Imported cabbageworms (also known as cabbage whites or small whites), cabbage loopers and diamondback moths are the three most significant caterpillar pests of Wisconsin cole crops, with the imported cabbage worm being the most significant. Diamondback moths are worldwide pests of cabbage and leafy greens, and have developed resistance to numerous insecticides and several products containing Bacillus thuringiensis (Bt). The cabbage looper attacks beets, celery, lettuce, peas, potatoes, spinach and tomatoes, in addition to cole crops.

Damage caused by these pests is generally of little economic importance in Wisconsin.

Identification

Imported cabbageworm adults, commonly referred to as the white cabbage butterfly, are white butterflies with black markings on the wing tips. Female butterflies have 2 black dots on each fore wing; males, which are smaller, have 1 dot per wing. Eggs are yellow and conical, laid individually on the leaf surface and occasionally on the stem. An adult butterfly can lay 300 to 400 eggs in her lifetime. Larvae appear as velvety green worms up to 1 inch long with a faint yellow stripe running down the back. The caterpillar is commonly found along the veins of leaves and easily blends into the foliage.

Cabbage looper eggs are round, white and about the size of a pin head. Eggs are typically laid singly on the outer fringes of lower leaves. Larvae hatch 3 to 6 days after being laid. Larvae are pale green with a white stripe down each side of the body. The cabbage looper reaches 1 to 1½ inches in length and has a distinctive green head capsule. It gets its name from the way it arches its body as it moves. The adult cabbage looper is a grayish-brown, night-flying moth with a wingspan of 1½ inches. The mottled brown fore wings are marked near the middle with a characteristic small, silver-white figure 8 or letter Y.

Diamondback moth eggs are tiny, flat, circular and cream-colored, laid singly or in small clusters on the leaves. A single diamondback moth can lay up to 300 eggs in her lifetime, but probably average about half that number. The larvae are small in size (up to 3/8 inches long at maturity) pointed at both ends and range in color from green to yellow. The diamondback larvae are commonly found on the leaf surface and will wiggle back and forth when disturbed, often falling from the plant. Adults are small grayish-brown, night-flying moths with a 1-inch wingspan. Diamond-shaped markings on the wings are evident when the wings are folded over the back at rest.

Behavioral differences between the caterpillars can aid in their identification. The cabbageworm will usually remain motionless when disturbed, whereas, the diamondback moth will wiggle its body around. The cabbage looper will either inch away or try to defend itself by whipping its head around.

Life cycle

Imported cabbageworms overwinter as chrysalae on plant debris and usually produce 3-6 generations in a season. Butterflies emerge in early May and begin laying single, small, yellow-orange eggs on any plant part that is above ground. The eggs hatch in about 5 days. The larve develop on cruciferous weeds and cole crops that are early planted. The caterpillar feeds and develops for approximately 11 to 20 days before forming a pupa from which the adult butterfly emerges after 6 to 11 days. Second generation butterflies emerge mid-July and larvae develop almost entirely on cultivated cole crops. This generation causes the most damage.

Cabbage looper adults overwinter in the south and migrate to Wisconsin from mid-July through September. In July, female moths lay single white eggs on the lower surfaces of leaves. Female moths can lay 200 to 300 eggs, usually over a 10 to 12 day period. Larvae hatch 3 to 6 days after being laid and begin to feed immediately. Larvae feed for up to 5 weeks before pupating. Moths emerge 10 to 14 days later, mate and lay their eggs, giving rise to the second generation, which causes the most damage to cole crops.

Diamondback moths overwinter as adults and are therefore an early season pest. However, cold winters increase mortality (except in protected sites). In early spring, females lay eggs on weeds in the mustard family. The first instar larvae (larvae between molts) mine between the leaf surfaces. After completing four larval stages they spin white silk cocoons on the lower portion of the plant. There are typically 3 to 5 generations of diamondback moths per year in Wisconsin.

Damage

Larvae of all three insects feed between the large veins and midribs on cole crop leaves. The imported cabbageworm will feed on all ages of leaves but prefers the younger leaves.  They feed along the edges of the leaves, leaving only thick veins behind. The cabbage looper often feeds between veins on the underside of lower leaves. Large loopers will make larger holes in the foliage and can burrow through 3 to 6 layers of tightly wrapped head leaves in cabbage. A good indicator of the presence of loopers and imported cabbageworms is fresh frass (droppings) on leaves.

Diamondback moth larvae initially mine the plant tissue and consume only the middle layer of the leaves. As larvae grow, they exit the leaves and consume all tissues except the thin layer of transparent wax. This type of damage resembles windowpanes and is characteristic of this pest.

Severe feeding damage stunts cabbage and cauliflower heads. Larval damage to the developing bud on young cabbage can cause the head to abort. Head boring by cabbage loopers is also common in early cabbage and can result in unmarketable heads. The copious quantity of greenish-brown frass, or excrement, produced by the larvae is problematic because it contaminates heads and foliage.

Scouting suggestions

Scout fields weekly throughout the season for damage. Check plants carefully, even if no feeding damage is apparent, to look for eggs that will hatch into small caterpillars several days to a week later. Examine the lower leaves of the plant for the larvae of each pest. Although feeding damage and fecal material are signs of activity, it is better to rely on larvae counts to determine the level of infestation. Caterpillars cause varying amounts of damage depending on the plant’s maturity, so the need for treatment changes as the crop grows. Keep a record of which insect is present, its life stage and the percentage of plants infested. This information will be useful for monitoring whether the population is increasing or decreasing.

Treatment thresholds are well established and based on the percent of infestation by any lepidopteran species. Economic thresholds (ETs) vary based on the stage of crop development. Cabbage, broccoli and cauliflower in the seedbed are particularly susceptible to damage. Therefore, control measures are warranted when 10% of the plants are affected. Between transplant and cupping, the ET is raised to 30%,  from the time plants begin to cup until early heading, if more than 20% of plants are infested, treatment is warranted. From early heading until harvest, the threshold again drops to 10% to protect market quality of the produce.

When you transplant broccoli or cauliflower and it produces flowers or curds, increase this threshold to 50%. When flowers or curds begin to develop, the threshold again drops to 10% to maintain marketable quality.

Degree day modeling

The development of many insects can be modeled using degree days, which are calculated from daily low and high temperatures. Visit the Vegetable Disease and Insect Forecasting Network (VDIFN) website for a convenient map-based interface for viewing many degree day insect models.

Control

Integrated pest management is the sound use of all available methods for insect control. These include cultural, biological and chemical controls. Effective IPM programs for caterpillars should be designed to prevent damage, encourage natural control and avoid resistance. Current insect management recommendations include scouting and precise timing of insecticide sprays. Insecticides should only be applied when the insect pest is present at damaging levels, present at a vulnerable stage of its life cycle, or present during a critical stage of crop development.

Cultural

The use of transplants free of larval contamination is a key step in avoiding damage. Turning under crop residues after harvest and rotating cole crops to different fields each year may disrupt the lifecycles of the caterpillar pests and other key pests of cole crops. Removal of weed hosts closely related to cole crops, including mustards, removes an additional food source for these caterpillars. It also prevents them from establishing populations before the cultivated crops are transplanted into the field.

Biological

Biological control occurs regularly in Midwest fields and can be highly effective in controlling populations of insect pests that feed on cole crops. The three caterpillar pests are all susceptible to parasitism and predation by natural enemies throughout the growing season. For specific guidelines on biological control, refer to the UW-Extension, Nutrient and Pest Management Program publication BioIPM Cole Crops Workbook.

Chemical

Bacillus thuringiensis (Bt) applied to early instar larvae can be very effective in controlling imported cabbageworms. There are more than 20 types of Bt, most of which are highly selective against caterpillars. Bt does not directly affect predators or adult wasp parasites and therefore does not disrupt the activity of these beneficial insects. Target early instar larvae and ensure good plant coverage to improve the effectiveness of insecticides. Use pest-specific insecticides in early to mid-season when diamondback moths and cabbageworms are prevalent. Spinosans are another reduced-risk insecticide option. Spinosans are biologically based materials that are quite selectively active on caterpillar pests but are safe to beneficials. Many chemical insecticides are also effective in controlling caterpillar pests of cole crops.

Refer to the UW-Extension publication Commercial Vegetable Production in Wisconsin (A3422) for a list of registered insecticides and management recommendations.

Other considerations

Resistance is of concern with all lepidopteran pests on cole crops. Extensive resistance to organophosphate, pyrethroid, and carbamate insecticides has been documented in the diamondback moth. Resistant larvae are easily transported into Wisconsin on transplants. Diamondback moth resistance to Bt has also been documented in parts of the United States, but has not been reported in Wisconsin.

The choice of pesticide, as well as its timing and placement can have a big impact on beneficial insect species as well as on the development of resistance in the pest species. Ways to manage these factors include: keeping pesticide treatments to a minimum; choosing a selective insecticide whenever possible, avoiding broad spectrum compounds; choosing pesticides with little or no residual activity; and spot treating or banding insecticides rather than broadcasting the applications to minimize non-target effects.

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