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Practical approaches to the biological control of crucifer pests

This section examines the major pests of cole crops, providing pest descriptions and detailed information on selected natural enemies. The discussions of the pests explore the possibilities for using natural enemies to obtain effective biological control and also describe other alternative control methods, emphasizing nonchemical approaches. The discussions conclude with suggestions for integrating biological control with other crop and pest management practices.

Some of the parasites and predators discussed occur naturally, and conservation of these existing natural enemies can be extremely important in suppressing pest populations. However, augmentation is often necessary to achieve effective control. Many suppliers suggest release rates and methods for commercially available natural enemies, but these are only general guidelines. The performance of natural enemies is dependent on murderous variables, including the crop plant variety and stage, the pest population, environmental conditions, release methods, the quality of the natural enemies released, and other insects present. Specific release rates must reflect these factors if pest populations are to remain below economic injury levels.

CABBAGE MAGGOT Delia (=Hylemya) radicum (Wiedemann)

The cabbage maggot is an important early-season pest that may cause serious damage, especially to young transplants. Maggots feeding plant roots stunt plant growth and introduce disease that can kill the plant.

Damage

Maggots feed on and tunnel through the roots, which causes stunting, yellowing, and wilting, especially on hot days. Seedlings can be killed rapidly, while transplants tend to wilt and die slowly. Once established, healthy plants can tolerate moderate maggot infestation. On root crops (radish, rutabaga, and turnip), maggots produce characteristic brown tunnels. Maggot feeding also allows for invasion of secondary pathogens, bacterial soft rot, and the fungus that causes black leg.

Description and life cycle

The adult cabbage maggots are small, bristly, gray flies that resemble house flies, but they are only half as long and have black stripes on the thorax. The female flies lay small white eggs in cracks in the soil at the base of the plant. After hatching the larvae burrow down to the roots. Larvae are small, white, legless maggots with blunt rear ends. They grow to 6-7 mill (1/4 inch). The maggots pupate inside brown, egg-shaped 6 m (1/4 inch) puparia (singular--puparium) after they have spent 3-5 weeks feeding and developing on the roots. Adult flies emerge 2-3 weeks later. In the upper Midwest there are three generations per year, with the insects Overwintering in the pupal stage.

Pest status

The cabbage maggot is native to Europe and was accidentally introduced into North America in the nineteenth century. It is now widespread throughout the United States and Canada. The host plants are primarily in the mustard family (cabbage, cauliflower, broccoli, Brussels sprouts, radish, rutabaga, turnip, and cruciferous weeds such as yellow rocket, wild mustards, and wild radish). Hoot crops (radish, rutabaga, and turnip) t suffer severe damage because the marketable produce is damaged directly. Crops planted in light, sandy soils usually suffer more damage than those in heavier or muck soils. The first generation of cabbage maggot, in the early spring, is usually the most economically damaging. Cabbage maggots cause little damage during the summer months partly because hot, dry weather will dry out and kill eggs and newly hatched maggots.

Natural enemies

Although numerous parasites and predators attack cabbage maggots, natural enemies usually do not reduce cabbage maggot populations sufficiently to prevent economic damage in commercial cabbage fields in the upper Midwest. Normal chemical control practices also interfere with natural enemy abundance and activity.

PARASITES

Aleochara bilineata Gyllenhal. This species of parasitic rove beetle (family Staphylinidae) was introduced from Europe into North America with its host. The beetles are 3-4 nm1 (~8 inch) in size, and they attack cabbage maggots in the pupal stage. The adults are all black except for the short wing covers, which are a reddish brown. They emerge in spring and deposit eggs in the soil near the roots of plants infested with cabbage maggots. The beetle larvae search out host puparia, gnaw a hole in the shell, and feed externally on the pupae. Beetle pupation occurs within the host puparia.

The adult beetles are also predators, destroying eggs or young maggots near the soil surface and maggots in the plant roots. In an extensive survey throughout Canada parasitism by A. bilineata ranged from O'/o to 63% at different locations.

Another very similar beetle, A. bipustulata Linnaeus, occurs in small numbers in some areas. It has the same life cycle as A. bilineata.

Trybliographa rapae (Westwood). This species of cynipid wasp is widespread in Europe, where it is presumably native, and in North America, where it probably was introduced accidentally with its host. The wasps also attack other species of flies related to cabbage maggots. The 3-4 mm (1/8 inch) shiny black females deposit eggs in early-instar cabbage maggots, and the wasp larvae complete development in the puparia. The wasps overwinter as fourth-instar larvae in the host puparia, and adults emerge in the spring.

T. rapae can destroy up to 45'/o of the cabbage maggot population, but parasitism is quite variable. Whenever this wasp and an Aleochara both parasitize the same cabbage maggot, the beetle always destroys the wasp and the maggot. In spite of this competition, T. rapae is able to maintain a substantial rate of parasitism even when the beetle is abundant.

Steinernema (=Neoaplectana) carpocapsoe (Weiser). This nematode infects a variety of soil-inhabiting insects, including larval cabbage maggots. The nematode populations that occur naturally in the soil are never high enough to provide control, but nematodes can be released as a biological insecticide (see Section 3). A high concentration of nematodes (as many as 3 billion per acre) and optimum conditions for infection are required to control cabbage maggots.

Other parasites. Many other wasp parasites attack larval cabbage maggots and emerge from the puparium. The ichneumonids Phygadeuon fumator (Gravenhorst) and Atractodes tenetricosus (Gravenhorst) and the braconid Aphaereta auripes (Prov.) are found occasionally, but they never contribute substantially to cabbage maggot mortality.

PREDATORS

Ground beetles (family Carabidae). Ground beetles are often the most numerous predatory insects found in cabbage or other crucifer fields. Many species of carabids occur in the upper Midwest. Common species known to be predators of the cabbage maggot include the seed-corn beetles Agonoderus lecontei Chaudoir and A. comma (Fabricius) , and the ground beetles Bembidion quadrimaculaum Linnaeus, all similar in shape and are usually shiny black or brown, but they vary in size and may have prominent patterns of lighter colors depending on the species. The adult seed-corn beetles are 5 - 7 mm (1/4 inch) and are brownish yellow with a blackish head and spots on wing covers. They are active during the day in cooler weather and on warm nights, moving in cracks and burrows in the soil surrounding infested plants. They are also very attracted to lights at night. The smaller ground beetle adults are mainly nocturnal and run quickly when disturbed. B. quadrimaculatum adults are 2.7-3.7 mm (1/8 inch) with a bronze head and black body with four yellowish irregular spots on the wing covers.

Both seed-corn beetles and ground beetles search in the area around the roots to locate eggs, then chew a hole in an egg to eat the contents. They are responsible for significant egg mortality, although they also feed on the maggots themselves and other prey if available. They can destroy up to '30% of the first-generation maggot eggs, but predation averages only 30-45%. The immatures live in burrows in the soil, feeding on microorganisms, and pupate in a cell excavated in the burrow.

Because these beetles are attracted to light they are easily lured into a crop. Beetle populations are higher and plants have less cabbage maggot damage in lighted plots than in unlighted plots. However, even with increased predation these beetles do not reduce cabbage maggot levels to below economic thresholds for commercial production of root crops such as radish and rutabaga, and the lights may lure other, undesirable insects into the crop.

Trombidium spp. Red garden mites, Trombidium spp., were reported as predators of cabbage maggot eggs in Michigan, Minnesota, and other locations around the turn of the century. These have not been noted in recent years.

PATHOGENS

Entomophthora spp. These fungi affect only the adult flies. They require very specific environmental conditions for development and infection, and therefore they are not a consistent yearly biological control agent. When an epidemic caused by one of these fungi happens at the peak of egg laying, it will greatly reduce the subsequent generation.

Possibilities for effective biological control

Naturally occurring parasites and predators are important factors in reducing cabbage maggot populations. However, natural biological suppression of the cabbage maggot usually is not sufficient to prevent considerable crop loss. Because the natural enemies in Europe and North America are virtually identical, introductions from Europe would be futile, but mass releases might be effective. In Russia and Denmark, seasonal releases of the parasitic staphylinid beetle, Aleochara bilineata, at quantities of 10,000-30000 per hectare gave 83-93% control, but problems existed with mass rearing. No specific natural enemies of the cabbage maggot are available commercially in the United States.

Careful timing, specific placement, and overall reduction of chemical applications can preserve the beneficial effects of naturally occurring parasites and predators. Using pesticides with little or no residual activity to control soil-inhabiting and foliar pests promotes the survival of beetles and other natural enemies.

In Russia insecticide sprays are not recommended after the rosette phase for cabbage when natural enemy populations are high (20 individuals per meter and 60-70% parasitism) and fly eggs are less than 50 or 100 per plant for unresistant and resistant plants, respectively. No such recommendations have been developed for use in the United States, but a number of alternative control measures can reduce the need for insecticide treatments that disrupt natural enemy activity.

Other alternative control methods

TIMING OF PLANTING

Plant at specific times to avoid peak fly emergence and egg-laying periods, especially of the largest and most damaging generation. This technique may be impractical for large commercial growers who must maintain continuity of harvest and marketing schedules, but it can be useful for home gardeners and growers with more flexibility. You can predict this period accurately by keeping track of thermal units. The number of thermal units accumulated differs with the temperature scale used; the following calculations and recommendations are based on degrees Fahrenheit. Cabbage maggots do not develop below 43F (the developmental threshold) and their development above 43F is directly proportional to temperature. Thermal units are a means of estimating the amount of "heat" above 43F during a particular day. By recording air temperatures, heat units can be calculated by this formula:

Thermal unit= (Maximum temp. + Minimum temp./ 2 )-Developmental threshold

For example, if the maximum and minimum temperatures for one day were 65F and 45F, respectively, then:

(65 +45/2)- 43 = 12 thermal units for one day.

Peak emergence of the first generation of cabbage maggots occurs when 300 thermal units (F) have accumulated. To predict emergence of this generation, keep an accurate record of any temperatures above 43F, beginning when the ground thaws. Record the temperatures daily, calculate the thermal units for each day, and continue adding them until 300 is reached. Emergence of each subsequent generation peaks at 1176 additional thermal units. Plant transplants at least a week before or after these peaks and plant seed at least three weeks before or one week after. Do not plant during the peak.

PLANT RESISTANCE

Some cabbage varieties appear to be tolerant of cabbage maggot injury. Early Jersey, Wakefield, and Penn State Ballhead were the least damaged in trials in Massachusetts. In trials in Wisconsin, Flat Dutch, Red Hollander, and Red Acre were the least damaged of 21 cabbage varieties, but the range of injury was not great (33% for the most resistant Flat Dutch to 53% for the most susceptible Savoy Chieftan). Although red cabbage varieties often have less damage than green varieties, resistance to cabbage maggot damage is not related to the color itself. Cabbage generally sustains less damage than do other cruciferous crops such as cauliflower, broccoli, Brussels sprouts, rutabaga, and turnip.

TILLING

Plowing under crop residues immediately after harvest helps reduce Overwintering within the field.

PHYSICAL BARRIERS

In small plantings physical barriers, such as tar-paper discs, pie plates, or other circular devices cut to fit snugly around the plant stem, can be used to prevent flies from laying eggs at the base of the plants . Flexible row covers over the entire plant or row of plants would also prevent fly access to plants. Note, however, that row covers will not exclude flies emerging from pupae that are already in the soil. Lightweight polyethylene or nylon row covers are available from most farm or garden supply stores or mailorder firms.

REPELLENTS

Researchers in the Netherlands are experimenting with slow-release formulations of naphthalene to be spread or sprayed around the base of cauliflower plants to repel egg-laying flies. The most promising formulation is naphthalene encapsulated in Curasol, a synthetic soil conditioner. This formulation gave better crop development than Curasol alone and almost equaled the effect of a standard insecticide treatment. However, these materials are not currently registered for this use in the United States.

Integrating cabbage maggot control with other crop and pest management practices '

Although naturally occurring parasites and predators significantly reduce cabbage maggot populations, they usually are not effective enough to prevent some damage. Of the commercially available natural enemies, the nematode Steinernema (=Neoaplectana) carpocapsae has potential against cabbage maggot; with improved encapsulation methods it may become feasible against cabbage looper and flea beetles as well.

In small plantings, barriers around the base of each plant can deter flies from laying eggs. Other useful cropping practices include choosing red varieties of cabbage, which are more resistant to both cabbage maggot and thrips than are green varieties, although they are more preferred by cabbage looper. Destroying and removing crop residues after harvest eliminates Overwintering sites not only for cabbage maggot, but also for diamondback moth and cabbage aphid.

Avoiding planting in the peak egg-laying periods of the fly can prevent much damage. If the crop must be planted at a particular time, regardless of fly populations, insecticides may be necessary to prevent crop damage. The currently registered insecticides are broad spectrum and will kill many other organisms besides cabbage maggots. Use precise placement around the base of plants or in a 2- to 4-inch band over the row to minimize disruption of natural enemies.

DIAMONDBACK MOTH Plutella xylostella (Linnaeus)

Diamondback moth

Order Lepidoptera Moths and butterflies

Family Plutellidae:

Diamondback moths

The diamondback moth is one of three major caterpillar pests of crucifiers. is an increasing problem in the upper Midwest because it is developing resistance to insecticides.

Damage

Because of its small size, this insect is often overlooked, with more attention paid to the larger and more voracious cabbage caterpillars. First instars feed in mines in the lower leaf epidermis, leaving small white tunnels. Subsequent instars feed from the undersides of the leaves, often leaving the upper surface epidermis intact as a "window" that later disintegrates. Larval feeding damage to the growing point of small plants can cause severe stunting. Small larvae imported on transplants from southern states may infest fields before local populations are active.

Host plants include both wild and cultivated plants in the family Cruciferae. Among the cultivated crops, cabbage, cauliflower, Brussels sprouts, broccoli, and collard especially are attacked. Ornamental crucifers such as wallflower, candytuft, stock, and alyssum may also be infested. Cruciferous weeds, such as wild mustard or radish, are an important food source early in the spring before crops are planted. Oilseed crops such as canola could provide an additional early food source.

Description and life cycle

Adult diamondback moths are small and nocturnal, with a wingspan of 1.2 cm (1/2 inch). The wings are held together over the back, producing a pattern of three diamonds along the top of the narrow body. The very small eggs are laid singly on both the upper and lower leaf surfaces and are difficult to find. First-instar larvae are leaf miners, but later instars feed on the leaf surface. The larvae are pointed at both ends; they grow only to about 8 mm (shy inch) by the fourth and last instar. Their behavior when disturbed distinguishes them from other caterpillars found on cabbage: they wriggle furiously or drop off the edge of the leaf on a silken thread. Three to five generations occur each year in the upper Midwest; more occur in southern areas. Diamondback moths are not thought to overwinter through severe winters, such as often occur in Michigan, Minnesota, and Wisconsin, but early infestations often occur near cabbage debris, indicating that Overwintering of adults in protected areas is possible. Importation on transplants and migration on weather fronts are common means of introduction in northern states.

Pest status

Until recently the diamondback moth was considered the least serious of the caterpillar complex on cole crops in the upper Midwest. Favorable weather conditions for population build-up, development of resistance to commonly used insecticides, and other factors have elevated the diamondback moth to major pest status. It is a cosmopolitan species that probably originated in the Mediterranean region. It was first reported in North America in Illinois in 1854 and in western Canada in 1885. It is now present throughout the United States and Canada.

Natural enemies

PARASITES

More than 90 species of parasites attack the larvae and pupae of the diamondback moth in different parts of the world. Egg parasites are uncommon. In the upper Midwest, only a few species of wasps attack this pest.

Diadegma insulare (Cresson). This ichneumonid wasp parasitizes the later instars of the host, but it emerges from the host pupa. The adult is 6 mm (1//4 inch) long and black with reddish brown legs and abdomen . It lays a single egg in a later-instar host caterpillar. The wasp larva develops within the caterpillar. When the caterpillar pupates, the wasp larva replaces the host cocoon with its own pupal case, which is easily recognized by the broad white stripe around it.

D. insulate is capable of causing high mortality rates of diamondback moth caterpillars on cabbage. These wasps were the most common parasites of the diamondback moth during a four-year study in Wisconsin, parasitizing 53-88% of the caterpillars collected. In southwest Virginia they caused 46-69% mortality in an unsprayed cabbage field.

In Russia parasite levels are included in treatment thresholds. Insecticides are not recommended when parasitism by a related species of wasp occurs on at least 60% of the plants and caterpillar numbers do not exceed seven per plant. No such recommendations exist in the United States.

A large population of D. insulate could be used to reduce diamondback moth populations early in the season through releases timed to coincide with the increasing levels of the pest. Commercially available prefed fertile adults should be released weekly at a rate of 150 pairs per acre.

Diadromus subtilicornis (Gravenhorst). This ichneumonid wasp is the most abundant pupal parasite that occurs in the upper Midwest. The 6 mm (1//4 inch) female wasp is black with a long ovipositor, which it uses to lay single eggs in prepupae and pupae. The adults sometimes also feed on the body fluids of caterpillars when laying eggs. This insect is found every year in low numbers, parasitizing 1-12% of the diamondback moth pupae.

Microplitis plutellae (Muesbeck). This species of braconid wasp is native to Europe and adjoining areas. It probably was introduced to North America at the same time as the first accidental introductions of its host. The female wasps lay eggs in the first three instars (figure l2). Although many eggs are laid in each caterpillar, only one wasp develops. Parasitized caterpillars continue maturing and are active; they do not die until shortly after the wasp larvae emerge. The wasp larvae pupate in light brown cocoons.

Because these wasps destroy their hosts so late, they do not have much effect on the damage done by the host caterpillar, although they can affect the abundance of successive generations. The wasps do not provide much control of the diamondback moth in the upper Midwest. hi Wisconsin it is the least common diamondback moth parasite, parasitizing less than 5% of the caterpillars, and often it is not found at all. M. plutellae is available commercially.

Cotesia (=Apanteles) plutellae (Kurdjumov). This braconid wasp is native to Europe and is very similar to M. plutellae. It is not known to occur naturally in the upper Midwest, but it is available commercially. The females lay eggs in the first three instars, and a single wasp develops in each host. The host caterpillar dies when the wasp larva emerges to pupate.

This wasp controls diamondback moth in Taiwan, India, and other parts of the world when used in conjunction with other parasites and Bt. It is effective only when temperatures are between 20C (68F) and 35C (95F) and would therefore be most suitable for use during the warmest part of the growing season in the Midwest. The wasps should be released weekly at a rate of 150-400 pairs per acre for three weeks and an additional 200 pairs per acre weekly thereafter. In cooler weather or when diamondback moth larvae are numerous, higher release rates or more frequent introductions may be necessary to achieve control.

Other parasites. A few other wasps occasionally parasitize these caterpillars, but they do not have any significant effect on populations of the diamondback moth. These are the eulophid Tetrastichus sokolowskii Kurdjumov; the chalcid Spilochalcis bifrons (Walsh); the ichneumonids Gelis tenellus (Say) and Campoletis sp.; and the pteromalids Dibrachys cavus (Walker) and Habrocytus sp. The egg parasites Trichogramma spp. will develop in diamondback moth eggs. Weekly releases of these commercially available wasps at 40,000-200,000 per acre during peak egg laying may control diamondback moth, as well as other caterpillars that may be present.

PREDATORS

Although spiders, birds, and many species of insects, including aphidlions (lacewing larvae), minute pirate bugs, and syrphid fly larvae, have been observed feeding on diamondback moth caterpillars, predators generally are not important in reducing populations of this pest.

PATHOGENS

Naturally occurring microbial diseases are not an important cause of mortality. The bacterium Bacillus thuringiensis (Bt), used as a microbial insecticide, is very effective against the caterpillars (see Section 3). However, some populations, especially in the southern United States, have developed resistance to Bt var. kurstaki. Using a minimum number of applications or rotating to the newly commercially available strain Bt var. aizawai will help delay development of resistance to Bt. Pheromone traps can be used to monitor adult flight periods to determine when larvae should be monitored directly.

Possibilities for effective biological control

Eliminating this pest entirely is unnecessary because plants can tolerate low levels, offering the potential for developing effective biological controls. Natural parasitism is often sufficient to suppress diamondback moth populations, especially later in the season. Releases of the wasp parasite Diadegma insulare could suppress caterpillar populations if natural populations provide insufficient control.

The microbial insecticide Bt is very effective in controlling this pest and is commonly used commercially. The use of Bt instead of chemicals early in the season may allow parasites to survive and exert greater control late in the season. University Extension publications provide recommendations for the use of several products containing Bt. The diamondback moth is managed together with the imported cabbageworm and cabbage looper as a single caterpillar complex, with regular crop scouting and treatments based on larval infestation threshold levels (see "Management Practices for Caterpillars") .

Other alternative control methods

FLOATING ROW COVERS

Protective canopies of lightweight polyethylene or nylon material can prevent adult moths from laying eggs on plants. Plants must be covered when transplanted to exclude early generation moths moving from wild host plants. Floating row covers are available from most garden supply stores or mail-order firms.

HAND PICKING

The recommendation made around the turn of the century to pick caterpillars from the plants, gather the pupae, and net the adults is obviously impractical for large plantings, but it is still an option for home gardeners.

SANITATION

Clean cultivation of the crop residue immediately after harvest helps reduce population build-up and migration to younger plants. It also eliminates potential Overwintering sites. Removing the fall refuse and any cruciferous weeds along field margins can further reduce Overwintering populations.

INTERPLANTING

Both greenhouse cage experiments and field research in the Philippines have shown that the presence of tomato plants can reduce the number of diamondback moth eggs laid in a cruciferous crop, probably because adults are repelled by the odor from the tomatoes.

Tilled plots tend to support higher diamondback moth populations than do mowed or untilled plots, perhaps in part because general predators (syrphids, lacewings, ladybird beetles) are more numerous in weedy plots.

NEED EXTRACTS

Extracts from seeds of the neem plant (Azadirachta Indira A. Juss) have repellent or insecticidal properties. It is not yet registered for use on food crops and has not been tested against the diamondback moth in the United States. In the West African republic of Togo, weekly applications of the seed extract increased yield and reduced diamondback moth infestation on cabbage. Although yield was slightly better than in plots sprayed with standard chemical insecticides, more heads were damaged in the neem plots. Neem leaf extracts were not effective.

Integrating diamondback moth control with other crop and pest management practices

Often natural control by wasp parasites and other beneficial insects will be sufficient, especially if some cosmetic damage (to wrapper leaves on cabbage) can be tolerated. Weedy, no-till plots encourage the presence of natural enemies and are less attractive to both diamondback moth and cabbage aphid. However, in addition to the obvious agricultural disadvantages of allowing a weed cover, weedy plots may increase flea beetle populations. Intercropping tomatoes with crucifers seems to deter both diamondback moth and flea beetles. In smaller plots, physical barriers such as floating row covers can exclude diamondback moth, imported cabbageworm, and cabbage looper. Hand removal is another option against the three caterpillars. Destroying and removing crop residue after harvest eliminates Overwintering sites for diamondback moth, cabbage maggot, and cabbage aphid.

Trichogramma wasps are commercially available and are effective against diamondback moth, imported cabbageworm, and cabbage looper. Refer to Section 3 for more complete information on Trichogramma use and species selection. Good control of the diamondback moth can be achieved with Bt, which does not adversely affect other crop and pest management practices. "Management Practices for Caterpillars" contains specific information on applying Bt.

IMPORTED CABBAGEWORM Artogeia (= Pieris) rapae (Linnaeus)

Imported cabbageworm

Order Lepidoptera: Moths and butterflies

Family Pieridae: Whites and sulfur butterflies

The imported cabbageworm is the most common of the three caterpillar pests of cabbage in the upper Midwest. It is a pest throughout the entire growing season.

Damage

Larvae feed on leaves, causing damage similar to that of the cabbage looper. Larvae usually feed on the upper leaf surface near the midrib, removing large, irregular areas and leaving only the large veins. Extensive feeding can kill small plants or delay crop maturity, but older plants can tolerate substantial defoliation until heading. Heavy feeding after this time often results in head stunting or abortion. Older larvae move into the center of the plant and may bore into the head, causing serious losses. Contamination of edible portions of the plant with the copious amounts of green frass (fecal material) produced by the larvae makes the product unmarketable.

Description and life cycle

Adult imported cabbageworms are day-flying white butterflies with black spots on the wings. Males have a single dot on each forewing and a black body, while females have two dots on each forewing and a white body. The females have a wingspan of 5 cm (2 inches) and are slightly larger than the males. The females lay small (1 mm; 1/32 inch) yellow-orange, bullet-shaped eggs singly on any aboveground plant part, usually on the leaf margin. The velvety green larvae grow to 2.5 cm (l inch) in length and have a faint yellow stripe down the back when older. The larvae usually pupate on the underside of the lower wrapper leaves. The pupal stage is a green to gray chrysalis about 2 cm (3/4 inch) by 6 mm (1/4 inch) with two angular projections at the head end. There are several generations per year in the Midwest. In northern areas the first butterflies emerge in early May from pupae that overwintered on plant debris The larvae of this relatively small first generation develop on cruciferous weeds and early-planted cabbage. The second-generation butterflies emerge in mid-July, and larval development occurs almost entirely on cultivated cole crops. This generation develops much more quickly than the first because of warmer temperatures and greater availability of food. The third generation starts in mid-August and ends in late September with Overwintering pupae. In more southerly areas, a longer growing season supports more generations per year; six occur around Columbia, Missouri.

Pest status

This insect is heavily parasitized in its native Europe, where it only occurs in sporadic outbreaks. In the United States, however, predators and parasites usually are not sufficient to keep cabbageworm populations below damaging levels. The imported cabbageworm was introduced into Canada from Europe before 1856, and subsequently spread into the United States. It now occurs throughout the temperate and subtropical regions of the world, including Europe, Asia, North and South America, Australia, and New Zealand. This insect feeds on all crucifers and cruciferous weeds, including mustard and yellow rocket.

Natural enemies

PARASITES

Trichogramma spp. These tiny wasps parasitize eggs of the imported cabbageworm, as well as eggs of many other Lepidoptera (see Section 3). Adult wasps are less than 0.5 mm (1/64 inch) long and therefore are rarely noticed. After a female wasp lays an egg in a cabbageworm egg, the host egg turns black as the larval parasite matures within it.

In 1965 - 67 Trichogramma evanescent Westwood was introduced from Poland into Missouri and southern California as the initial phase of a biological control program on imported cabbageworm. Although natural populations of T. evanescent are ineffective in controlling early generations of the host, this parasite has great potential as a control agent through mass releases. Under favorable weather conditions, experimental releases were effective in controlling natural host populations, with parasitism reaching 100% and remaining at that level as long as parasite releases were continued A release of 600,000 adult T. evanescent resulted in 99% parasitism of imported cabbageworm eggs on a 1/3-acre cabbage planting. High rates of parasitism were also achieved with fewer adults on smaller plots. Releases of larval parasites that are not commercially available were necessary to control the host population when weather was unfavorable for T. evanesces, especially when the plants began to mature.

Several species and strains of Trichogramma are available commercially at relatively low cost (see Section 3). Research indicates that species and strains vary widely in adaptability to different weather conditions and ability to control caterpillars. T. pretiosum is probably the most suitable species commercially available for cole crops.

Cotesia (=Apanteles) glomerata (Linnaeus). This species of braconid wasp develops within the larvae of the imported cabbageworm. The adults are black, are 3 mm (1/8 inch) long, and feed on nectar of flowers and juice of cabbage leaves . Females "sting" small caterpillars, generally first instars, to lay eggs inside the host larvae. Numerous wasp larvae (16-52) develop within each caterpillar. The caterpillar continues its own development until the wasp larvae emerge from its body to spin their cocoons in a group . The mass of yellow to orange cocoons is often mistaken for insect eggs. In Europe C. glomerata is primarily a parasite of another caterpillar, Pieris brassicae, but because P. brassicae does not occur in North America, C. glomerata parasitizes the imported cabbageworm.

An English strain of this parasite was introduced into the United States in 1883, but additional accidental introductions may have occurred previously. The parasite spread rapidly to become generally distributed throughout the United States.

Parasitism by this wasp is quite variable. During a four-year study in Wisconsin, parasitism by C. glomerata averaged only 14.5% with a range of 0-21.2%. However, the wasps parasitized 86% of the caterpillars in one commercial red cabbage field in 1983. An effective parasite population level that would preclude spraying has not been determined in the United States. In Russia C. glomerata provides effective control when it parasitizes at least 60-70% of the caterpillars and when there are fewer than 10-12 caterpillars per plant.

Native C. glomerata populations have very little effect on the first imported cabbageworm generations, because (1) parasite development is not synchronized with that of the host; (2) the host density in the first two generations is too low for the parasites to increase at a sufficient rate to suppress subsequent populations before economically important injury occurs; and (3) densities of parasites in the spun" are low because of mortality during the winter and because of hyperparasites. Furthermore, parasitized caterpillars continue to feed, consuming approximately one and a half times as much as unparasitized caterpillars before they are killed by the emerging parasite larvae. Cotesia is a significant mortality factor later as parasitism increases over the subsequent generations, although host defense mechanisms frequently kill the parasite eggs. Rates of parasitism reach high levels only late in the season, after the caterpillars have injured the crop. C. glomerata is ineffective when populations of C. rubecula are present (see below). C. glomerata is not commercially available.

Cotesia (=Apanteles) rubecula (Marshall). This species of wasp is very similar in appearance to C. glomerata, but it is a solitary instead of gregarious parasite. C. rubecula is host specific to imported cabbageworm and therefore is a more efficient parasite than C. glomerata. Also, C. rubecula kills the host sooner, which reduces pest damage.

This species is much more important than C. glomerata in Europe, but it is established only in isolated pockets in the Americas, including British Columbia and Ontario, Canada. Pupae of C. rubecula were imported to the United States from Vancouver in 1967 and adults were released in Missouri. Establishing the species proved difficult because it does not always overwinter effectively, but periodic releases provided effective control. A strain of the species imported from Yugoslavia has been established in Michigan since releases in 1985. It has displaced C. glomerata there and has spread at least 20 miles from the original site. Even if the species does become established, however, additional releases may be necessary to provide economic control. Currently, the wasp is not commercially available.

A critical requirement for control by parasites is an adequate supply of hosts (i.e., the pests), which may require adding hosts to the environment. Mass releases of fertile hosts ensure caterpillars for the parasites even when host populations are continuously disturbed by farming practices. In USDA experiments in cabbage fields in Missouri, this method of releasing supplemental populations of imported cabbageworm butterflies with the parasites Trichogramma evanescent and C. rubecula controlled host populations sufficiently to prevent economic injury, yielding a marketable crop. Few of the cabbage heads were damaged by the imported cabbageworm, and most or all of the plants produced Grade A No. l heads, compared with none in the check plots. Egg parasitization in the second and third generations averaged 97% and 98%, respectively. Larval parasitization was 61 % and 50% during the second and third generations.

Pteromalus puparum (Linnaeus). This gregarious eulophid wasp develops inside host pupae. Adult wasps are 3-4 mm (1/8 inch) long and feed on flower nectar . Females are shiny black, while males are a metallic greenish bronze and are normally smaller than the females. The wasps fly short distances of less than 2.5 cm (1 inch), which gives them the appearance of hopping. Females lay eggs in either the prepupae or pupae of the host, and 200-400 offspring are produced within each cabbageworm pupa (see figure 7). Parasitized pupae are dull brown compared with unparasitized green or gray pupae that change to yellowish white. One or more exit holes in the pupae indicate the parasites have emerged. The wasps will parasitize other host species when the imported cabbageworm is scarce Because the wasps attack only the pupae, caterpillar feeding is not reduced and significant damage can occur, especially if cabbageworm population levels are high. The benefit of this parasite is in reducing the number of adults, and thereby reducing the size of the subsequent generation.

This parasite was accidentally introduced into the United States from Europe in the late 1800s and is now found throughout the country, although its abundance varies in different geographical areas. Naturally occurring parasites usually reduce cabbageworm populations, but not enough to prevent damage until late in the season. The rate of parasitism varies considerably throughout the year, from field to field, and from season to season. Average parasitism in Wisconsin over four years was 13%, but ranged from 0% to 38%. In Minnesota parasitism was 90% among late August pupae. In commercial cabbage fields in southwestern Virginia, 5()% of the pupae in late July and 67% of Overwintering pupae were parasitized. Augmentative releases would probably help, but this wasp is not available commercially. Lespesia sp., Phryxe vulgaris (Fallen), and Sarconesiopsis sp. These three species of parasitic flies attack imported cabbageworm caterpillars. The adult flies are similar to house flies, but they are larger and more bristly. The females lay eggs on the outside of host caterpillars. When the fly larvae hatch, they bore into the caterpillars. A single maggot develops within the host caterpillar and emerges from the host pupa to form a smooth brown pupa on the plant leaves. The caterpillars complete the larval stage before they are killed by the emergence of the fly larvae. Consequently, the flies do not reduce the damage of the current generation, but they do have an impact on subsequent generations.

The tachinid flies Lespesia and Phryxe are occasionally the only insects parasitizing imported cabbageworm larvae in Wisconsin, with rates of up to 68% and 30%, respectively. Sarconesiopsis is a sarcophagid fly that occurs only sporadically and in low numbers. None of these flies are commercially available.

PREDATORS

Predators are an important, and sometimes the major, cause of mortality of eggs, larvae, and pupae. There are no species that attack imported cabbageworm exclusively. General predators known to feed on eggs include ants, lacewing larvae, ladybird beetle adults and larvae, tarnished plant bug adults and nymphs, and trombiculid mites. Larval predators include damsel bugs, green lacewings, paper wasps, shined soldier bugs, tarnished plant bugs, twospotted stink bugs, daddy longlegs, and jumping spiders. Pupal predators include ants, cantharid beetle larvae, ground beetles, paper wasps, spined soldier bugs, tree crickets, twospotted stink bugs, yellowjackets, and daddy longlegs. There are many other occasional predators. See Section 3 for descriptions of the more important general predators.

PATHOGENS

In Europe many pathogens, including viruses, fungi, and microsporidians, affect the imported cabbageworm. These pathogens rarely cause important natural infections in the United States.

Bacillus thuringiensis Berliner. This bacterium, commonly called Bt, can provide adequate control of the imported cabbageworm when applied regularly throughout the growing season (see Section 3). Bt applications should be directed at small larvae. Thorough plant coverage is important because ingestion is necessary for Bt to affect the caterpillars.

Granulosis virus. The granulosis virus of the imported cabbageworm (ARGV) occurs naturally, but epidemics sufficient to provide economic control of caterpillars rarely develop unaided. This virus is not commercially available in the United States and is not likely to be I registered for use in the near future.

Experimentally it has been an effective control agent for larvae on cabbage, but weekly applications are necessary because of degradation of the virus by sunlight. Third-instar larvae are more susceptible to the virus than any other larval instar. Damage to the outer foliage of cabbage plants treated with ARGV was often quite extensive, but the quality of the heads harvested was equal to those treated with chemical insecticides and was sufficient for processing. Treatment applications based on population threshold criteria (two or more eggs per plant) were very effective. ARGV takes 7--12 days to kill caterpillars, whereas Bt and chemicals are more rapid. Some damage occurred during the period after application and before the death of the d caterpillars, but the virus killed most caterpillars before they could migrate to the head.

Entomogenous fungi. Several fungi infect the imported cabbageworm and other caterpillars. All fungi must have favorable conditions of high humidity to infect caterpillars and none spread well enough naturally to cause epidemics. Fungi are not important natural enemies of the imported cabbageworm.

Possibilities for effective biological control

Despite the abundance and wide distribution of Cotesia glomerata and Pteromalus puparum, natural populations of these parasites provide little economic control of the imported cabbageworm, and natural populations of Trichogramma evanescens are ineffective in controlling caterpillar pests at low host densities (during early generations of the host). Practices to conserve natural enemies might help improve natural control. The Russian technique of planting a few mustard plants in cabbage fields to provide food for adult wasps may increase the effectiveness of parasites of cabbageworm.

Control with insect parasites may be feasible, however, through parasite releases at timely intervals. Trichogramma has great potential as a control agent in mass releases. Parasitism can reach 100% and can effectively control natural host populations as long as releases are made under favorable conditions during the 10- to 1 2-day peak butterfly egg-laying period for each generation. The method of using supplemental releases of both imported cabbageworm butterflies and parasites (T. evanescens and C. rubecula) worked extremely well for scientists in Missouri, but many growers would be reluctant to release more pests among their crops in order to achieve control. If conditions are less than optimal, released insects may not survive, may not attack enough hosts, or may not build up to large enough populations to suppress the hosts. In those cases parasite releases do not provide economic control.

The microbial insecticide Bt is commonly used commercially as an alternative to chemical insecticides for control of the imported cabbageworm. Several products are available, and University Extension publications provide recommendations for their use. Applications need not be made on a regular schedule, but they can be timed based on numbers of eggs or caterpillars on the plants. The adult flight periods are easily observed to determine when scouting for eggs should begin. Cabbage looper, diamondback moth, and imported cabbageworm are managed together as a single caterpillar complex, with regular crop scouting and treatments based on larval infestation threshold levels (see "Management Practices for Caterpillars"). Finally, the granulosis virus has great potential but currently is not registered or available commercially.

Other alternative control methods

FLOATING ROW COVERS

Protective canopies of lightweight polyethylene or nylon material prevent adult butterflies from laying eggs on plants. The material is easy to stretch over the rows, and it is available from most farm or garden supply stores or mail-order firms.

HAND PICKING

Frequent plant inspections and removal of all visible caterpillars may be a practical way to avoid damage in small home gardens.

TILLING

Turning the soil in the fall and early spring exposes Overwintering pupae to predators and desiccation.

EARLY PLANTING

Early-season varieties of cabbage are not damaged as greatly as late-season varieties because they are harvested before the large second generation reaches damaging levels. Late varieties are set out at the peak oviposition period of the second generation, which may create a serious problem. Early planting is especially useful for broccoli and cauliflower. These crops mature more quickly than many cabbage varieties, and early planting is usually sufficient to avoid cabbageworm damage.

INTERPLANTING

Although some organic growers advocate planting cole crops with herbs that are said to have repellent properties, this method of insect control is not scientifically proven. In tests conducted at Virginia State University, perimeter plantings of marigold, nasturtium, pennyroyal, peppermint, garden sage, and thyme had little effect in protecting cabbage from imported cabbageworm.

REPELLENTS

Organic gardeners have suggested several mixtures of common household substances as controls for the imported cabbageworm, but most have not been tested for effectiveness. A sprinkling of salt, rye flour, or a mixture of salt and wheat flour directly on wet plants is said to kill or gum up the caterpillars. A mixture of mint, green onion, garlic, horseradish, hot peppers, peppercorns, and pure soap blended in water is recommended in Rodale's Encyclopedia of Natural Insect and Disease Control as a repellent drench. Another supposed repellent, spooning a little sour milk into the center of each cabbage plant, actually increased damage in tests at the University of Illinois.

Integrating imported cabbageworm control with other crop and pest management practices

Early planting and harvesting reduces potential infestations of both imported cabbageworm and flea beetles. Trichogramma wasps are commercially available and are effective against imported cabbageworm, diamondback moth, and cabbage looper. In small gardens the use of floating row covers or removal of caterpillars by hand, along with tolerance for a slightly blemished product, may be sufficient. In larger plantings or when moderate feeding damage is not acceptable, Bt can adequately control the imported cabbageworm if applications are made when larvae are small. "Management Practices for Caterpillars" contains specific information on applying Bt. If large caterpillars are present, a chemical insecticide may be necessary to prevent economic damage. Unlike Bt, which does not interfere with other crop and pest management practices, chemical insecticide applications will affect all insects in the field, including natural enemies of this and other pests. The use of chemicals with short persistence will result in the least disruption of beneficials.

CABBAGE LOOPER Trichoplusia ni (Hubner)

Cabbage looper - Order Lepidoptera:

Moths and butterflies

Family Noctuidae

0wlet moths and underwings

The cabbage looper is one of three caterpillars that are very destructive on cabbage and other cole crops. This insect is mainly a late season pest in the upper Midwest.

Damage

Loopers damage cabbage by eating holes in the leaves, boring into heads, and contaminating heads with brass. When small, the larvae feed primarily on the underside of lower leaves. Older larvae move deeper into the plant, especially under hot, dry conditions, burrowing through several layers of leaves to feed on the heart leaves and heads on cabbage and the heads on cauliflower. Cabbage plants can tolerate substantial feeding before heading without yield loss, but feeding on the head or inner wrapper leaves significantly reduces quality and commercial value Broccoli and cauliflower are generally less tolerant than cabbage, and they can tolerate no direct damage once heads begin to form. Cabbage looper is generally the most difficult to control of the caterpillar pests of crucifiers.

Description and life cycle

Adult cabbage loopers are nocturnal moths. They are 2.5 cm (1 inch) long and grayish brown. Their mottled forewings are marked with a small, silver-white figure eight. The round white eggs are laid singly, mainly on the older leaves. Looper larvae are pale green caterpillars with a light stripe down each side. They grow to 4 cm (11/2 inches) in length. The caterpillars move with a characteristic "looping" motion produced by holding on with the front legs and arching the middle portion of the body to bring the prolegs (hind legs) forward, and then extending the front of the body while holding on with the prolegs. Pupation occurs in a loosely woven cocoon attached by one side to a plant leaf. Cabbage loopers do not overwinter in the upper Midwest, but adults migrate into the area from the South from mid-July through September.

Pest status

In the Midwest cabbage looper is normally a late-season pest of crucifers, with ,damaging populations occurring in August and September. The species is subtropical in origin (including subtropical areas of the United States), but it is now widely distributed throughout Eurasia and North America. In the United States it only overwinters along the Gulf coast and in the Southwest and then moves throughout the country from these breeding grounds each year. This insect can be found on many other crops in addition to all crucifers, including beet, celery, cotton, lettuce, mint, and potato.

Natural enemies

PARASITES

About 70 different insects parasitize the cabbage looper, but only a few of these are prevalent in the Midwest.

Trichogramma spp. These tiny wasps are the major parasites of cabbage looper eggs. They are very important in preventing major crop damage because they kill their hosts before the insects can cause plant damage. Natural parasitism of looper eggs is generally under 20% in crucifers, but supplemental releases have shown potential for suppressing cabbage loopers on venous crops in California, Missouri, and Florida. In some cases releases resulted in substantial control, but in other cases releases were ineffective. There are several species and strains of Trichogramma, which vary considerably in their ability to control the cabbage looper and in their adaptation to different environmental conditions and crops, but the differences are not well understood.

Trichogramma are readily available in large quantities from commercial suppliers. T. pretiosum is probably the most suitable species commercially available for field and vegetable crops .

Copidosoma floridanum (Ashmead) This small encyrtid wasp attacks eggs of the cabbage looper, but continues its development in the caterpillar, eventually killing the last-instar larva right after cocoon formation. The black adult wasp is about 1 mm (1/32 inch) long and is rarely noticed on plants . The female lays 1-2 eggs within the host egg. The wasp egg then divides to give rise to as many as 2,000 wasp larvae, which develop within the body of the growing, eating caterpillar. The caterpillar completes its development and spins its cocoon, but it fails to pupate. Instead, it curls into a characteristic S-shape and numerous wasp larvae can be seen inside the caterpillar body . Parasitized caterpillars feed longer than unparasitized ones, increasing the amount of damage done to the plant, but the wasp does prevent the cabbage looper from maturing and producing offspring.

Parasitism by this wasp is erratic, but it tends to increase as looper populations increase. Parasitism is higher in warm, dry years than in cooler, wetter years with fewer looper generations. In some cases there is no parasitism by this wasp.

Voria ruralis (Fallen). This tachinid fly attacks the cabbage looper caterpillar. The adult fly resembles a house fly, but is larger and more bristly . The female flies lay eggs on a caterpillar and one or more maggots develop within the host . Death does not occur until the caterpillar is fully grown, so the parasite reduces neither the feeding period nor the damage.

In Wisconsin parasitism of the cabbage looper by this fly is variable, ranging from 0% to 45% with an average of 16%. Natural populations do not provide effective control of cabbage looper on commercially grown vegetables. Supplemental releases of laboratory-reared flies have increased the effectiveness of this parasite in experimental tests, but the fly is not available commercially.

Cotesia marginiventris (Cresson). This small brown braconid wasp is native to the West Indies, but it is now one of the most abundant parasites of caterpillars in soybean and other crops throughout much of the southern and midwestern United States. It parasitizes many types of noctuid moth caterpillars, including the cabbage looper. The females will lay eggs in the first three larval instars, but they prefer to oviposit in 2-day-old first instars. A single wasp develops in each caterpillar in 7-8 days. The host caterpillar dies when the wasp larva emerges to spin a yellow cocoon nearby.

This wasp provides effective control when the rate of parasitism is high because it kills cabbage loopers when they are small, preventing a significant amount of feeding damage. Although this wasp is not normally found in crucifer plantings, it is available commercially for release to control cabbage looper. The wasps should be released weekly at a rate of 150-400 pairs per acre for three weeks beginning when the first cabbage looper adults are detected and an additional 100-200 pairs per acre weekly thereafter.

Steinernema (=Neoaplectana) carpocapsoe (Weiser). This nematode is parasitic on a wide variety of insects, including caterpillars. Foliar application, which is necessary for cabbage looper control, currently does not provide reliable results. Consequently, this nematode did not give good control of cabbage looper caterpillars in experimental tests. Improved encapsulation methods may increase its effectiveness.

PREDATORS

Predation is a major mortality factor for cabbage loopers, mainly for the eggs and small caterpillars. The most abundant insects that consume cabbage loopers are ladybird beetles. Other generalist predators that may be important include syrphid fly larvae, green lacewing larvae, minute pirate bugs, bigeyed bugs, paper wasps, and ground beetles (see Section 3). However, many of these feed primarily on aphids and therefore destroy few loopers when the preferred aphids are present.

Supplemental releases of some insect predators may be effective for controlling the cabbage looper in certain crops, but little research has been conducted in this area. In general, lacewing larvae released in the field as either eggs or young larvae do not survive well and are ineffective in suppressing cabbage looper on cabbage or collard. Experimental releases of the damsel bug Reduviolus alternates and the bigeyed bug Geocoris punaipes have reduced defoliation and increased yields in soybean.

Certain crops, such as white clover, strip planted among crucifers may provide a reservoir for large numbers of predators that can move into the cruciferous crop to prey on cabbage loopers.

PATHOGENS

The cabbage looper is susceptible to 20 species of pathogens, although most are not effective in regulating populations of this insect in the field. Following are the most important viral, bacterial, and fungal pathogens, which can be useful in reducing damaging populations of the cabbage looper.

Nuclear polyhedrosis virus (NPV). Cabbage looper populations can be infected naturally with NPV disease, but generally only after the pest has reached damaging levels. Virus infected loopers turn a chalky white and appear half-dead. They may climb to the top of the plant and lie on or hang from the leaves, where they eventually turn black and liquefy (figure 20). Birds and other animals may be important in dispersing the virus. Although the field efficacy and potential of the looper NPV as a microbial insecticide has been established, whether the material is used alone or in conjunction with chemical insecticides, it is not currently available commercially. Registration of this virus for commercial use is not anticipated in the near future.

A homemade virus solution for use on small plantings is recommended in Rodale's Encyclopedic of Natural Insect and Disease Control: collect diseases loopers (living white or dead black), liquefy with water in a blender, dilute to a sprayable consistency, and apply. Ten NPV-infected looper; should be enough to protect an acre. When environmental conditions are right, a single application of NPV will control loopers for the entire season.

Bacillus thuringiensis Berliner. Although is occurs naturally, Bacillus thuringiensis (Bt) does not cause epidemics by itself and therefore must be used as a microbial insecticide (see Section 3). Bt can provide good control of the cabbage looper on cabbage if applications are made when larvae are small. Pheromone traps can be used to monitor adult flight and plants can be scouted for eggs to determine the best timing of Bt applications. The cabbage looper, the diamondback moth, and imported cabbageworm are managed together as a single caterpillar complex, with regular crop scouting and treatments based on larval threshold levels (see "Management Practices for Caterpillars").

Nomuraea rileyi. This fungus is considered a potential microbial control agent, but it is not produced commercially in the United States. Under the appropriate environmental conditions natural populations of fungi can cause high amounts of caterpillar mortality. It requires high relative humidity to spread efficiently from infected caterpillars. Insufficient moisture at the time caterpillars die prevents the fungus from producing the spores that will infect other caterpillars. Weekly applications in experiments in South Carolina produced larval mortality and reduced damage, but were insufficient for control as none of the cabbage heads in the plots could be classified as USDA No. 1. Considerable research remains to achieve commercially acceptable control with this fungus.

Possibilities for effective biological control

The cabbage looper often is not well regulated by its parasites, and natural mortality is usually insufficient to keep loopers below economically damaging levels on crucifiers. Because it is native to southern parts of the United States, the possibility of obtaining biological control through the importation of natural enemies is very low. Supplementary releases of parasites, such as Trichogramma, or predators have potential for managing cabbage looper populations. Currently the best means of biological control of the cabbage looper is with the microbial insecticide Bt. Consult your state's Extension publications for specific recommendations.

Other alternative control methods

FLOATING ROW COVERS

Protective canopies of lightweight polyethylene or nylon material prevent adult moths from ovipositing on plants. The material is easy to stretch over the rows, and it is available from most farm or garden supply stores or mail-order firms.

PLANT RESISTANCE

No crucifers are truly resistant to attack by the cabbage looper. Differences in damage between varieties are usually due to the amount of foliage, stage of maturity, height, and general physical condition of the plant. However, loopers prefer some plants over others when several are available together. They lay fewer eggs on Chinese cabbage, mustard, rutabaga, and turnip than on cabbage, broccoli, or cauliflower when all are planted together. They seem to prefer red cabbage varieties over green varieties for egg laying.

HAND PICKING

Removal of caterpillars to prevent feeding damage may be practical in a small home garden. Inspect plants thoroughly and frequently to eliminate newly hatched caterpillars or those that may have escaped earlier detection.

REPELLENTS

Ground up cabbage tissue, looper brass liquefied in water, and extracts of other host plants deterred cabbage loopers from laying eggs on cabbage in laboratory experiments. No estimate of the potential benefits for control in the field can be made from this limited work.

Integrating cabbage looper control with other crop and pest management practices

Of the commercially available natural enemies, Trichogramma wasps are effective against the caterpillars (cabbage looper, diamondback moth, and imported cabbageworm). The nematode Steinernema (=Neoaplectana) carpocapsue (Weiser), which can be effective against cabbage maggot, may become feasible against cabbage looper and flea beetles with improved encapsulation methods. In small plantings, floating row covers can exclude cabbage looper, diamondback moth, and imported cabbageworm. Hand removal may also be practical against all the caterpillars. Green varieties of cabbage are less preferred by cabbage looper than are red varieties, although they are more susceptible to thrips and cabbage maggot.

Bt can adequately control cabbage looper if applications are made when larvae are small; this does not interfere with other crop and pest management practices. "Management Practices for Caterpillars" contains specific information on applying Bt. If large cabbage looper larvae are present, a chemical insecticide may be necessary to prevent economic damage. Such insecticide applications will affect other insects in the field, including natural enemies of the cabbage looper and other pests, so short-persistence materials should be used if possible to reduce the impact of the insecticide on beneficials.

ONION THRIPS Thrips tabaci Lindeman

Onion thrips Order Thysanoptera:

: Family Thripidae

Common thrips

Thrips are small insects that create whitish scratches or brownish blotches on cabbage leaves. Injury from thrips is most severe in hot ,dry weather Several species of thrips cause plant damage, but on cabbage the onion thrips is most common.

Damage

Adults and nymphs use a "punch and suck" feeding technique that leaves whitish scratches or bronzed blisters with a rough texture on cabbage leaves. This injury shows up as dark blotches on processed cabbage. The thrips may damage only certain layers of the outer third of the head, making detection difficult.. Heavy infestations may cause cabbage heads to be lightweight and misshapen. Description and life cycle

Adult onion thrips are very small (2 mm; 5/64 inch) and slender, pale yellow or brown, with wings fringed with long hairs. Eggs are inserted in the leaves. The nymphs resemble the adults but are smaller and lack wings. There are four nymphal instars. Thrips prefer tight spaces on plants, such as the closely appressed leaves of a developing cabbage head. Thrips overwinter in both the adult and nymphal stages in plant debris in fields or along margins. There are five to eight generations per year in the upper Midwest.

Pest status

The onion thrips is a cosmopolitan species that was first detected in North America in 1872. It attacks a wide variety of plants and is most severe in hot, dry weather. Severe damage occurs when thrips reproduce rapidly without being noticed inside cabbage heads. Detection of onion thrips in cabbage plantings is difficult because of their small size and reclusive habit. Thrips are commonly a problem in areas adjacent to winter wheat, oat, or alfalfa; they migrate to cabbage as the grain plants senesce or the alfalfa is harvested.

Natural enemies

Very few natural enemies are specific to onion thrips, and pathogens rarely affect this insect.

PARASITES

Most of the parasitic wasps that attack thrips are tropical or subtropical species that cannot survive winters in the upper Midwest. Therefore, they are a possibility only for releases or for establishment in greenhouse culture. Thnipobius semiluteus Boucek is a enlophid wasp that parasitizes larval greenhouse thrips; it is commercially available for releases in avocado in southern California, but no research has been conducted on its use in cole crops. Ceranisus (=Thripoctenus) brui (Vuillet), another enlophid wasp, occurs in Japan, Indonesia, Europe, and the West Indies. It was introduced and established in 1933-34 in Hawaii, where onion thrips are the most important vectors of yellow spot virus of pineapple. This species is not commercially available.

PREDATORS

Predatory phytoseiid mites and minute pirate bugs (Orius spp.) are the most common predators of onion thrips. Several other general insect predators will feed on thrips occasionally.

Neoseialus (=Amblyseius) barker) (= mckenziei) (Schuster and Pritchard). This tiny predaceous phytoseiid mite feeds on shops and other mites. The adult mites are about the size of the dot of an i. The mites glue their shiny white eggs to leaf hairs on the underside of leaves. The white, translucent larvae that hatch from the eggs are active, but non-feeding. The larvae develop into pale brown nymphs, and finally into light brown adults. Both the nymphal and adult stages are feeding stages, during which the mites are very active and are found on the plant in the same places that thrips prefer. Under warm conditions (25C; 77F), this mite develops from egg to mature adult in eight days. A single mite consumes two to three thrips each day, and about 85 thrips during its lifetime. The mites prefer thrips, but if thrips are not available they will eat spider mites, pollen, or other predatory mites. The mites do not injure people, animals, or plants. N. barken and another predatory mite, N. cucumens (Oudemans), are available commercially for thrips control on greenhouse and outdoor crops, especially cucumber. Both species must be applied to the crop as soon as an infestation is discovered, because the predators do not reproduce as fast as the thrips. Timely and regular applications of large numbers of predatory mites (300-600 mites per square yard), beginning at planting, are necessary to keep thrips populations below damaging levels.

In experimental releases on cabbage, the mites colonized mature cabbage heads, survived there, and reduced thrips populations in proportion to the number of mites released. These releases did not achieve commercially acceptable thrips control, but additional research might discover release methods to improve control.

Orius tristicolor (White). This species of minute pirate bug has a wide distribution in North America. The 3 mm1 (1/8 inch) adults are black with distinctive white markings on the wings, while the smaller nymphs are light brown. This bug is an important predator of many types of arthropods, but it prefers thrips and mites. Minute pirate bugs do not occur in high enough numbers to provide natural control of thrips on cabbage. They are available commercially as adults.

Possibilities for effective biological control

No effective means of biological control of thrips specifically on cabbage or other cole crops is currently available. Predatory mite releases have the potential for providing thrips control, but the potential for using a wasp parasite is limited.

Other alternative control methods

FIELD LOCATION

Avoid planting next to or down wind from wheat or alfalfa fields. Thrips tend to develop large populations on these crops and will migrate to cabbage as the grain plants senesce or when the alfalfa is cut.

PLANT RESISTANCE

Using cabbage varieties that are not highly susceptible to damage reduces shops problems. Although none of the varieties that have been tested are immune to thrips injury, some are more resistant to thrips damage. In general, thrips prefer varieties with extremely tight heads over those with loose heads. Fresh-market cabbages generally are tight-headed and tend to have more thrips problems than processing types used for sauerkraut. Red varieties often have fewer thrips than do green varieties. Varieties that have had less-than-average damage in field tests include Bravo, Brutus, Green Cup, Super Elite, Vantage Point, and Zerlina. The varieties Excel, Hinova, Little Rock, Market Prize, Quisto, Super Green, and Super Pack have had greater-than average thrips problems. Grand Prize had thrips problems only when left in the field too long.

TIMELY HARVEST

Backyard gardeners can avoid thrips problems by harvesting cabbage when the head is small, before thrips have had a chance to cause damage. If you harvest the heads too late to avoid damage, you could peel away and discard the affected layers. This is not an option for commercial growers who must meet weight standards (50 Ibs per 20 heads) and cannot sell peeled cabbage.

Integrating thrips control with other crop and pest management practices

The best means of avoiding thrips problems is by planting cabbage varieties that are less susceptible to infestation and damage. Red varieties are more resistant to both thrips and cabbage maggot than are green varieties, although they are more preferred by cabbage looper. Yellow sticky traps placed around the field are useful in monitoring the initial influx of thrips migrating into cabbage from many host crops. However, identification of thrips on sticky traps is often difficult and messy, and visual observation may be preferred. Because no effective biological controls are currently available, you must use insecticides when thrips infestations are heavy. Spray at cupping if possible. The insecticides currently registered for use against thrips are nonselective and will also kill beneficial insects that may be suppressing other pest populations.

CABBAGE APHID Brevicorynr brassicae (Linnaeaus)

Cabbage aphid

Order Hemiptera (Homoptera) Family Aphididae: Aphids

The cabbage aphid occurs every year in the upper Midwest, but it is usually a pest only later in the season (often after frost) as aphid numbers accumulate and the natural control of parasites and predators fails.

Damage

When aphid numbers are high, leaves cup or curl, plants become stunted and yellowed, and consequently cabbage plants do not form a marketable head. Heavy infestations on seedlings and small plants often result in death, and surviving plants have reduced yield potential. Another major problem is contamination with aphid bodies, which are very difficult to remove even with washing, especially on broccoli and cauliflower. Aphids may also transmit viruses, but viral diseases of cole crops are seldom a problem in this region.

Description and life cycle

Cabbage aphids are dull green to powdery blue, with a waxy covering that gives them a grayish white appearance. The aphids occur in large clusters on the undersides of the leaves, around the youngest leaves and flowering parts, o, in the centers of cabbage heads. Both winged and wingless adults occur, but the winged forms do not have the characteristic waxy coating. Each wingless female typically gives birth to 80-100 live young. Up to 15 generations are possible each season in the upper Midwest. Winged individuals can fly to other plants and are produced in response to crowding or deterioration of the food source. This insect overwinters as black eggs on remnants of its hosts.

Pest status

The cabbage aphid is a perennial pest of a number of cruciferous crops, with serious outbreaks occurring sporadically. It is widely distributed throughout the world, and it occurs throughout the United States and Canada on many cultivated and weed plants of the mustard family.

Natural enemies

PARASITES

Diaeretiella rapae (M'lntosh). This small aphidiid wasp is the most important parasite of the cabbage aphid. The dark brown adult female is 3 mm1 (1/8 inch) long (figure 21) and, over a lifetime, deposits an average of 85 eggs internally in separate aphids. It prefers half-grown nymphs over first instars or adults. The wasp larva that hatches from the egg consumes the body contents of the aphid. By the time the wasp larva is fully grown, the empty body of the host aphid turns into a hardened, light brown shell called a mummy . The adult wasp emerges through a circular hole cut in the back of the mummy. The parasite overwinters as fully grown larvae within host mummies. Although the cabbage aphid is its main host, D. rapae will also parasitize the green peach aphid (Myzas persicae), which may infest cruciferous crops.

Although this wasp parasitizes large numbers of cabbage aphids, it is ineffective in controlling the cabbage aphid because of lack of synchronization with its host. By the time the parasite population builds up to a significant level, the aphid numbers have already exceeded threshold levels. The effectiveness of the parasite is also reduced by hyperparasites, several species of wasps that develop on and kill D. rapae. Releases of this commercially available wasp could provide effective control.

PREDATORS

Ladybird beetles, syrphid fly larvae, lacewing larvae, and other predators are often effective in destroying cabbage aphids, especially small colonies (see Section 3). These predators seem to be favored by wet weather, which is unfavorable to the aphid. In England syrphids are the most effective control agent of the cabbage aphid.

PATHOGENS

Entomogenous fungi. Fungal epidemics cause cabbage aphid populations to decline, on occasion killing entire colonies. This occurs only when the weather is wet and/or humid, which is necessary for the development and spread of the fungi. Fungicide sprays may kill entomogenous fungi.

Possibilities for effective biological control

Natural enemy populations are often numerous enough to keep aphid infestations below economic levels. The most effective control agents of cabbage aphids vary according to environmental conditions. In some cases wasp parasites are the most effective; in other cases predators, especially Syrphidae, or weather conditions and fungi cause the most mortality. Occasionally the natural enemy populations do not build up fast enough to prevent heavy aphid infestations from causing damage.

Other alternative control methods

INTERPLANTING

Mixing cultivars reduced cabbage aphid populations in small broccoli field plots in California. Planting a preferred (tall) variety as a border 15 days before planting the main crop resulted in fewer aphids than planting a single variety. The taller varieties functioned as a protective barrier: as the aphids flew into the field, they settled on the tall border rather than on the shorter plants.

Intercropping with beans or grass reduced cabbage aphid infestations by over 6()% on Brussels sprouts and cabbage in England. Separation of rows of host and non-host plants by 50 cm (20 inches) or less gave the best results. Plant the intercrop earlier than the crucifies, so that the intercrop is large enough to disrupt the host-finding behavior of the aphids by the time the crucifer is planted.

Some organic gardeners suggest planting mint or laying old, woody mint plants as a mulch among crucifers to repel aphids, but this has not been scientifically proven.

More natural enemies are found in weedy plots than in weed-free crops, and aphids are more attracted to crops grown on a background of bare soil, which is normal agricultural practice. Weeds among a crop, even in small numbers, may be advantageous in both reducing aphids and increasing natural enemy activity, although there are obvious agricultural disadvantages in allowing a vigorous weed cover.

SANITATION

Destruction of crop remnants at the end of the season will reduce Overwintering cabbage aphid eggs and thus the subsequent year's population. Around field borders, eliminate alternate host plants on which the aphid can overwinter, including mustards and related weeds.

PHYSICAL REMOVAL

A fine, forceful spray from a garden hose can knock some of the insects off the plants and reduce aphid populations.

REPELLENTS

Some organic gardeners suggest that sprinkling finely ground limestone or diatomaceous earth over plants can drive away or kill aphids. This method of aphid control has not been scientifically proven.

INSECTICIDAL SOAP

Insecticidal soap applied at recommended rates is very effective in controlling cabbage aphids. Sprays must cover the aphid bodies to be effective. There is no residual activity after sprays have dried.

Integrating cabbage aphid control with other crop and pest management practices

Wasp parasites, predators such as ladybird beetles and syrphid fly larvae, and fungal epidemics often achieve biological control of the cabbage aphid naturally. Weedy, no-till plots encourage the presence of these natural enemies and are less attractive to both cabbage aphid and diamondback moth. However, in addition to the obvious agricultural disadvantages of allowing a weed cover, weedy plots may increase flea beetle populations. Destroying and removing crop residues after harvest eliminates Overwintering sites not only for cabbage aphid, but also for diamondback moth and cabbage maggot.

When aphid numbers are high enough to cause damage, an insecticide treatment may be necessary. If possible, apply treatments in the early stages of infestation. Insecticidal soap does not have significant effects on other insects. Most chemical insecticides registered for use against cabbage aphid will affect other insects, including the natural enemies of the aphid and of other pests. Aphid parasites within the mummified body of their aphid hosts are the least susceptible to insecticides, offering an opportunity for selective timing of insecticide application. If parasites are to survive when insecticides are applied to the crop, applications should be timed so that the maximum number of parasites are within the mummified aphids. Experimentally, the use of certain less toxic or less-persistent insecticides allowed higher wasp parasite survival within treated mummified aphids. Combining low-toxicity insecticides with selective timing can help preserve naturally occurring parasites to prevent aphid resurgence after insecticide treatment in the field.

FLEA BEETLES Phyllotreta spp

Flea: beetles Order Coleoptera: Beetles : Family Chrysomelidae: Leaf beetles

Flea beetles are occasional pests of cole crops in the Midwest, especially in fields that are weedy or surrounded by weeds. The feeding of the adult beetles causes most of the damage.

Damage

The adults chew out small circular holes or pits in the leaf tissue. Flea beetles may cause plan stunting or death when they occur in large numbers Ol1 seedlings, although they usually do not cause economic damage on older plants. Larvae of most species feed on roots, but usually they are not handful.

Description and life cycle

Several species of flea beetles will feed oil cruciferous crops. The most common species the cause damage On cabbage are Phyllotreta cruciferae (Goeze) and the striped flea beetle, P. striolata (Fabricius). The adults are small, hard beetles that range in size from 2 to 6 mm (1/16 to 1/4 inch) in length and may be jet black, black and yellow striped, or metallic blue-green depending on the species. They all have large hind legs enabling them to jump considerable distances when disturbed. The very small, white, cigar-shaped eggs are laid in the soil. The larvae, which feed o roots, have thin white bodies and brown head capsules. The white pupae are enclosed in earthen cells. Most species have one or two generations per year, entering into diapause in August. They overwinter as adults in the soil or under plant remains.

Pest status

Flea beetles can cause serious damage to young plants, but seldom to mature plants. P. striolata was introduced to North America before 1801 and was widespread across the continent by the early 1900s. P. Cruciferae was introduced on the west coast of North America it the early 1920s and is now prevalent throughout the Midwest and the canola-growing prairie provinces of Canada. Neither beetle is an important pest in their native Europe. These species feed primarily on cruciferous plants.

Natural enemies

Flea beetles may be parasitized by wasps and nematodes and attacked by beetles, but these natural enemies are not common. No pathogens are known to affect this insect.

PARASITES

Microctonus vittatoe Muesebeck. This native braconid wasp attacks adult beetles. It is generally distributed throughout the eastern half of the United States. The small black females (3 mm 1/8 inch) lay fertile eggs without mating; therefore males are extremely rare. The females insert eggs into the midsection of adult flea beetles, and the single wasp larva develops within the beetle body. The full-grown larva pushes itself out the end of the beetle to pupate in a silken cocoon in the soil. Not only does the emergence of the wasps kill the adult beetles, but also the larval wasp feeding sterilizes female flea beetles, which greatly enhances this species' impact on the flea beetle population.

M. vittatae normally causes less than 5% parasitism, but in some areas levels may reach 45%. It is considered the major factor controlling striped flea beetles from north of the Mason Dixon line into southern Canada.

The related European braconid Townesilitus (=,Microctonnus) bicolor (Wesmael) parasitized up to 50% of adults in summer in Germany. Releases in Canada did not result in establishment of the parasite.

Nematodes. Several naturally occurring nematodes affect flea beetles in Europe, where parasitism is sometimes as high as 90%. In the Midwest, an all allantonematid , Howardula phyllotratae Oldham, infests less shall 3% of adult P. striolata. This nematode only reduces the reproductive capability of the host but does not kill the beetle. A mermithid nematode also occurs but exerts no significant control.

The nematode Steinernema (=:Neoaplectana) carpocapsae (Weiser) is capable of infecting flea beetles. However, field applications of nematodes in water suspension had no effect on larval P. cruciferae populations because the infectivity of the nematodes declined sharply within a few days of the treatment. New encapsulation technology may increase nematode persistence enough that S.carporapsae could have an effect on flea beetle populations.

PREDATORS

Many species of general predators occasionally feed on adult flea beetles, but none exert any significant pressure on flea beetle populations. These insects include lacewing larvae; the melyrid beetle, Collops vittatus Say; a bigeyed bug, Geocoris Mullahs (Say); the western damsel bug, Nabis alternatus Parshley; the spined soldier bug, Podisus maculiventris (Say); a nabid bug, Nabicula americolimbata (Carayon); and the northern fall field cricket, Gryllus pennsylvanicus Burmeister.

Possibilities for effective biological control

Few natural enemies of flea beetles provide substantial control. Importation of parasites from the beetle's native Europe holds some promise. Although the one attempt to introduce a European wasp in Canada was unsuccessful, improved release methods and sites could enhance the chances for success. Also, other parasite species may be found that are more suitable. Nematodes appear to have limited potential as biological control agents for crucifer flea beetles. Other alternative control methods

EARLY PLANTING

Planting early in the spring may avoid high populations of adult flea beetles when plants are small and most susceptible to damage.

PLANT SELECTION

Flea beetles seem to prefer certain species or varieties over others. In general cauliflower is the most preferred, followed by turnip, radish, and cabbage. Certain commercial cultivars of broccoli, cabbage, and other crucifers were considered resistant in experimental plantings. However, the varieties available to the adult flea beetles and the stage of plant development will modify host-plant preference.

SANITATION

Flea beetles are often a problem in weedy areas. Removal of weeds in and around the field may help reduce flea beetle populations.

INTERPLANTING

Planting tomatoes in rows adjacent to crucifers interfered with the orientation of flea beetles to their host plants in experimental plots.

Integrating flea beetle control with other crop and pest management practices

No effective biological controls for flea beetles are currently available. Insecticides are the only alternative for controlling damaging populations. The insecticides registered for use against flea beetles are broad spectrum and will kill other insects, including natural enemies of other crucifer pests. Treatment of small, very susceptible plants with non-persistent materials will reduce the negative impact on beneficials.

Some general cropping practices may help keep flea beetle populations below damaging levels. Early planting and harvesting reduces potential infestations of both flea beetles and imported cabbageworm. Clear cultivation seems to deter flea beetles, although weedy no-till plots are more effective against diamondback moth and cabbage aphid. Intercropping tomatoes with crucifers is somewhat effective against both flea beetles and diamondback moth. Of the commercially available natural enemies, the nematode Steinerneina (=Necaplectana) carpocapsae may have a limited effect against flea beetles; with improved encapsulation methods it may be more effective as a foliar application against cabbage looper as well.

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