8.8 Some important natural enemies in tea

Transcription

1 8.8 Some important natural enemies in tea The natural enemies are presented here according to their major groups: predators, parasitoids, and diseases. To get an idea of which natural enemies might help control a given pest, check the following tables: Pests P r e d a t o r s Spiders Ground beetles Ladybird beetles Rove Ants Lacewing Hover fly Praying beetles larvae larvae mantis ("aphid average tiny lions") size x x x?? x Dragon flies Green leafhopper Mosquito bug x?? x x x Mites x x x x x Thrips? x x x? x x? x Aphids x x x x Scales? x Caterpillars x x? x x? moths Red borer Termites? x Predatory mites 111

2 Pests Parasitoids Parasitoids that attack eggs Parasitoids that attack larvae or nymphs Diseases Bacillus thuringiensis (bacteria) Fungi Green leafhopper x Mosquito bug Mites x Thrips x Aphids x x Scales x x Caterpillars x x x x x Red borer x? Termites Viruses 112

3 8.8.1 Predators Lady Beetles - Coccinellidae Also called ladybugs, ladybird beetles or coccinellid beetles. There are many different species of lady beetles. Some are easy to see and brightly colored. Others are tiny (only a little larger than red spider mites), and colored black. What they eat: aphids, mites, scales, thrips, green leafhopper, and other small insects; also insect eggs. Which stages attack other insects: Ladybeetles have a 4-form lifecycle (egg/larva/pupa/adult). Both larvae and adults are predators. Description and life cycle Adult lady beetles are round to oval in shape, with hard shiny covers over their wings. Different species of lady beetles have different colors or different markings. Many common ladybird species are small (5-12 mm long) with red, orange or yellow forewings marked with black spots or lines (see the picture below). But, other ladybeetles are tiny (not much bigger than spider mites) and colored black. These tiny black ladybeetles are important predators of mites (and perhaps thrips). Life cycle of the lady beetle (from: Hoffmann et al, 1993) 113

4 Predator of thrips and mites: the tiny black lady-bird beetle, Stethorus sp. Actual length of these beetles is only 2-3 mm. Top, left: Adult beetle, magnified. Compare its size with the red spider mites in the top-left and bottom-right corners of the photograph. Top, right: Adult beetle, greatly magnified (note the many hairs on the body). Bottom: Larva (immature stage) of lady-bird beetle. Source: Le Thi Thu Huong Biological control of red spider mite (Oligonychus coffeae) on tea plants in Thai Nguyen province, Vietnam. Master of Engineering thesis, Asian Institute of Technology, Bangkok. Drawing from J.E. Lawrence Order Coleoptera. In: F.W. Stehr. Immature Insects Vol. 2. Kendall/Hunt, Dubuque, Iowa, U.S.A. Female adults lay 200 to more than 1000 eggs in a few months time. The more food there is, the more eggs it lays. That way, it can keep up with the pest insect populations. Eggs are usually deposited near prey such as aphids, often in small clusters in protected sites on leaves and stems. The eggs are small (about 1 mm), cream, yellow or orange in color. The larvae have a very different appearance from the adults. They are dark and look a bit like an alligator with 3 pairs of legs. There are usually 4 larval instars. The last larval instar pupates attached to a leaf or other surface. Pupae may be dark or yellow-orange in color. Pupal stage takes about 3 to 12 days, depending on temperature and species. Adults live for a few months up to a year. There are several generations in a year. 114

5 Effectiveness Both larvae and adults of lady beetles are predators: they eat aphids, small caterpillars, mites and insect eggs. Many lady beetles prefer a diet of aphids but may switch to other prey when there are not enough aphids. Lady beetles are voracious feeders. As an adult, they may eat as many as 50 aphids per day. Each larva eats 200 to 300 aphids as it grows. Lady beetles can consume many prey in a day and can also travel around quite far (larvae may travel up to 12 m) in search of prey. They are effective predators when the pest population is high: one adult may eliminate all aphids from a seriously infested plant in just a few days. Lady beetles are thought to be less effective when pest densities are low. There may also be some crop damage before lady beetles have an impact on an aphid population. Because of their ability to survive on other prey or on pollen when there are not so many aphids, lady beetles are very valuable. For example, in Thailand, the Dept. of Agricultural Extension rears ladybeetles and distributes them to vegetable farmers for field releases. Lady beetles (including Stethorus, the tiny predator of mites and thrips) can be purchased from many companies in the U.S. and Europe. How to protect and help lady beetles Like many other natural enemies, lady beetles are easily killed by broad-spectrum insecticides. Avoid the use of these pesticides as much as possible! Adult lady beetles also feed on pollen and therefore having some flowering plants or weeds in or near the field will attract them. Lady beetles benefit from shelter for protection from adverse weather conditions and for refuge when crops are harvested. This shelter can simply be some plants around the field. Ground beetles - Carabidae What they eat: soil-dwelling insects; insect eggs; small caterpillars and other soft-bodied insects like green leafhopper and thrips. Which stages attack other insects: Ground beetles have a 4-form lifecycle (egg/larvae/pupae/adult). Both larvae and adults are predators. Description and life cycle There are many species of ground beetles. Adults may be very small (about 3 mm) to large (12-25 mm). Many are active mainly at night. These are dark, shiny beetles, often with big eyes and threadlike antennae. During the day, they hide under stones or debris. They can adult ground beetle (other species look different) run rapidly when disturbed or when in search of prey. Other species are active during the day, and are usually brightly colored or metallic in appearance. Although many species hunt on the ground, some species hunt in the branches and leaves of tea bushes. You can catch ground beetles with pitfall traps in the field. See Section 8.6 (above). Eggs are usually laid singly on or in the soil near prey. The eggs are about 0.5 mm long. Some species lay only a few eggs, others may lay hundreds of eggs. Generally, the more food 115

6 there is for a ground beetle, the more eggs it lays. That way, it can keep up with the pest insect populations. The larvae usually have large hard heads with large jaws for holding and piercing prey. They look very different from the adults. Pupae of most species are found in the soil. Effectiveness The larvae and adults of several ground beetle species eat many pests. Their ability to cover large distances in search of prey makes them a valuable addition to other natural enemies. How to protect and help ground beetles Ground beetles are easily killed by (broad-spectrum) insecticides. Avoid using these pesticides when possible. Permanent vegetation around the field borders, and also mulch, can provide hiding places for the adult beetles and can help them through periods when the tea field is disrupted (for example, pruning). Lacewings - Chrysopidae There are several species of green lacewings (Chrysopa and Chrysoperla spp.). Another species is the brown lacewing, which is brown in color and about half the size of the green lacewing. Because in several areas in South-East Asia, the common green lacewing Chrysoperla carnea is the predominant species, this important predator is discussed in this section. What they eat: aphids, spider mites (especially red mites), thrips, small caterpillars, and other small insects; insect eggs. Which stages attack other insects: Lacewings have a 4-form lifecycle (egg/larvae/pupae/ adult). Both larvae and adults are predators. Description and life cycle Adult green lacewings are pale green, about mm long, with long antennae and bright, golden eyes. They have large, transparent, pale green wings and a delicate body. Adults are active fliers, particularly during the evening and night and have a characteristic, fluttering flight. Adults feed only on nectar, pollen, and aphid honeydew, but their larvae are active predators. Adult green lacewing. Source: Shepard, B.M. et al Insects and their natural enemies associated with vegetables and soybean in southeast Asia. Quality Printing Company, Orangeburg, South Carolina, U.S.A. 116

7 Each egg is laid at the end of a long silken stalk attached to a leaf. Eggs are small (less than 1 mm) and pale green in color, turning gray in several days. Each female lays several hundred eggs, sometimes in clusters. The larvae, which are very active, are gray or brownish and alligator-like with well-developed legs. Their mouthparts are large pincers with which they suck the body fluids from prey. Larvae grow from less than 1 mm to about 6-8 mm, through 3 instars in about 2 3 weeks. Larvae of some lacewings cover their body with "trash" (bits of dead leaves, or even dead insects). Mature larvae spin round, silken cocoons, usually in hidden places on plants. The pupa forms inside the cocoon. After days, adults emerge from the cocoons. The life cycle is faster at higher temperatures. There may be two to several generations per year. Larvae (immature stages) of lacewings. Actual size is about 1 cm long. Note the long mouthparts that look like pincers. Sources: left drawing from C.A. Tauber Order Neuroptera. In: F.W. Stehr. Immature Insects Vol. 2. Kendall/Hunt, Dubuque, Iowa, U.S.A. Right drawing from Borror, Triplehorn, and Johnson An introduction to the study of insects, 6th edition. Harcourt Brace Johanovich College Publishers, Fort Worth, Texas, U.S.A. Source: Hofmann et al, 1993; and Weeden et al. website 117

8 Effectiveness Lacewing larvae eat a wide range of pests but prefer aphids. The larvae are sometimes called aphid lions, and have been reported to eat between 100 and 600 aphids each. The appetite of lacewing larvae In a trial from India it was found that during development, each larva of Chrysoperla carnea consumed an average of 419 aphids (Aphis gossypii), 329 pupae of whitefly (Bemisia tabaci) and 288 nymphs of jassid (Amrasca biguttula). In all cases, 3rdinstar larvae consumed the major portion of the total number consumed (60-80%) (Balasubramani et al, 1994). Purchase and release There is potential for commercialization of Chrysopa sp. for use against a variety of pests and a lot of research is ongoing on rearing methods and field effectiveness in South-East Asia. In Thailand, the Dept. of Agricultural Extension rears lacewings and distributes them to farmers for field release. In the U.S. and some European countries like the Netherlands, C. carnea and C. rufilabris are available commercially. C. carnea is recommended for dry areas, C. rufilabris for humid areas. Larvae are likely to remain near the release site if aphids or other prey are available. Newly emerging adults, however, will disperse in search of food, often over great distances, before laying eggs. Predator of predators!?! C. carnea predation on other predators can disrupt aphid control in some crops. That is the negative side of a predator that eats such a wide range of insects. How to protect and help lacewings Because young larvae are susceptible to drought, they may need a source of moisture. Adult lacewings need nectar or honeydew as food before egg laying and they also feed on pollen. Therefore, plantings should include flowering plants (e.g. at borders of the field), and a low level of aphids can be tolerated to attract and conserve lacewings. The green lacewing appears to have some natural tolerance to several chemical insecticides. Still, when lacewings (and other natural enemies) occur in the field, it is advisable to avoid using pesticides. Lacewings can be purchased from many companies in the U.S. and Europe. 118

9 Hover flies - Syrphidae Hover flies get their name because the adults often hover in one spot. They are also called syrphid flies or flower flies. What they eat: aphids, small caterpillars, sometimes thrips, possibly leafhopper nymphs. Which stages attack other insects: Hover flies have a 4-form lifecycle (egg/larva/pupa/adult). Only the larvae are predators. Description and life cycle Adults of the hover fly eat only pollen and nectar from flowers. They look like bees or wasps and are usually seen near flowers. Many species have compact, flattened bodies, large eyes and black and yellow stripes on the body. They vary in size from 9-18 mm. The female lays single, small (about 1 mm), white eggs that lie flat on leaves or shoots near or among aphid populations. Females can lay several hundred eggs. The larvae hatch in 2-3 days. The larvae are small maggots without legs, and they look like tiny slugs. They vary in color from cream to green to brown, depending on the species and the prey consumed. There are 3 larval instars. The larvae suck out the inside liquids of aphids and small caterpillars until only the skin remains! In about 2 weeks, the larva develops into a pupa which usually is pear-shaped and is cream, green or brown in color. The pupa usually is attached to leaves or stems, sometimes in the soil. The period from egg to adult varies from 2 to 6 weeks, depending on the temperature, species and availability of aphids. If there are many aphids for the hoverflies to eat, there can be more generations. Adult hover fly. Larva of hover fly eating an aphid. Source: B.M. Shepard et al Insects and their natural enemies associated with vegetables and soybean in Southeast Asia. Quality Printing Company, Orangeburg, South Carolina, U.S.A. 119

10 Effectiveness Larvae of the hover fly are voracious eaters. One larva may eat up to 400 aphids during its development! On a small scale, larvae can keep aphid populations in check but it is unknown if they manage to control aphids in large fields. How efficient is one hover fly larva? A study example. To check how effective one hoverfly larva is, you can count how many prey it eats in a day. Collect a few hoverfly larvae from the field (use wetted hair brush to handle them). Put one larva per potted plant with a known number of prey e.g. aphids on a leaf. Count how many prey is left after 24 hours. Place the same number of aphids in a jar without the predator to check how quick the aphid population grows when no predator is around. Use hoverfly larvae of different sizes (there are 3 larval stages) and compare the appetites of each stage. Discuss if hoverflies can keep up with aphid population growth. How to protect and help hover flies As for almost all natural enemies, hover flies are easily killed by broad-spectrum insecticides. Avoid use of these pesticides when possible. Adult hover flies need flowering plants to feed on. They are attracted to weedy borders and garden plantings. Flowers have an important function in attracting hoverfly adults. Spiders - Araneae There are many species of spiders and they can be roughly divided into two main groups: (1) hunting spiders that actively hunt in search of prey and (2) spiders that make webs and wait for prey to be caught in the web. What they eat: Hunting spiders mostly eat walking insects such as aphids, mites, caterpillars, and other soft-bodied insects on the leaves. Web-making spiders mostly eat flying insects like adults of moths and leafhoppers, although some tiny spiders make webs on the leaf surface and eat small walking insects like thrips and mites. Both types of spiders may also attack other natural enemies. Which stages attack other insects: Spiders have a 3-form lifecycle (egg/nymph/adult). Both nymphs and adults are predators. Description and life cycle Spiders are not insects but instead belong to the order Araneae that have 8 rather than 6 legs. Like many other predators, the more prey adult spiders can consume, the more eggs will be laid by the female. This allows these predators to increase their numbers when the pest population increases. 120

11 The number of eggs spiders can lay varies from a few to several hundreds, depending on the species. Some spiders carry the eggs in a little sac until the young spiders hatch from the eggs (e.g. wolf spiders Lycosidae). Others guard the location where the eggs are deposited (e.g. lynx spiders) or place the egg mass in the web or on leaves, covered with fluffy silk. Nymphs hatch out of eggs and shed their skins several times as they grow into adults. Spiders may live up to 4 months, depending on the species. Effectiveness Most hunting spiders are very mobile and spend a lot of time searching for insects. Spiders are voracious predators: it depends on the species how many prey it can eat on a day. Some spiders can eat as many as 5 large insects per day! How to protect and help spiders Mulching, especially organic mulch, can increase the number of spiders because spiders can hide in the layer of mulch and they find protection from sun and rain. Spiders are easily killed by broad-spectrum insecticides. Avoid using pesticides as much as possible. Praying mantids - Mantidae Also called praying mantis. What they eat: flies, mosquito bugs, moths and small spiders. Which stages attack other insects: Mantids have a 3-form life cycle (egg/nymph/adult). Both nymphs and adults are predators. Description and life cycle The adults are good flyers and can travel long distances. Adults are light green to brown in color and can be 5-10 cm long. The eggs are laid in a papery mass ( egg case ) attached to a twig. The nymphs look like small adults. Both adult and nymphs have large front legs that they hold in a praying position. adult praying mantid Effectiveness Mantids are active predators of a wide range of insects. They can eat many insects per day, and therefore help reduce pest numbers. Nonetheless, mantids are usually not considered to be effective in controlling pest populations. The reason may be that they eat so many different kinds of insects, or that the number of mantids is often low. How to protect and help mantids As for almost all natural enemies, praying mantids are easily killed by broad-spectrum insecticides. Avoid use of these pesticides when possible. When the egg cases of praying mantids are seen attached to trees or places outside the field, they may be carefully removed and placed in the tea field. Mantids can be purchased from many companies in the U.S. and Europe. 121

12 Predatory Ants There are many different kinds of ants. Some ants are pests that eat seeds and other plant parts. Some ants are pests because they protect aphids and eat their sugary excretions. But some ants are useful predators that eat pest insects. Predatory ants can bite pluckers and other agricultural workers. What they eat: Caterpillars, aphids, mosquito bugs, thrips, and other soft-bodied insects on leaves; termites and other insects that live in the soil. In forest trees, ants are known to attack caterpillars that bore in tree trunks. So, in tea, ants may eat the caterpillars of the red borer. Which stages attack other insects: Ants have a 4-form lifecycle (egg/larva/pupa/adult). Only the adults are predators. Description and life cycle Ants live in colonies or nests. They often dig tunnels under the soil, covered with a small mound of soil. Some types of ants locate their colonies inside the trunks of trees, either dead or alive. Some build nests of leaves in the branches of trees. Adult ants are easy to recognize. Their bodies are hard and dark, with a narrow waist, and usually with no wings. It is possible to confuse termites with ants, but termites have mostly soft, pale bodies that are shaped like a tube (no waist ). Flying adult ants with wings appear at some times of the year, often after the first rains. Flying adults look like small wasps, but have: antennae bent like an elbow one or two lumps or knobs sticking upwards from the waist (between the hind legs and the abdomen). Only ants have these lumps. Wasps do not have them. And few wasps have antennae that are bent like an elbow. 1 lump sticking up 2 lumps sticking up Examples of ants, showing 1 or 2 "lumps" sticking upwards from the narrow waist. Note also the antennae that are bent like elbows. Source: Borror, Triplehorn, and Johnson An introduction to the study of insects. Harcourt Brace Johanovich, Fort Worth, Texas, U.S.A. 122

13 All the immature stages are found only inside the colony. The queen ant (much larger than the other ants) lays small white eggs inside the colony or nest. These hatch into white larvae that have no legs. Adult ants inside the colony take care of the larvae and feed them until they form pupae. New adults emerge from the pupae and help continue the work of finding food and taking care of the colony. If you damage a colony, you can sometimes see adults carrying away the white larvae or pupae. Effectiveness The effectiveness depends on the type of ant. Many predacious ants are voracious predators, carefully patrolling the entire plant and eating all pest insects. But remember: some types of ants are actually pests, because they protect aphids and scale insects (they use these insects like cattle, drinking their sugary secretions). When you see ants in your tea, observe them carefully to see what they are eating. How to protect and help predatory ants Avoid using insecticides that kill a wide range of insects. Ants that make colonies in the soil are affected by the way you manage the soil. Observe your field (and neighbors fields) to see if useful predatory ants are more abundant, or less abundant, in places with: mulch cover crops weeds Ants that nest in the leaves can search for pests more easily if they have bridges between bushes. If the tea bushes are big enough to touch each other, then ants can easily move from one bush to another. If bushes are too small to touch, try tying some thin strips of bamboo or thin branches to connect one bush (where you find ants) to another bush (where pests need to be controlled). Chinese farmers used these bamboo bridges between fruit trees centuries ago, which is the earliest written record of farmers intentionally manipulating natural enemies. Predatory mites What they eat: Mostly mite pests (spider mites, purple mites, yellow mites, etc.). They eat eggs, nymphs, and adults of pest mites. Some predatory mites eat thrips. Predatory mites will also eat other predatory mites. Which stages attack other insects: Mites have a 3-form life cycle (egg/nymph/adult). Both nymphs and adults are predators. Description and life cycle Adult predatory mites look something like spider mites. They are very small and have 8 legs. But, their legs are longer than the legs of spider mites, and they run faster. They are white or almost transparent, but become red if they eat red spider mites. Like most predators, they are less abundant than their food; you will find about 1 predatory mite for every pest mites. Adults lay small round white eggs on the leaves, near pest mites or other food insects. 123

14 Nymphs hatch from eggs and shed their skins several times as they grow into adults. It takes days for eggs to develop into mature adults. There are several generations per year. Predatory mite (Phytoseilus sp.), near a leaf vein and some spider-mite eggs. Predatory mites (Typhlodromus sp.) eating red spider mites. Effectiveness This is one of the most effective predators of mite pests. But, their effectiveness is reduced by the fact that they usually do not become common until after pest mites have become abundant (about a 2-4 week delay). So, they help reduce damage from mite pests, but do not prevent it from starting. How to protect and help predatory mites Avoid pesticides (including fungicides) that kill a wide range of insects. The following pesticides are known to be very toxic to predatory mites: carbaryl pyrethroids (including permethrin, cypermethrin, fenvalerate, and others) mancozeb and maneb (two fungicides) dinocap Farmers can help predatory mites by planting shade trees and (perhaps) by spraying water with backpack sprayers. Thesis research (Huong 1999) showed that predatory mites (and other natural enemies of pest mites) were more abundant in fields with shade trees and where water was sprayed onto the plants by sprinklers. Predatory mites can be purchased from many companies in the U.S. and Europe. 124

15 Rove beetles Rove beetles are also called staphylinid beetles. This is a very large and diverse group of beetles (many different sizes and shapes). What they eat: In tea, the most important rove beetle lives on the leaves and eats eggs and active stages of mites, thrips, and other tiny insects on leaves. Other, larger rove beetles live in the soil and eat soil insects. Which stages attack other insects: Rove beetles have a 4-form lifecycle (egg/larva/pupa/ adult). Both larvae and adults are predators. Description and life cycle The species that eats mites and thrips is tiny (only about twice as big as a spider mite) and black. Like most rove beetles, the adult has very short wing covers that do not cover the abdomen. The wings are folded underneath these wing covers. Adults seem to be most active on the leaves from about 10 a.m. 1 p.m., and are also active at night when the weather is dry. Adult females probably lay eggs on the leaves near mites or thrips. Larvae hatch out of eggs and eat mites, shedding their skins several times as they grow. Mature larvae form pupae probably on the leaf but perhaps in the soil. View from above View from below View from above Wings are folded underneath these short wing covers Tiny black rove beetle (about 1 mm long). Source: J.S. Ashe, Aleocharinae Image Database, website address Drawing by Sara Taliaferro. Made available thanks to NSF-Peet Grant DEB to J.S. Ashe. Effectiveness These are voracious predators of spider mites and thrips, capable of eating many mites or eggs per day. Like predatory mites, their effectiveness is reduced by the fact that they usually do not become common until after pests have become abundant (about a 2-4 week delay). So, they help reduce damage from mites and thrips, but do not prevent it from starting. 125

16 How to protect and help rove beetles Avoid using insecticides that kill a wide range of insects. Farmers can help the tiny black rove beetles by planting shade trees and (perhaps) by spraying water with backpack sprayers. Research in Thai Nguyen province (Huong 1999) showed that natural enemies of pest mites were more abundant in fields with shade trees and where water was sprayed onto the plants by sprinklers. Farmers can help the larger rove beetles that live in soil by using mulch, which provides shelter to soil predators. The tiny black rove beetles (Oligota spp.) can be purchased from several companies in the U.S. and Europe Parasitoids Parasitoids are insects (usually tiny wasps or flies) that lay their eggs inside, or on the skin of, pest insects. The parasitoid larvae develop inside or on a single pest insect, eating all or most of its body and eventually killing it. This is why parasitoids are important as natural enemies of insect pests. Adult parasitoids fly freely, feeding on nectar from flowers and searching for pest insects. Parasitoids are usually smaller than their host and they usually attack only one, or a very few, different types of pests. Also, they usually attack only one stage of the pest insect: either eggs, larvae, or pupae. Parasitoids are often named according to their stage preference, for example egg parasitoids attack only eggs of a particular insect. Only female parasitoids search for hosts, because only they lay eggs (in or near the host). SEX: male or female...? In wasps, the sex of a parasitoid off-spring is determined differently than for other animals. In parasitic wasps, females come from fertilized eggs and males come from unfertilized eggs. So if a female does not mate with a male wasp, she will produce only males. If she does mate, she will produce a mix of both males and females, usually more females. And that is important because only females are able to parasitize other insects! Males are only useful for mating...! Whereas insect predators immediately kill or disable their prey, pests attacked by parasitoids die more slowly. Some hosts are paralyzed, while others may continue to feed or even lay eggs before they die from the parasitoid attack. Parasitoids, however, often complete their life cycle much more quickly and increase their numbers much faster than many predators. Parasitoids are dependent on the pest population. Unlike predators, which often feed on a wide range of different insects, parasitoids cannot increase their own population without their host insects. It is therefore always good to have at least a few pest insects in the field. They serve as food for the natural enemies! 126

17 Parasitoids can be the dominant and most effective natural enemies of some pest insects, but because they are so small, their presence may not be obvious. This is why it is so important to monitor fields. During Agro-Ecosystem Analysis, be sure that farmers learn to recognize the presence of parasitoids (see the boxes below). Otherwise, the friends of the farmer will never be noticed, and in fact, might be treated with pesticides instead of with gratitude! The life cycle and reproductive habits of beneficial parasitoids can be complex. For example, some parasitoids lay their eggs inside the eggs of pests, but the parasitoid larvae do not emerge until te pest has already turned into a caterpillar or pupa. In some species, only one parasitoid will develop in or on each pest, while in others, hundreds of young larvae may develop within a single host. Farmers should consider the contribution of parasitoids when they are deciding whether or not a pest population needs to be sprayed. If all the pests are parasitized, they are going to die anyway; there is probably no need to spray. To determine what percent of the pests are being killed by parasitoids, it is often necessary to rear samples of pest insects to see if any adult parasitoids emerge (see boxes above). Be sure to keep specimens for at least one month and even after it looks like everything has already emerged. A parasitoid parasitized?? Yes, unfortunately, it is possible. A parasitoid of insect pests can itself be parasitized by other parasitoids: this is called hyperparasitism. Hyperparasitoids are even smaller than parasitoids. Hyperparasitism can be common, and may reduce the effectiveness of some beneficial species, especially in case of introduced natural enemies (those natural enemies that are brought into a field from outside). Little can be done to manage hyperparasitism. Related exercises from CABI Bioscience/FAO manual 1.6 Show effects of beneficials incl. natural enemies 127

18 Parasitoids that attack aphids These are tiny wasps in the families Aphelinidae, Encyrtidae, and Braconidae (subfamily Aphidiinae). Which pests they attack: Aphids (they lay their eggs inside aphid nymphs). Which stages attack other insects: These wasps have a 4-stage life cycle (egg/larva/pupa/ adult). The adult females lay eggs inside aphids, then the larvae eat and kill the aphid. Description and life cycle Adult wasps are very small, only from 1-3 mm long. Their bodies (at least the head and thorax) are dark and hard, and they have long antennae. Adult wasps are usually active on bright, clear days. They may not fly on very hot, windy or rainy days. Females lay up to several hundred eggs. They prefer to lay eggs in aphid nymphs rather than adults. One wasp larva hatches inside each parasitized aphid. As the wasp larvae grow, the parasitized aphids begin to look like mummies : swollen, dead aphids that are glued to the leaf surface. Aphid mummies are brownish in color and have a hardened, papery skin. The larva inside the aphid may spin a cocoon under the dead aphid or form a pupa there, or it may spin a cocoon and form a pupa inside the mummified aphid. The adult wasp emerges after cutting a circular hole in its cocoon. Adults live from 1 to 3 weeks and there can be many generations per year. Life cycle of Diaeretiella rapae 1: Female lays egg in aphid 2: larva develops inside aphid 3: pupa in swollen aphid mummy 4: adult wasp emerges from mummy From: Hoffmann et al, 1993; and Weeden et al. website 128

19 Effectiveness Parasitoids can have a big impact on aphid infestations. Females of some species may parasitize hundreds of aphids in one day. If young aphids are parasitized, they will usually die before reproducing. Aphids parasitized as older nymphs or as adults may reproduce before dying but produce fewer young aphids than normal. Recognizing parasitoid activities in aphids If there are many aphids present on a tea bush (or even on weeds), check to see if any of the aphids have been transformed into mummies (swollen, dead aphids that have turned brownish and hardened). If possible, use a hand lens to search for mummies. When mummies are found, place the leaf with aphids in a jar (insect zoo), together with a piece of tissue to avoid condensation. Close the jar with a fine netting. Monitor changes in aphids and mummies over the next few days. Other aphids that have been parasitized, will transform into mummies in the next few days. Adult parasitoids (possibly Diaeretiella, described later in this chapter) will emerge from the mummy. After emergence, you can see the exit hole towards the end of the aphid s abdomen. Monitoring just the number of aphid mummies will not give the right estimation of the degree of parasitism by wasps. It does not account for the parasitized aphids that have not yet become mummies. Also, parasitized aphids may move away from the feeding site or fall off the plant. But: it does tell you if the parasitoids are present and active, and it gives an idea of the amount of parasitism. How to protect and help parasitoids that attack aphids 1. Most parasitoids are even more susceptible to pesticides than predators. Adult parasitoids are usually also more susceptible to pesticides than their hosts. Immature parasitoids may sometimes be protected when they are inside the host or in their egg or cocoon, but when the host is killed, the immature parasitoid will also be killed. 2. Many adult wasps feed on nectar from flowers. When there is a lot of this food available for the adult wasps, they live longer and lay more eggs, resulting in a higher percentage of parasitism. Adult wasps are attracted to flowering plants near the field, such as mustard plants. They also benefit from a source of water nearby. 3. To help spread the parasitoids in the field, farmers can redistribute leaves containing aphid mummies. Check if the parasitoids are still inside the mummies: when you see a circular hole in the mummy or in the cocoon of the parasitoid, the adult has emerged and it is useless to redistribute that leaf. 129

20 Parasitoids that attack caterpillars Which pests they attack: In tea, all of the caterpillars are attacked by parasitoids. This includes the red borer caterpillar, which might seem to be safe inside its tunnel in the wood! Most parasitoids are small wasps in the families Ichneumonidae, Braconidae, Eulophidae, and Chalcididae. A few parasites are hairy flies in the family Tachinidae. Which stages attack other insects: Like other parasitoids, these have a 4-form life cycle (egg/larva/pupa/adult). The adult females lay their eggs in the pest caterpillars, and the parasitoid larvae eat and kill the pest caterpillars. Description and life cycle Adults are either small slender wasps (10-25 mm long, with 4 wings) or flies (only 2 wings) that look more or less like houseflies but often have many stiff hairs. Female adults lay small white eggs either by injecting them into the pest caterpillar (wasps) or by gluing them onto the skin of the caterpillars (flies). The parasitoid larvae burrow inside the body of the pest caterpillar to feed. Parasitized caterpillars may seem sick and not move much, but some parasitized caterpillars continue to feed and behave almost normally. After feeding inside for 1-3 weeks, the parasitoid larvae crawl out of the caterpillar and spin small silk cocoons. The caterpillar usually dies when they crawl out. The parasitoid cocoons are often touching the dead caterpillar. Inside each cocoon the parasitoid forms its pupa. When it is mature, the adult parasitoid emerges from the cocoon to search for more pests to attack. Pest caterpillars with larvae or cocoons of parasitoids. Before coming out, the parasitoid larvae fed inside the caterpillar. Sources: drawing from Borror, Triplehorn, and Johnson An introduction to the study of insects. Harcourt Brace Johanovich, Fort Worth, Texas, U.S.A. Photograph from Shepard, B.M. et al Insects and their natural enemies associated with vegetables and soybean in southeast Asia. Quality Printing Company, Orangeburg, South Carolina, U.S.A. Effectiveness These parasitoids can be very important in controlling caterpillar populations. How to protect and help parasitoids that attack caterpillars Avoid using broad-spectrum insecticides for any insect pests. 130

21 Recognizing parasitoid activities in caterpillars Parasitoid wasps are difficult to spot because they are very small and they are fast flyers. However, they leave evidence of their activities. For example, parasitoids that attack caterpillars leave two types of evidence: 1. Caterpillars that act sick or sluggish (they contain parasitoid larvae that are feeding on them). 2. Some common parasitoids usually make a white or brown silk cocoon next to a dead pest caterpillar. The dead caterpillar has a hole in it, indicating where the parasitoid larva emerged before it spun its cocoon. Often, these cocoons are wrongly thought to be eggs of insect pests. When parasitoid cocoons or sick caterpillars are found, it is useful to have farmers collect many pest caterpillars of similar age. Place the cocoons inside clear plastic bottles and observe what emerges from the cocoon during the next 1-3 weeks. Draw the cocoons, the dead caterpillar and the type of adults that emerge, using color pencils. Having farmers witness the process of parasitoids emerging will help farmers understand the importance of the parasitoid cocoons (Ooi, 1999). Another easy-to-observe example is parasitoids that attack aphids (see the box on page 129). Parasitoids that attack eggs Parasitoids that attack eggs are so tiny that they can complete their development (from egg to adult) inside the pest egg! These parasitoids are tiny wasps in the families Eulophidae, Scelionidae, Trichogrammatidae, and Mymaridae. Which pests they attack: The eggs of thrips, leafhoppers, and perhaps caterpillars. Which stages attack other insects: Like other parasitoids, these wasps have a 4-form lifecycle (egg/larva/pupa/adult). The female adults lay their eggs inside the eggs of pest insects. The parasitoid larvae then grow inside the pest egg, eating it and killing it. Description and life cycle These parasitoids are so small that you will probably never see them, but they are included here because of their importance. Adults are tiny wasps (less than 1 mm long) with 4 wings. The edges of the wings are often surrounded by a fringe of long hairs. Adult females lay their own eggs inside the eggs of pests such as thrips and leafhoppers. The parasitoid larvae and pupae develop inside the shell of the pest egg, eventually killing it. When mature, the adult parasitoid breaks out of the now-dead pest egg and flies off to search for more pest eggs. 131

22 Effectiveness These are one of the most important natural enemies of thrips and leafhoppers. How to protect and help parasitoids that attack eggs Avoid using broad-spectrum insecticides for any insect pests Insect Pathogens Pathogens are bacteria, viruses, fungi, and nematodes. Insects, like humans and plants, can be infected with pathogens that cause diseases. Insect pathogens generally kill, reduce reproduction, slow the growth, or shorten the life of a pest insect. Unlike chemical insecticides, microbial insecticides usually take longer to kill or weaken the target pest. But under certain conditions, such as high humidity or high pest populations, these pathogens can cause disease outbreaks that decimate an insect population. Most insect pathogens are not harmful to beneficial insects, and none are toxic to humans. Pathogens are most effective when pest populations are very high. Pathogens are difficult to manage because their presence and effectiveness strongly depends on factors like moderate temperature and high humidity. During the dry season, for example, you will almost never see aphids killed by a fungus because that fungus needs a high humidity for survival and spread. Most pathogens are too small to be seen by human eyes. Only the symptoms that insectpathogens cause can be seen with the eyes: for example a dead insect covered with fungus spores like hairs or dust or a dead insect which is black and spills fluid out of the body. Some pathogens have been mass produced and are available commercially for use in standard spray equipment. These products are often called microbial insecticides, microbials, or biopesticides. The best known microbial insecticide is probably the bacterium Bacillus thuringiensis or Bt, which is available in many different formulations. In addition, several viruses (such as nuclear polyhedrosis virus or NPV) are increasingly being used in Asia because farmers can produce them on-farm. Below, some common pathogens of tea insect pests are described. 132

23 Bacillus thuringiensis (Bt). Bacillus thuringiensis (Bt) is a bacteria that occurs naturally in the soil. Nonetheless, to control insects, Bt usually is purchased as a microbial insecticide and applied via a backpack sprayer. Bt is sold under various trade names, including Dipel, Javelin, Biobit, MVP, Xentari, and Agree. In addition, Bt products are now produced in many Asian countries, for example, in Vietnam and Thailand. Which pests does it infect: Caterpillars. Advantages of Bt: Effective against many caterpillar pests Safe for natural enemies and non-target insects Can be used until close to the day of harvest. Can be applied using conventional spray equipment. Good spray coverage is essential because the bacteria must be eaten by the insect to be effective. Disadvantages of Bt: Only effective against caterpillar pests. Takes several days to kill caterpillars (although they stop eating after only about 6 hours) Mode of action and symptoms The toxin inside the bacterium is only effective when eaten. The Bt is sprayed over the leaves and when the insect eats the leaves, it will also eat the Bt bacteria. The toxin inside the bacteria damages and paralyzes the gut of the insect. The toxin can only affect insects that have a specific gut structure, that s why Bt only kills caterpillars. Within about 6 hours after eating Bt, affected caterpillars become inactive and stop feeding. Within a few days more, the caterpillars die from the combination of starvation and damage of the gut by the toxins of the bacterium. Before dying, caterpillars turn soft, may have a watery excrement, and the head capsule may appear to be overly large for the body size. After death, the body turns brownish-black as it decomposes. Using Bt = patience?!? It was noted that some farmers concluded after spraying Bt that it didn t work because the caterpillars were still alive. Some farmers even sprayed a chemical insecticide only one day after applying Bt. However, only when looking more closely, they found that the caterpillars were actually hardly eating anymore, they were just sitting on leaves, not moving very much. This is most important: when they stop eating, they stop damaging the crop! Bt is a stomach poison and the toxin paralyses the stomach. Death by starving takes some time and, caterpillars will be dead after three days. Also see box below about testing Bt. (pers. comm. Dr. P.Ooi, 1999) 133

24 Effectiveness Bt can be an extremely effective bio-insecticide. But, to obtain effective control of the caterpillars, it is essential to apply Bt: At the susceptible stage of development. Young caterpillars are usually most susceptible. This is because caterpillars must eat sufficient quantities of Bt to be killed. When they eat just a little Bt, they may not die but their growth is retarded. Further, older caterpillars may be protected from Bt if they bore into the plants where Bt will not reach them. In the right concentration (follow the label instructions). At the correct temperature. Bt only works at temperatures above 15 o C. So, don t apply during very cold weather. In the evening if possible. Bt formulations are deactivated by sunlight. This is a reason that Bt is only effective for one to three days. Apply in the evening to make the Bt last as long as possible. Be sure to spray the undersides of leaves, where Bt will be protected from sunlight. Not right before a rainstorm. Rain or overhead irrigation can also reduce effectiveness by washing Bt from the leaves. So, don t apply if you think it will rain within 12 hours. How to protect and help Bt Bt formulations are applied like an insecticide: for short-term control. In fact, the bacterium contained in commercial formulations is dead. Therefore, conservation methods, which are important for predators and parasitoids, are not relevant for Bt. Sunshine and Bt, not a good match...!? The Philippines highland vegetable FFS programme included a specific experiment for farmers to observe the effect of sunlight on the efficacy of Bt products. They compared feeding and death rates when DBM larvae were placed on cabbage leaves which had been sprayed with Bt at different times of day and hence received different sunlight exposures. The results showed that sunlight deactivates Bt. By discussing the results, the farmers were able to decide the best time of day to apply biopesticides, avoiding application when the sun is strongest (CABI, 1996). 134

25 Testing Bt with farmers: a case from Dalat, Vietnam Testing Bt is not like testing chemical insecticides. Chemical insecticides kill caterpillars quickly. So, when you are testing chemical insecticides, you only need to count the number of caterpillars that are left alive. The insecticide that has the fewest living caterpillars is the most effective. But, Bt works differently from insecticides. Caterpillars affected by Bt may live for 2-3 days after the Bt was sprayed. So, if you only count how many caterpillars are alive 2 days after spraying, you will be disappointed. Most caterpillars will still be alive, and it will seem that the Bt did nothing. But this conclusion will be wrong, because in fact the caterpillars are so sick that they are not damaging the crop! Therefore, when testing Bt, it is not enough to count the number of caterpillars that are alive. You must also measure the effect that Bt has on the caterpillars' feeding and health. Caterpillars affected by Bt usually stop feeding after 6 hours. Farmers can prove that the caterpillars have stopped eating by measuring that there is less damage to the leaf and less frass production. At 24 hours after exposure to Bt, larvae are very sick. Farmers can prove that the caterpillars are sick by measuring that the caterpillars do not move much, even if touched. Larvae die after about 3 days. It is important to help farmers learn to measure whether Bt is working, by measuring if there is (A) less damage on the leaf, (B) less frass production, and (C) less caterpillar activity. To help farmers measure these three variables, it is often useful to use a scoring system. The following scoring system was found to be very useful by vegetable farmers in Dalat, Vietnam: A. Leaf damage 1 = low 2 = moderate 3 = high B. Frass production 1 = none 2 = little 3 = much C. State of 1 = dead 2 = very sick and 3 = active caterpillars dying Farmers used this system to give a score to each of the experimental plots (in other words, a score for each of the different types or concentrations of Bt that were used). Before beginning the job of assigning scores, the farmers discussed among themselves to agree on how to choose which score was most suitable (for example, how much frass was "little" and how much was "much"). Farmers in Dalat were very excited about this scoring system because it provided a better opportunity to study how Bt works. They learned that in Bt studies, just counting the number of caterpillars still alive is not enough and may even lead to a false conclusion that Bt is not effective. (Ooi, 1999 (appendix 2)) 135

26 Fungi There are fungus species that can infect and kill insects. These fungi are called insectpathogenic fungi or entomopathogenic fungi. These fungi are very specific to insects, often to particular insect species, and do not infect animals or plants. Most insect-pathogenic fungi need humid conditions for infection and development but some fungus species can also infect insects when it is dry. There are other beneficial fungi that attack the pathogens that cause plant diseases. These beneficial fungi are called antagonists. An example of an effective antagonist is Trichoderma (Gliocladium). But, this section describes insect-pathogenic fungi only. Antagonists are described in Chapter Ten. There are several insect-pathogenic fungi naturally present in tea fields. These may control some insect species under favorable conditions (high humidity and moderate temperatures). For example, they help control sucking insects like aphids that are not susceptible to bacteria (e.g. Bt) or to viruses. Farmers can help to protect and help these naturally-occurring fungiby being extra careful when considering pesticide applications, especially fungicides. Fungicides applied to control plant diseases can quickly kill the beneficial insect-pathogenic fungi! For example, this may be why copper-based fungicides (applied to control blister blight) can increase problems with mites; the fungicides may also be killing fungi that attack the mites. Other insect-pathogenic fungi are commercially available in some countries in formulations that can be applied using backpack sprayers. Some experiences from Asia are listed below. Some common insect-pathogenic fungi are described on the following pages. But remember: It is not important to know all the scientific names of insect-pathogenic fungi! What is important is that you can recognize a fungus in the field, and realize that it is killing insects and helping the farmer! Free help from Mother Nature! When an insect has been killed by a fungus, the fungus emerges from the insect s body to produce spores that can sometimes be seen as a dusty or cottony appearance. When you find many insects dead and covered with fungus, you can consider making your own bio-insecticide from these dead insects. Collect as many as you can find in the field, put them in a jar with water, crush them a little and stir well. This will release fungus spores into the water. Filter the water slightly to remove large insect parts. The remaining solution can be used to test its effectiveness in insect zoos. Spray the solution over insects that are placed in a jar. Check if these insects become infected over the next days. Spray another jar of insects with pure water as a control. 136