiana (Bb Plus(r)) for control of aphids, thrips, and whiteflies available from Juanco SPS Ltd.; Trichoderma asperelum (Trichotech(r)) available from Dudutech (K) Ltd. and Trichoderma spp (Rootgard(r)) available from Juanco SPS Ltd. for control of Fusarium wilt; Pochonia chlamydosporia (Klamitech(r)) available from Dudutech (K) Ltd. and Paecilomyces lilacinus (Pc Plus(r)) available from Juanco SPS Ltd. for control of plant parasitic nematodes.
|Grasshoper killed by a fungal infection|
(c) A. M. Varela
Description: A wide range of viruses has been identified attacking insects, and some of them have been used as biological pesticides. Viruses are commonly found among caterpillars and sawflies. The most common types are granulosis viruses and nuclear polyhedrosis viruses. They are species specific. Viruses need to be ingested (unlike fungi) in order to enter and replicate within the insect body. Caterpillars or sawflies infected by nuclear polyhedrosis viruses become flaccid, die rapidly, and hang limply by the abdominal legs from leaves. At the slightest touch the insect body ruptures spilling the virus particles onto leaves infecting other insects. Other viruses act more slowly; infected caterpillars appear pale or have a chalky colour, become swollen and the development is retarded. Insects infected with viruses usually die prior to reaching adulthood and may turn black or brown after death due to secondary infection with bacteria.
Nematodes are long, thin round worms, so tiny that they usually can only be seen under the microscope. Insect-infecting nematodes are found in the soil, and infect many different types of insects living or foraging in the soil or on the ground such as the larvae of some moths, butterflies, beetles and flies, as well as adult crickets and grasshoppers.
The most commonly studied nematodes belong to the groups Steinernematidae and Heterorhabditidae, and have been used in the biological control of insect pests. These nematodes are associated with bacteria, which are released from the nematode into the body cavity of the insect, where they multiply, killing the insect within 48 hours. The nematodes then feed on the insect liquefying tissues, develop and reproduce for several generations inside the cadaver. When the food supplies run out, juvenile nematodes move into the soil to find new hosts. The bodies of insects killed by nematodes are yellow, orange or bright red in colour.
In some cases, farmers can produce homemade microbial pesticides by collecting diseased larvae and crushing and mixing them with water in a blender. They then filter out the large tissue masses to leave the liquid for spraying the crop. The pathogen will infect and kill other pests in the crop.