Imported Cabbageworm

Authors: PJ Liesch and Russ Groves, UW-Entomology
Revised: 01/30/2026
Item number: XHT1032

The imported cabbageworm (Pieris rapae) is the most important pest of brassica/cole crops in Wisconsin. All cole crops, including cabbage, broccoli, Brussels sprouts, cauliflower, and rutabaga, are susceptible to attack by this insect.

Appearance: Imported cabbageworm adults are the white butterflies commonly seen flying in great numbers on warm summer days. Female butterflies have 2 black dots on each forewing, while the smaller males have only one dot per wing. Larvae are velvety green worms up to one inch long, with a faint yellow stripe running down the back and a wrinkly appearance when viewed up close. While the adults can be conspicuous as they fly, the caterpillars blend in on plants and are the destructive life stage of this insect.

Symptoms and Effects: Imported cabbageworm larvae feed on the leaves of brassica/cole crops between the large veins and midribs. Feeding occurs primarily on the upper leaf surface near the midrib producing large, irregular holes. As older imported cabbageworm larvae move toward the center of the plant, they may remove all but the main leaf veins. Severe feeding damage will stunt cabbage and cauliflower heads. Larval damage to the developing buds on young cabbages can cause heads to abort. Imported cabbageworm damage to root crops is generally of little economic importance. The copious quantity of greenish-brown frass (droppings) produced by larvae can also be a problem as it contaminates heads and foliage.

Life Cycle: Imported cabbageworms overwinter as pupae on plant debris. The adult butterflies emerge in spring and begin laying small, yellow-orange eggs singly on any above-ground plant part. Within a week, the eggs hatch. The larvae develop on cruciferous weeds and early-planted cole crops. The second generation butterflies emerge in mid-July and larval development occurs almost entirely on cultivated cole crops. This generation causes the most damage. There are usually three generations per season with the second generation being the most damaging to cole crops grown in Wisconsin.

Scouting Suggestions: Scout fields weekly throughout the growing 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 plants for the larvae as well. Although feeding damage and droppings are signs of activity, it’s better to rely on larval counts to determine the level of infestation. Caterpillars cause varying amounts of damage depending on the maturity of the plant, so the need for treatment changes as the crop grows. Keep a record of the 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 in commercial situations and are based on the percent infestation by any caterpillar pests and varies based on the stage of crop development. Cabbage, broccoli, and cauliflower in the seed bed are particularly susceptible to damage and therefore when 10% of the plants are affected with cabbage loopers, diamondback moths, or imported cabbageworms, control is warranted. For cabbage between transplant and cupping, the ecomonic threshold (ET) is raised to 30%. Once the plants have begun to cup, until early heading, if greater than 20% of plants are infested, treatment is warranted. From early heading until harvest, the threshold drops back to 10% to protect the market quality of the produce. For broccoli and cauliflower between transplant and first flower or curd, the threshold is increased to 50%. However once flowers or curds begin to develop, the economic threshold drops back to 10% to maintain a high level of quality.

Management: Effective integrated pest management (IPM) programs for imported cabbageworm should be designed to prevent damage, encourage natural control, and avoid resistance.

Non-chemical management: Many non-chemical options exist for managing imported cabbageworms and other caterpillar pests.

Start clean: Starting your own crops from seeds indoors is low-risk for imported cabbageworms. However, if you obtain transplants from elsewhere (e.g., greenhouse or other supplier), check plants to make sure they are free from imported cabbageworm and other pests.
Good clean-up: Since imported cabbageworm can overwinter amongst plant debris, good clean-up in fall and/or plowing in spring can reduce the possibility of overwintering adults.
Physical barriers: Fine mesh netting (e.g., row cover) can be used as a physical barrier to keep adults from reaching crops to lay eggs. This is often most effective in home gardens and other small-scale growing situations.
Hand-removal & squishing: In small-scale settings, hand-picking, squishing, or knocking caterpillars into a container of soapy water can be a consideration. Keep in mind that imported cabbageworms and other caterpillars on cole crops tend to blend in well on the greenish foliage. While inspecting plants, keep an eye out for the tiny, pale, individually-laid eggs, which can also be crushed by hand.
Encourage natural enemies: Many natural enemies (predators and parasites) will attack imported cabbageworms. These are often quite effective in preventing buildups of cabbageworm populations. Keep in mind that the use of broad-spectrum pesticides can harm populations of these natural enemies, which could hinder their activity.

Chemical management: A number of options exist for both small-scale and commercial situations. For best results, target early instar (small) larvae and ensure good plant coverage to improve efficacy when using insecticides. Use pest-specific insecticides in early to mid-season when imported cabbageworms are prevalent so that natural enemies are conserved. Resistance is a key concern with all lepidopteran pests on cole crops.

Home gardeners have a number of spray options available, including organic options such as insecticidal soaps, horticultural oils (e.g., neem oil) and pyrethrins. One particularly useful organic ingredient is Bacillus thuringiensis var kurstaki, which can be very effective at controlling small imported cabbageworms. Conventional sprays for homeowners include ingredients from the pyrethroid group (e.g., cypermethrin, permethrin, etc.) and carbaryl.

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