Natural enemies
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Natural enemies
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Introduction
Areas of land which have not been cultivated or disturbed contain hundreds or
thousands of species which tend to form a balance, with each of them depending
on some of the others. Although large outbreaks of plant-eating pests do sometimes occur in natural systems, any one particular species is less likely to build up a large population if the organisms which feed on it are also present - in other words, its natural enemies The natural enemies of plant pests are considered as farmers' friends.
Various beneficial organisms that can help the farmer to keep pests (and some diseases) under control and prevent them from causing economic damage.Predators are one type of natural enemies which tend to keep the population of their prey in check. They catch and eat other insects and mites, including pest species. Parasitoids are another type of natural enemies. They lay eggs in or on other species of insect (called hosts) and the larval stage kills the host as it feeds on it and develops. The third major group of farmers' friends is pathogens. These are fatal or debilitating diseases to arthropod pests and include fungi, nematodes, bacteria, viruses, and other microbes. Fungi, particularly Deuteromycetes, can infect pests externally under favourable conditions, but other pathogens must be ingested to be effective as control agents. Pathogens are very specific to their hosts. Pathogens can be used as biopesticides because they can be applied in similar ways to chemical interventions.
Beneficial living organisms which reduce pests and diseases are usually present in any crop unless broad spectrum pesticides (which kill a wide range of arthropods) have been used. These so-called natural enemies can be conserved by taking care with farming practices so that they are not killed or are actually encouraged. If numbers of such biocontrol agents are still not sufficient to keep pests at acceptable levels, it is possible to release additional beneficial organisms of the same type - a process known as augmentation or inundation. Farmers who collect ladybird beetles in field margins and release them on their crop are practising augmentation. Alternatively if suitable types of beneficial organisms are not present in the crop, they can be introduced. Where introduction involves a local beneficial organism which has simply not yet reached a particular crop, this is known as inoculation. If the introduced beneficial organism is from outside the area (typically from the country or area where the troublesome pest originated) and becomes established as the controlling factor for the pest in the new area, it is known as classical biological control. A new balance is created so that the pest becomes less important.
How farmers can help to keep the balance
Farmers can help to keep the balance in their favour in trying not to harm predatory arthropods such as ladybirds, spiders and hover fly larvae which feed on plant-eating pests such as aphids and caterpillars. Some of the ways to do this are:
- Use control pesticide products only when necessary and then not broad spectrum ones (note that most botanical pesticides are broad spectrum)
- If control products are used - use them selectively
- Growing flowering plants which provide nectar and pollen to farmers' friends such as adult parasitoid wasps, hover-flies and ladybird beetles adults by having living fences (hedges) around the crop to provide shelter and refuge for farmers' friends should be encouraged. These are called refugia, and examples include beetle banks (grassy areas near crops) flowering plants and unsprayed field edges.
- Mulching to provide refugia for ground-living farmers friends such as predatory beetles
Conservation of natural enemies
When crops are grown, it inevitably disturbs the natural balance, especially where
the crop is a monoculture i.e. all one species of plant. However, the beneficial
effect of predators and parasitoids continue to be critically important. If they are
correctly managed, they will help prevent some of the pest problems which
farmers encounter. An example of how farmers can help to keep the balance in
their favour is to try not to harm predatory insects such as ladybirds, spiders and
hoverfly larvae which feed on plant-eating pests such as aphids and caterpillars.
These predators can be found on most crops together with parasitoid wasps (and
occasionally parasitoid flies) which lay their eggs in/on pests. In IPM systems
which aim to minimise dependence on pesticides, it is essential that the farmer
can distinguish these natural enemies from pests and can use farming techniques
which will conserve and encourage natural enemies.Like humans, insects also suffer from diseases which can weaken or kill them. Types of fungal, bacterial and viral pathogens which only inffect insects and are safe for humans and animals have been identified. Some of these are commercially produced as bio-pesticides and some can be prepared on the farm.
Augmentation and inundation with natural enemies
Sometimes there are predators and other natural enemies present which are feeding on the pests, but they are not able to control them effectively, particularly those pests that are capable of breeding very quickly. Farmers can augment the number of natural enemies by bringing them in from outside the field, for example, ladybird beetles or parasitised aphids which contain young parasitoids. Some types of natural enemies can be specially bred in large numbers, and then released onto the crop in order to attack and control the pest. The natural enemies inundate the pest population. Many of the advances in this technique have been against pests of crops which have been economically important for a long time, such as cotton. For example, the egg parasitoid wasp called Trichogramma has been bred in laboratories to allow huge numbers to be released when eggs of the African bollworm are present on the crop. The same bollworm can be a serious pest of tomatoes, so it may be possible to use the same biocontrol tactics developed for cotton. Aphid parasitoids could theoretically be produced and released in a similar way, but these technologies are often not yet available to small-scale vegetable growers.
Using insect pathogens in pest control sprays
Naturally-occurring pathogens (fungi, bacteria and viruses) that kill insects can be obtained from diseased insects and incorporated into sprays applied to control the pest. This is a type of inundation. Pathogen-based sprays are not yet widely available for vegetable pests except for Bt (Bacillus thuringiensis) a bacterium which kills larvae of moths and butterflies (caterpillars). One example under development is a virus which kills diamondback moth caterpillars - a serious pest of brassicas. The pathogen is called Plutella xylostella granulovirus (PlxyGV). The pathogen has the important advantage of being highly specific. In other words it does not harm other arthropods such as natural enemies so it works together with the natural processes which limit pest numbers. This contrasts with most pesticides which also kill natural enemies. Farmers sometimes use a type of home-made biopesticide - they gather diseased pests, crush and mix them with water, then spray the liquid onto the crop. The fungi, bacteria or viruses which were infecting the collected pests will infect other pests in the crop and kill them.
The types of natural enemies: Predators, parasitoids and pathogens.
Predators
These are organisms that prey and feed on other organisms. They often feed on various stages of the host (pest): eggs, larvae, pupae and adult. Each predator kills and feed on a number of prey individuals during their development (larvae to adult). Most adults are also predators.
For example: ladybird beetles, dragonflies, predatory mites, predatory bugs, predatory wasps and spiders.
Parasitoids
Organisms that during the larval stages feed on pests (external parasitoids) or in the pest (internal parasitoids). They complete their development on a single host, killing it. In their adult stages they are mostly free-living (with few exceptions) and feed on pollen and nectar or other sugary substances such as honeydew.
The most common parasitoids are parasitic wasps and flies.
Pathogens
Organisms that can cause diseases of pests. They include fungi, bacteria, viruses and nematodes. They can be important in controlling pest populations in agricultural systems. However, naturally occurring pathogens often are too rare to serve as important control agents or occur when the damage has already been done.
Some pathogens such as the bacterium Bacillus thuringiensis (Bt) and the fungus Trichoderma viride are commercially available in many countries, including Kenya.
Other fungi such as Zoophthora, Verticillium and Entomophthora can be readily found in the field at particular times of the year, infecting aphids, beetles, caterpillars, grasshoppers and whiteflies.
Natural systems support a wide range of plants and animals. The plants provide food for plant-eating animals such as antelopes and many insects. The plant-eating animals are themselves eaten by a wide range of predators. Plant-eating animals that feed on a crop that we value (for example aphids that feed on cabbage), are considered pests. Plant-eating organisms are also eaten by organisms called natural enemies, which can be considered "farmer's friends".
| Type of organism | Example 1 | Example 2 |
| Plant | Tomato | Cabbage |
| Plant-eating animal | Caterpillar (pest) | Aphid (pest) |
| Predatory or parasitic animal | Parasitic wasp (natural enemy) | Hover fly larva (natural enemy) |
These feeding levels (see tableabove) exist in all stable natural and agricultural habitats. The number of animals from each feeding level is kept in check by complex interactions. In agricultural systems the balance between the four feeding levels is frequently upset and pest numbers increase rapidly, resulting in pest outbreaks. Reasons for this can be:
Monocultures: Planting of large tracts of land with one single crop type is referred to as monoculture. This makes it easy for pests to find the crop and once they have found it, to develop quickly because their food source is plentiful and closely spaced. Monocultures also contain few shelters and alternative food sources for natural enemies.
Poorly adapted crop cultivars: Many plant cultivars grown for food crops in tropical regions have originated from Latin America and Asia and are not always adapted to local growing conditions and thus have low levels of natural resistance to pests and diseases.
Use of pesticide: Use of pesticides can cause several problems. In many instances, natural enemies are more susceptible to pesticides than the pest itself and are thus harmed or killed while the pest is not much affected. This causes pests to multiply much more quickly.
Pests may also develop resistance to pesticides. This occurs in situations where pesticides are used frequently and the same type of product is used for long periods. A farmer said: "I've been spraying once a week for the last month and the crop is being destroyed by pests. What is going on?" This is because some of the pests are less susceptible to the pesticide and after a period of heavy pesticide use, they still persist because they have developed resistance to the pesticides. Increased frequency of spraying only affects natural enemies. Furthermore, other beneficial insects, such as pollinators, are also harmed.
Although proper identification of pests and beneficials is important it is not always easy. Take for instance ladybird beetles, which are one of the most talked about and valuable farmers' friends because of their ability to eat large numbers of plant sucking aphids. But some ladybird species do eat plants and can cause crop damage! These species usually look dusty or slightly hairy - as compared with the very shiny species which are usually farmer's friends. To make matters more difficult, most insects look very different at different stages in their development. For example, ladybird larvae look like tiny spotted crocodiles and are very different from the adults, but they also eat many aphids, scales and young caterpillars. This highlights the importance of being able to identify that "good and bad" on the crop no matter what life cycle stage is present.
Insect Life Cycles
Most crop pests are arthropods, which means they have external skeletons and
jointed legs. Within this group, there are insects which have 6 legs as adults and
mites which have 8 legs as adults (but 6 legs when younger). Arthropods are
usually considered to be pests if they eat any part of the crop plant (chewing
pests) or if they suck crop plant juices (sucking pests). The damage may be
caused by adults or by the younger stages called larvae or in some cases
nymphs. Other serious pests of vegetables include nematodes, tiny worms that
live in and feed on plant roots (plant parasites) and larger animals such as rats,
monkeys and even hippos.Life stages of arthropods Arthropods can look very different at different stages in their life cycle. It is useful to understand these different forms so that pests can be identified regardless of the stage they have reached in their life cycle. Arthropods go through a process called metamorphosis which means change of form. Some have complete metamorphosis where the young stages look completely different from the adults - see the butterfly in the left hand picture below. Others go through incomplete metamorphosis where the young look a little like small versions of the adult, but without fully developed wings - see the bug in the right hand picture below.
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| Insect Life Cycles |
| © agLearn.net |
It is also possible to find out whether an organism is a friend or a pest by setting up an insect zoo to investigate what the organism is doing on the crop. This involves putting the insect in a small jar together with leaves from that plant and any known pests which were also on that leaf. If it eats or sucks the leaves, it is a pest and if it eats the pests it is a natural enemy.
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Find out which natural enemies are present.
In most cropping systems natural enemies are already present or close by and do not need to be introduced. It is crucial that natural enemies can be recognised and distinguished from pests.
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Insect zoo
To find out who is eating who or what, an "insect zoo" can be set up. A small group of suspected pests and natural enemies are kept together in ventilated jars or other small containers with a small portion of the crop, for example kale leaves. Always make sure that the insects have fresh food. Keep the zoo in a shaded place to avoid high temperature. Take care that the environment inside the zoo does not get too dry or too humid. Maintain the insect zoo all of them have gone through their complete development, so that you can see who eats who or what. Insect zoos can be used to study the life cycle of insects and their behaviour. They are also very useful to study the effects of predators or Bt. -
Select appropriate cultivars
It is important to choose crop cultivars that are not highly susceptible to pests and diseases. Information on the susceptibility of different cultivars can often be obtained by asking other farmers, extension staff or relevant research institutes. If the information is not available, it may even be useful to run on-farm trials of different cultivars. -
Conserve natural enemies by cultural means
There are many ways to encourage natural enemies on your farm: - Providing food sources for adult stages of natural enemies such as flowering plants (fennel, thistles, coriander, Indian mustard and other flowering brassicas) close to the crop
- Mixed cropping systems provide food and shelter and attract a wider range of natural enemies. Green manure plant such as legumes and sun hemp grown in rotation with other vegetables also improve soil fertility.
- Live fences (trees, hedges) act as windbreaks and provide shelter for natural enemies.
- Mulches around plants provide attractive environments for ground-living predators such as beetles and spiders.
Predators
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| Adult ladybirds mating |
| © Jim Occi, www.insectimages.org |
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 7-10mm 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).
T 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.
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| Ladybird beetle larva |
| © Clemson University, insectimages.org |
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.
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| Brown lacewing (Micromus timidus) |
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).
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| Hover fly adult |
| © Jonny N.Dell, retired, Courtesy of Bugwood.org |
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.
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| Hover fly larva. Close-up of larva preying on aphids. |
| © Clemson University, www.insectimages.org |
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
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| Different types of assasin bugs present on crops. |
| © 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®) from Dudutech (K).
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| Praying mantis |
| © A. M. Varela |
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.
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| Rove beetle. |
| © A. M. Varela, icipe |
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.
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.
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| Predatory wasp |
| © A. M. Varela |
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.
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| Ants attacking a cutworm caterpillar. |
| © A.M. Varela |
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.
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.
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| Weaver ant nest on a citrus tree |
| © A.A. Seif, icipe |
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).
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| Predatory thrips |
| © A. M. Varela |
Description: Thrips are tiny insects (0.5 to 3 mm in length), they have slender bodies, short antenna, and have piercing-sucking mouthparts, 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.
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| Mite Phytoseiulus longipes a predator of the tobacco spider mite (real size-1mm). |
| © icipe |
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, 21-27°C 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.
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| A predatory mite (Phytoseuilus persimiles) (red) feeding on spider mites |
| © 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 fungicides.
Commercial availability: Phytoseiulus persimilis is available from Dudutech (K) Ltd, Koppert Biological Systems (K) Ltd and Real IPM Ltd.(K)
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| Amblyseius swirskii |
| © Koppert Biological Systems |
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.
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| Crab spider |
| © Edward L. Manigault, Bugwood.org |
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.
Parasitoids
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| Parasitic wasp emerged from caterpillar. Note emergence hole in mummified caterpillar |
| © 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.
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| Parasitic wasp Oomyzus laying eggs in diamondback moth caterpillar |
| © icipe |
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.
Commercial availability: Parasitic wasps commercially available in Kenya: Aphidius transcapicus (Aphitec®) for control of aphids, Trichogrammatoidea sp. (Grammatech®) for control of moth and butterfly eggs, Diglyphus isaea (Diglytech®) for control of leafminers and Encarsia formosa (Encartec®) for whitefly control. These parasitic wasps are available from Dudutech (K) Ltd. Eretmocerus eremicus (Ercal®) for whitefly control is available from Koppert Biological Systems (K) Ltd.
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| Braconid wasp |
| © Scott Bauer, USDA Agricultural Research Service, Bugwood.org |
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| Tachinid fly , a parasitoid of caterpillars. Note brown fly puparium and dead caterpillar. |
| © A. M. Varela |
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.
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.
Pathogens
Bacteria:
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® and Florbac® (Bt. var. aizawai), Thuricide HP® 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
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| Grasshoper killed by a fungal infection |
| © A. M. Varela |
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®) for control of aphids, thrips, and whiteflies available from Juanco SPS Ltd.; Trichoderma asperelum (Trichotech®) available from Dudutech (K) Ltd. and Trichoderma spp (Rootgard®) available from Juanco SPS Ltd. for control of Fusarium wilt; Pochonia chlamydosporia (Klamitech®) available from Dudutech (K) Ltd. and Paecilomyces lilacinus (Pc Plus®) available from Juanco SPS Ltd. for control of plant parasitic nematodes.
Viruses:
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:
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.
- Aglearn.net, the Network for Sustainable Agriculture, Thailand. Vegetable IPM Course, Natural Enemies. www.aglearn.net
- Entomophatogenic nematodes. en.wikipedia.org
- Hajek, A. (2004). Natural enemies. An Introduction to Biological Control. Cambridge University Press.
- Natural enemy gallery. www.ipm.ucdavis.edu
- OISAT: Online Information Service for Non-Chemical Pest Management in the Tropics. www.oisat.org
- Van Mele, P. and Cuc, N. T.T. (2007). Ants as friends. Improving your tree crops with weaver ants. (2nd Edition). Africa Rice Centre (WARDA), Cotonou, Benin and CABI, Egham, U.K. 72 pp. ISBN: 92-913-3116.
- Verkerk, R. (2001). Farmers' Friends - recognition and conservation of natural enemies of vegetable pests: a field guide for extension staff and trainers in Zimbabwe. Biology Department, Imperial College of Science, Technology and Medicine, UK. ISBN: 0-9540132-0-4 Online version
- Wabule, M.N., Ngaruiya,P. N., Kimmins, F.K. and Silverside, P. J. (Eds) (2003). Registration for Biocontrol Agents in Kenya. Proceedings of the PCPB/KARI/DFIDF CPP Workshop. Nakuru, Kenya, 14-16 May 2003.
- Way, M. J. and Khoo, K.C. (1992). Role of ants in pest management. Annual review of Entomology. Vol 37: 479-503.