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Generalist natural enemies

This section discusses many of the common general predators, parasites, and pathogens that may contribute to the biological control of insect pests on crucifers These generalists attack a wide variety of insects or other arthropods instead of just one or a few closely related species. General predators are very important in small, diverse plantings and are often present in gardens even before pests arrive. hi larger plantings natural populations provide some control, but augmentation with commercially produced natural enemies may be necessary for control of specific pests.

Conservation of natural enemies requires avoiding the use of broad-spectrum pesticides. Providing favorable habitats that supply nectar, pollen, alternate hosts, and Overwintering sites near the planting may further encourage natural evenly activity Sweet alyssum (Lobularia maritime [L.] Desv.), coriander or cilantro (Coriandrum sativum L.), and goldenrod (Solidago spp.) are particularly attractive to adult syrphid flies and small parasitic wasps. Sprayable powders such as Pred-Feed or Good Bug Powder Meal contain food supplements to attract and increase predators; these can be applied directly to crops. Unfortunately, not all conservation methods are compatible with conventional agronomic practices. Large plantings are not conducive to interplanting, for instance, and providing weedy spots to harbor alternate prey may actually aggravate certain pest problems. Conservation methods must therefore reflect specific field conditions and production practices.

Parasites

Trichogramma spp. (family Trichogrammatidae). There are numerous species of Trichogramma wasps that attack the eggs of over 200 species of moths and butterflies. These almost microscopic wasps (0,5 mill; 1/64 inch) are very important in preventing crop damage because they kill their hosts before the insects can cause plant damage. The female Trichogramma lays an egg within a recently laid host egg , and as the wasp larva develops, the host egg turns black. Each female parasitizes about 100 eggs and may also destroy additional eggs by host feeding. The short life cycle of 8-10 days allows the wasp population to increase rapidly. These wasps are harmless to people, animals, and plants.

Tricogramma are readily available in large quantities from commercial suppliers. There are several species and strains of Trichogramma, which vary considerably in their ability to control different insects and in their adaptation to different environmental conditions and crops. Determining the best species or strain to release may be difficult. Most suppliers provide detailed instructions for the strain selection, release, and use of Trichogramma, but their recommendations may not always be accurate. The most suitable species commercially available for release in field and vegetable crops is probably T. pretiosum, but again, results may vary considerably.

The wasps are shipped as immatures inside moth eggs glued to small cards that can be attached by hand to infested plants. Renal application is possible for large acreages. Keep the cards in a warm, humid place out of direct sunlight until the emerging adults can be seen as small dots moving around in the closed container. A few tiny caterpillars may also be found in the container because it is very difficult to obtain 100% parasitization of the moth eggs, but these are harmless in cole crops. When most of the adults have emerged, place the containers in a shaded spot upwind of the areas where moths are suspected or egg laying is occurring. The adult wasps will fly onto the plants in search of new host eggs to attack. Do not put the cards out before the wasps have emerged because ants and other predators may eat them. The emerging wasps will have the best chance of finding and parasitizing eggs when the weather is moderate. The best time to release is early morning or evening when direct sunlight will not hit the cards. Avoid making releases under extremely hot, cold, rainy, or windy conditions.

Whenever possible, releases should begin at the time of the first moth flight, before the pest population has built up. Pheromone traps for diamondback moth and cabbage looper, black light traps for cabbage looper, or visual inspection for imported cabbageworm butterflies are useful for monitoring adult flight. Regular scouting to determine the appearance of caterpillar eggs is a more accurate method to determine when hosts for Trichogramma are available.

Frequent releases made over several weeks result in better parasitism and control than a single large release. Releases of 5,000 Trichogramma made at weekly intervals over 3-6 weeks will usually be sufficient for the home garden of up to 2,000 square feet (186 square meters). For each additional 2,000 square feet, release 5,000 more Trichogramma. Up to several hundred thousand wasps should be released per acre

Nematodes. There are several insect-killing nematodes that occur naturally in the soil and are parasitic on a wide variety of soil-inhabiting insects, including many major crop pests. Steinernema (=Neoaplectana carpocapsae (Weiser) is one common species with considerable potential as a biological control agent because of its wide host range, rapid action, and environmental safety. Many species of caterpillars, root maggots, beetles, cutworms, weevils, cockroaches, ants, and borers are susceptible to this nematode. This nematode does not affect aphids, whiteflies, scales, or slugs. Some other important nematode species that affect insects include S.feltiae (=Neoaplectana bibionis), S. (=N.) glaseri, and Heterorhabditis bacteriophora (=heliothidis). These nematodes are harmless to people, animals, beneficial aboveground insects, and earthworms. (The scientific names of two nematodes were revised in 1992: what used to be called S. feltiae is now called S. carpocapsoe and what is now called S. feltiae used to be called N. bibionis. Be sure to clarify with suppliers which nematode is appropriate for your situation.)

Nematodes are minute animals that look like long, slender worms under the microscope . They are about 0,5 mm (1/64 inch) long, transparent, and practically invisible to the naked eye. They seek out and reach insects underground or in protected plant parts where moisture is present. Moisture is essential for the movement and persistence of nematodes. The nematodes enter an insect and release a bacterium that kills the host within 24--48 hours The bacteria serve as food for the nematodes, which complete their development in 1-3 weeks inside the dead insect. The next generation of nematodes leaves the insect in search of new hosts. Unlike plant-parasitic nematodes, insect-killing nematodes do no damage to plants.

Although these nematodes occur naturally in soil, rarely are enough of them present to provide adequate control of most insect pests. They can be applied to crops in large quantities as a biological insecticide. BioSafeTM (from Biosys in Palo Alto, CA) is a formulation of S. carpocapsae that is available to the homeowner for control of several common lawn and garden pests, including root maggots. Other suppliers also offer packages of this nematode. The suggested release rate for home gardens is 10 million nematodes to treat 200-300 square feet or an application of a 3-inch band on each side of up to 450 linear feet of plant row. For large field applications, the recommended rate is 1-3 billion nematodes per acre. Some suppliers offer the other species of insect-parasitic nematodes mentioned above.

To apply the nematodes, mix the package contents with water as directed on the package and spray or sprinkle the solution on the soil along garden rows or around individual plants. Nematodes can withstand pressures of up to 300 psi and therefore can be applied with the same equipment used for the application of chemical pesticides, such as small pressurized sprayers, mist blowers, and helicopters. Nematodes should be applied to moist, but not saturated, soil. This may require watering the area to be treated before application. Application in the early evening or morning is recommended to avoid exposing the nematodes to extreme heat and sunlight. Nematodes are most effective at 18-32C (65-85F), so they will not be very effective if applied in early spring before soil temperatures are above 18C' (65F).

It may take 10-30 days for populations of pests such as white grubs, mole crickets, billbugs, and root weevils to decline, but for other insects found closer to the soil surface or on above-ground plant parts, control can be achieved in 2-5 days after treatment. Applications on clay soils may be less effective than on sand or loam soils because the nematodes have difficulty migrating through clay soils. More than one application of nematodes may be necessary if soil conditions are not suitable for nematode persistence, especially against insects that feed over a long period (e.g., root weevils) or have more than one generation per year (e.g., root maggots).

Predators

Ground beetles (family Carabidae). Ground beetles are important predators; they may be the most numerous predatory insects in certain agricultural systems. Hundreds of species of e carabids occur in the upper Midwest, most of which are dark, shiny, and somewhat flattened, with slender legs for running. Some are an iridescent blue or green. They are commonly found under leaves or debris, in cracks in the soil, or running along the ground. Adult ground s beetles run quickly when disturbed, but they seldom climb and rarely fly. Many are nocturnal and some are attracted to lights at night. hi general, arable land contains more carabids than does land planted to permanent crops such as clover or alfalfa. Most species are more common on clay than on sandy soils.

The adults are fierce predators that chew up their prey with their large, sharp mouthparts. Caterpillars, grubs and adults of other beetles, and fly maggots and pupae are common prey for many ground beetles. They can consume their body weight in food daily. Eggs are deposited either on objects above ground or in cavities made in the soil. The three larval instars live in debris or in burrows in the soil. The larvae are also predaceous, although in some species the adult and larval foods are very different. The majority of species overwinter as adults, and adults of some species may live for two or more years. Calosoma sycophants, a large, bright green beetle, was imported from Europe to New England for the biological control of the gypsy moth in 1905. The larva feeds day and night, consuming 50 caterpillars during its two-week developmental period. The adult will eat several hundred caterpillars during a life span of two to four years. There are also several native species of Calosoma.

Lady beetles (family Coccinellidae). The lady beetles or ladybird beetles are a large group of important natural enemies. They are frequently called ladybugs, but these insects are not true bugs; therefore the other common names are preferred. Most are predaceous as both larvae and adults, but a few are fungus or pollen feeders and a few, such as Mexican bean beetle, feed on plants. Most of the common predatory species feed primarily on aphids, scale insects, mealybugs, and whiteflies, although they occasionally eat other types of prey, such as spider mites and insect eggs. Adult lady beetles also feed on nectar and pollen.

Lady beetles overwinter as adults. Some species, such as our native convergent lady beetle, Hippodamia convergens Guerin-Meneville, congregate in enormous Overwintering clusters. Other species overwinter singly or in small clusters. In spring they seek out the aphids or other prey that will be both adult and larval food. The eggs are laid adjacent to the prey on the leaf surface, often on end. The eggs of the aphid feeding species are usually yellow to orange in color and 1 1.5 mm (1/32-1/16 inch) long .

Some species scatter eggs individually, while other species lay eggs in compact clusters of 10-20 or more. Eggs usually hatch in 3-7 days.

The larvae of the aphid-feeding species are somewhat slender, with the body tapering to a point at the rear . Depending on species and instar, they will be 3-15 mm (1/8-5/8 inch) in length. They are usually black or dark gray, but they may have conspicuous red, yellow, orange, or blue markings. The prominent legs are held out to the sides. Larval lady beetles normally consume 500-1000 aphids or similar prey during their growth. If prey is abundant and temperatures warm, most lady beetle larvae complete development 2-4 weeks after egg hatch and pupate where they were feeding. The pupal period lasts about one week. The entire life cycle takes about 4-6 weeks, with two to three generations per year.

There are many species of beneficial lady beetles in the North Central United States. The convergent lady beetle is one of the most common throughout the United States, and it is a very important predator of aphids and other pests. Coleomegilla maculata (DeGeer) is commonly found on crucifers in the upper Midwest (figure 28). Other important aphid predators include the twospotted lady beetle, Adalia bipunctata (Linnaeus); the sevenspotted lady beetle, Coccinella septempunctata Linnaeus; and the transverse lady beetle, C. transversogutatta richardsoni Brown.Lady beetles have been available by mailorder purchase from commercial vendors for many years. These usually are the convergent lady beetle, which were mass collected from Overwintering congregations in mountainous areas of the West. Unfortunately for the purchaser, the normal behavior of such lady beetles is to disperse after Overwintering, and to seek areas where there is abundant prey, especially aphids. Beetles usually disperse rapidly upon release and decline in number within a few days. Although this adds to the overall number of beetles in the vicinity, the immediate benefit near the release site is negligible.

Syrphid or hover flies (family Syrphidae). The syrphid or hover flies are common and important natural Enemies of aphid and other small, slow-moving insects. The adult flies are often seen on or hovering near flowers. They are small to medium in size (8 - 20 mm; 5/16-3/4 inch), often with a striped yellow-and black body resembling bees or wasps . The larvae of many common species prey on aphids, scales, and other insects. These pale green to yellow maggots have a slug-like appearance , and the larger species attain a size of up to 10-15 mm (3/8-5/8 inch) in length. The larvae consume as many as 400 aphids each during their development. Some species pupate on the foliage near the feeding site, whereas others leave the plant and enter the soil to pupate. The pupa is enclosed within a puparium, which is the hardened skin of the last larval instar. The puparia are often tear-drop shaped, smooth, and tan colored. The life cycle of species studied takes 2-4 weeks to complete.

Lacewings. There are two types of lacewings, the more common green lacewings (family Chrysopidae) and the brown lacewings (family Hemerobiidae). The larvae of both types are general predators that feed on a wide variety of small insects.

Green lacewings. Several species of Chrysoperla are important predators. The light green adult lacewing has long, slender antennae, golden eyes, and large, veined, gauze-like wings that are 1.3-2 cm (1/2-3/4 inch) long . It is a slow-flying, nocturnal insect that feeds on nectar and pollen, and it emits a foul-smelling fluid from special glands if captured. The female lacewing lays eggs usually in groups on leaves, each egg held away from the leaf surface on the end of a slender stalk . A female lays up to 30() eggs over a period of 3-4 weeks, but often it does not survive that long in the field. Freshly laid eggs are green, but they change to whitish gray as they get close to hatching. The larva, commonly called an aphidlion, resembles a greengray alligator with mouthparts like ice tongs (figure 33). An aphidlion seizes and punctures its prey with the long, sickle-shaped jaws, injects a paralyzing venom, and sucks out the body fluids. After feeding and growing to 1 cm (3/8 inch) in length during a 2-3 week period, the larva spins a spherical, white silken cocoon in which it pupates. The adult emerges in about 5 days through a round hole that it cuts in the top of the cocoon. The insect overwinters as a pupa within its cocoon.

An aphidlion is a voracious feeder and can consume up to 200 aphids or other prey a week. In addition to aphids, it will destroy mites and a wide variety of soft-bodied insects, including insect eggs, thrips, mealybugs, immature whiteflies, and small caterpillars. Aphidlions will also consume each other if no other prey is available. Green lacewings are available from many commercial suppliers. They are usually offered as eggs but may be sent as larvae or adults, too. Eggs are usually sent in a carrier such as rice hulls, wheat bran, corn grits, or vermiculite to cushion and separate the emerging larvae during shipment, along with moth eggs for food so the larvae will be less likely to eat each other. The carrier also makes it easier to distribute the very tiny eggs evenly by sprinkling the contents onto infested plants. Applications on larger acreages may require ground vehicles or aircraft for distribution. Release the lacewings as soon as possible after the majority of the eggs have hatched. The newly hatched larvae will be about the same size as the eggs, so you may have difficulty seeing them. Making releases early in the morning or late in the day when it is cooler, or on a cloudy day, increases the chances that the lacewings will survive. Because aphidlions are cannibalistic, lacewings purchased as larvae must be shipped in individual containers, which increases the cost of the product. Lacewings released as prefed adults ready to lay eggs can fly away immediately, so you must take great care to ensure their establishment in the infested area.

Providing an adequate food supply and suitable adult habitat can contribute to lacewings remaining and reproducing in the crop. The released aphidlions move around a lot and will travel 80-100 feet in search of prey. Once their food source is exhausted they will leave the area. The predatory larvae feed for 2-3 weeks before they become adults. The adults must have a source of nectar, pollen, or honeydew to feed on in the general vicinity of the pest area to stimulate egg laying, or they will leave. Small patches of unmowed grass and flowering plants near the area will provide sources of nutrition as well as a sheltered, humid spot in which the nocturnal adults can hide during the day without dehydrating. A sprayable food supplement, such as Pred-Feed or Good Bug Powder Meal, applied directly to the crop may encourage green lacewings (and other predators) to remain in the crop. Additional releases can provide a continuous supply of larvae if adults do not stay and reproduce.

The number of lacewings needed for effective control depends largely on the pest population and climatic conditions. For control of moderate aphid infestations in home gardens, 5-10 lacewing eggs per plant or 1,000 eggs per 200 square feet are recommended. General release recommendations for most crop situations start at 5,000 per acre for each application, but much higher rates may be necessary. Two or three successive releases made at two week intervals are better than a single release. Suppliers usually make recommendations based on specific situations. These insects are extremely effective under certain conditions, especially in protected or enclosed areas such as a greenhouse, but they may fail to survive and provide control when conditions are not favorable.

Brown lacewings. These insects are garden and woodland inhabitants that resemble green lacewings, but they are smaller and less common. The adults are brown instead of green and are only 6 mm (1/4 inch) in length . The oval eggs are stuck singly or in clusters on leaves and the bark of trees. The larvae consume aphids, mites, thrips, scales, and other small, soft-bodied insects. Unlike the green lacewing adults, brown lacewing adults are also predators, mainly of aphids. They can be quite effective in natural control but are not available commercially.

Stink bugs (family Pentatomidae). The stink bugs constitute a large and well known group easily recognized by their shieldlike appearance. In addition to the common, plantfeeding pests that produce a disagreeable odor, this group contains some predaceous members. The shined soldier bug, Podisus maculiventris (Say), feeds on caterpillars and other insects. It is reddish brown with two short spines sticking out from the middle edges of its body. The twospotted stink bug, Perillus bioculatus (Fabricius), attacks cutworms and other caterpillars, as well as Colorado potato beetles. This bug is generally yellow or red with a black Y on its back .

The biology and habits of these two bugs are very similar. Both the adults and nymphs are predaceous, feeding in the same manner. The bugs pierce the prey insect with sharp mouthparts and suck the body fluids from it. After coming out of hibernation in the early spring, the adults feed on plant sap until hosts are available. Females lay a total of about 150 eggs over a two-month period in dense clusters of 15-30 eggs on leaves. The first-instar nymphs remain together in a group and feed almost exclusively on plant sap, but later instars disperse and feed only on other insects. One nymph may consume as many as 36() host eggs during its development.

Both of these bugs are commercially available. The suggested release rate for spined soldier bugs is five nymphs for every foot of garden row.

Nabis, Geocoris, and Orius (order Hemiptera). The beneficial predators Nabis, Geocoris, and Orius are among the most abundant general predators in many agricultural situations. Although species in each group feed on plants as well as insect prey, they do little if any damage to the crop plants.

The damsel bugs (family Nabidae) are common in many gardens and crops. Adults are slender, 8-10 mm (5/16-3/8 inch) long, and usually tan or black. The nymphs are similar, but they lack wings and are smaller . The piercing mouthparts curve downward from the front of the head. They feed on many types of insects, including aphids and small caterpillars.

The bigeyed bugs, Geocoris spp. (family Lygacidae), occur in many habitats and feed on many types of prey. They often occur in agricultural settings, especially where the use of broad-spectrum insecticides is minimal, and in many non-crop situations. Bigeyed bugs are small (2-4 mm 1/16-3/16 inch) and usually dark brown or black They consume numerous aphids, flea beetles, small caterpillars, whiteflies, other small insects, and spider mites during nymphal development and as adults. Supplementary green plant material and seeds in their diet improves development, reproduction, and survival. Sunflower seeds scattered on plants have enhanced Geocoris populations in experimental fields.

The minute pirate bugs, Orius spp. (family Anthocoridae), are common in gardens and many agricultural situations, including pasture land, field margins, and crops, especially where broadspectrum insecticides are not routinely used. The black, 2-5 mm (1/16-1/4 inch) adults are ovoid and somewhat flattened, with distinctively patterned black and white wings . The eggs are laid in leaf tissues with one end slightly sticking out. The nymphs are pinkish yellow to light brown . Both nymphs and adults are very active, have prominent beaks, and feed on thrips, mites, aphids, whiteflies, and caterpillar eggs, as well as plant pollen. Orius sp. and Orius tristicolor (White) are available commercially. They are shipped as adults in a carrier such as bran, rice hulls, or vermiculite, along with a food source. Shake the carrier onto the plants, and the bugs will readily disperse and locate prey. Suggested release rates vary considerably with the crop and the pest.

Spiders (various families). Spiders are not insects, but belong to the arthropod class Arachnida. All spiders are predaceous, mostly on insects. There are about 50 families of spiders in the United States; about 15 of these are frequently encountered in crops. Spiders are more abundant in small fields surrounded by undisturbed vegetation than in large fields.

An insect in virtually any active stage of its life cycle can fall prey to a spider. Some spiders will even eat insect eggs or pupae. However, each spider species is likely to catch a certain type of prey, based on the method of capture. Spiders capture their prey in three ways. The largest group of spiders constructs a web of some sort, which is used to capture the prey. Common families of web spinners include the orb weavers (family Araneidae), the sheetweb spiders (family Linyphiidae), the combfooted spiders (family Theridiidae), and the funnel web spiders (family Agelenidae). The hunting spiders, the second group, do not construct a web to capture their prey, although they may construct a silken refuge. These spiders are very active and often run down their prey to capture it. Examples of hunting spiders include the wolf spiders (family Lycosidae), the jumping spiders (family Salticidae), the lynx spiders (family Oxyopidae), and the twoclawed hunting spiders (family Clubionidae). The third group COtlsists of one large family, the crab spiders (Thomisidae). This group uses the ambush method of prey capture. These spiders are common on flowers and vegetation and sit motionless until their prey comes within easy grasp.

The importance of spiders as natural enemies in pest control is questionable. Several studies have indicated that spiders help regulate pest populations, but because they are generalist predators they do not specifically select pest insects. Spiders typically have a single generation per year, and therefore they are unable to increase their numbers rapidly in response to the buildup of a multi-generational pest. Spiders are part of the overall natural enemy complex that helps keep pest populations from rapidly expanding, but no single general predator species alone can have this effect.

Spiders are beneficial natural enemies and precautions should be taken to prevent their destruction. Broad-spectrum insecticides not only directly kill spiders, but they also kill many nonpest insects that the spiders eat during periods of low pest numbers. Therefore, apply insecticides only when needed and use the most selective materials. Many cultural crop practices also disrupt spider activity. Harvesting and tillage can destroy much of the Overwintering spider population, especially when conducted in late summer or fall. Pathogens

Bacillus thuringiensis Berliner. Bacillus thuringiensis, commonly called Bt, is a naturally occurring, soil-inhabiting bacterium that is nonpolluting and safe to humans. Under certain conditions, mainly when food sources are exhausted, the bacterial cell forms a heat- and desiccation-resistant spore inside the cell during a complex process called sporulation. At the time of sporulation it also forms a parasporal body, or crystal, which is the source of Bt's insecticidal properties. The crystal is toxic only when activated by insect gut juices. In susceptible insects, the crystals dissolve in the gut, releasing toxic compounds. The spores later germinate and bacteria grow within the insect body. The first observable reaction after an insect ingests Bt is paralysis of the gut and mouthparts and cessation of feeding; this reaction may happen within a matter of minutes. Though feeding stops quickly, death of the insect generally occurs 30 minutes to 3 days after ingestion.

There are more than 20 types of Bt, which differ in both specificity and potency against a range of target insect species. The majority of these are highly selective and active against caterpillars (larvae of moths and butterflies). Two exceptions are Bt var. israelensis, which is toxic to the larvae of mosquitoes, blackflies, and some other types of flies, and Bt var. tenebrionis, which affects certain beetles. Bt does not directly affect predators or adult wasp parasites, but it does kill parasitized caterpillars, thereby reducing parasite numbers.

The first field test of Bt was against the European corn borer in 1927. It showed considerable promise, but the use of Bt as a microbial insecticide was not pursued further until the 1950s. Since then Bt has become the most widely used, commercialized microorganism for insect control, accounting for about 90% of world sales of microbial insecticides, with numerous registered formulations. Although Bt can cause natural epidemics in enclosed environments such as granaries, it does not spread well enough naturally and is inactivated too quickly by sunlight to have significant effects on insect populations under field conditions. This means Bt must be applied whenever pest populations warrant, and in a manner similar to conventional insecticides, for adequate pest control. Because Bt has no contact action, but has to be eaten to be effective, it must be applied where the target insects are feeding. It is not effective against insects feeding within plant tissues, such as cabbage looper larvae in the cabbage head. However, if insects feed in an exposed location at some time during their larval development, they may be susceptible at that time. Bt is usually more effective against earlier instars than later instars.

Overview of biological control of crucifer pests

Biological control can be a highly effective means of managing many of the pests that attack crucifiers. However, it is not based on recipes containing set responses to particular pest problems. Many variables contribute to the effectiveness of biological control, and every field presents a unique combination of these variables. Growers must therefore become familiar with the specific conditions of their fields. What problems have occurred in the past? How much damage can be tolerated before the market value of the crop begins to drop? What crops are grown in neighboring fields, and what control methods are practiced there? These are just a few of the issues to consider as you plan your biological control system. The best approach may be to coordinate several biological control methods rather than relying on a single solution. Consider all the options and evaluate whether they are compatible with each other and with your chosen control practices.

One way to gain experience and confidence in biological control methods is to experiment with trial plots while maintaining more traditional practices on your main fields. As methods prove effective in the trial plots, implement them on a larger scale. This approach delays the full implementation of a biological control system, but it minimizes the risks of changing to new practices and allows you to gain the experience necessary to succeed with biological control.

In small plantings or home gardens, biological and cultural control methods that would not be practical on large plantings can be used to produce pesticide-free crucifiers. Floating row covers can protect crops from numerous pests, and hand picking can eliminate many of the larger pests. The diversified surroundings of such plantings may encourage the development of more numerous natural enemy populations. Interplanting crops that are unattractive to pests, such as tomatoes for diamondback moth and flea beetles, can contribute to the diversity of the environment while deterring the targeted pests. If such practices succeed in suppressing pest populations, chemical controls will be unnecessary. Tolerance of minor feeding damage also reduces the need for pesticide use, particularly in home gardens.

Even for large-scale commercial plantings, biologically based control of crucifer pests is a realistic option. Certain cultural practices can help keep pests below damaging levels. Choosing a cultivar with some resistance to a particular pest may reduce infestation levels of that pest. No-till cultivation may be appropriate to encourage the presence of natural enemies and to deter diamondback moth and cabbage aphid. Conversely, clear cultivation is less attractive to flea beetles and is easier to maintain. Destroying and removing crop residues after harvest eliminates Overwintering sites for cabbage aphid, diamondback moth, and cabbage maggot. Releases of commercially available natural enemies--most notably parasitic wasps--are particularly effective if only one or two pests are an immediate problem. The major pests of crucifers are the leaf-feeding caterpillars, including, the cabbage looper, the diamondback moth, and the imported cabbageworm. Trichogramma wasps attack all three caterpillars, and microbial insecticides based on Bacillus thuringiensis can effectively control these caterpillars if applications are made while larvae are small. An early-season pest, the cabbage maggot, can be avoided by timing planting when flies are not prevalent.

Naturally occurring parasites, predators, or pathogens often suppress many of the other pests, especially cabbage aphid. If broad-spectrum insecticides destroy the natural enemies of these pests, however, economically damaging pest populations may result.

Chemical insecticides will sometimes be necessary to prevent economic damage. For some insects, such as thrips and flea beetles, no effective biological controls are currently available. If cultural practices do not prevent damaging populations of these pests from developing, the only control alternative is chemical insecticides. For other pests, such as cabbage looper, imported cabbageworm, or cabbage aphid, insecticides become necessary to control damaging populations if biological controls fail. Insecticides will probably remain an integral part of an overall crop management program, but their usage should be carefully directed. Using selective insecticides with low toxicity to natural enemies when possible and specific placement and timing of application will reduce the impact on beneficial organisms. The successful integrated pest management program will be biologically based, but it will use carefully considered cultural and chemical controls when necessary and if properly coordinated with biological control.

Copyright 1997


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