How to Control Cabbage Worms

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Cabbage worms are the green caterpillars that chew holes through the leaves of cabbage, broccoli, kale and related crops. Most control comes down to three steps: scout early and often, block the white butterflies that lay the eggs, and remove the larvae by hand or with a labeled biological spray. This guide explains how to control cabbage worms using two university extension pages, one from Wisconsin and one from Maryland. Both name the same main pest, and both describe non-chemical methods first.

Quick answer

Scout weekly. Look for tiny pale eggs on the leaves and for dark green droppings under the plants.
Cover young plants with fine mesh or floating row cover so the butterflies cannot lay eggs on them.
Handpick larvae, or knock them into a container of soapy water on a small planting.
If you use Bacillus thuringiensis (Bt) or another spray, follow the product label and aim it at small caterpillars.

How we chose these sources

We picked two extension pages that cover the same pest and come from two separate universities. The University of Wisconsin page is an integrated pest management article by UW-Entomology authors. The University of Maryland page is a vegetable insect resource listed under its Home and Garden Information Center. Both were opened on 2026-10-29 and read in full. Neither is a retail page, and neither was chosen because it sells a product.

The two pages agree on the identity of the pest, the damage it causes and the main non-chemical controls. They differ on a few details, such as how many generations occur in a season. The draft reports those differences instead of picking one version. Where a claim appears in only one source, the draft says so.

Identify the caterpillar before you act

Most people who search this topic are looking at green caterpillars on cole crops, and the first job is to confirm that they are the right pest. The Wisconsin page names the imported cabbageworm (Pieris rapae) as the most important pest of brassica and cole crops in that state. It says cabbage, broccoli, Brussels sprouts, cauliflower and rutabaga are all susceptible. The larvae are velvety green worms up to one inch long, with a faint yellow stripe down the back. The page adds that the caterpillars blend in on the plants, which is why the damage is often noticed before the worm is.

The Maryland page gives a closer description of the final larval stage. It says the caterpillar is about 1 1/4 inches long and is velvety green, with a single yellow stripe down the center of the back and yellow spots or a broken yellow line along each side. The adult is a white butterfly with a two-inch wingspan, usually with black tips and two or three black spots on the wings. The adults are the butterflies commonly seen flying over the garden on warm days.

The adult butterfly is not the one that damages the plants. For the life change from caterpillar to adult, see how caterpillars turn into butterflies during metamorphosis. The Maryland page says that adult butterflies feed only on nectar, causing no damage, and that the larvae are the feeding stage. The Wisconsin page makes the same point when it says that the caterpillars are the destructive life stage. Once you know that, the control plan can focus on the eggs and the larvae.

Other caterpillars can look similar on the same plants. The Wisconsin page lists cabbage loopers and diamondback moth larvae as separate pests that also attack cole crops. This guide covers only the imported cabbageworm, because that is the pest both pages describe in detail. If the caterpillar you find does not match the descriptions above, compare it with the pest descriptions on the extension page for that insect before you choose a control.

Read the damage and the droppings

The feeding pattern is the first clue. The Wisconsin page says that imported cabbageworm larvae feed on leaves between the large veins and the midribs. Feeding happens mostly on the upper leaf surface near the midrib and produces large, irregular holes. As older larvae move toward the center of the plant, they may remove everything except the main veins. The Maryland page describes the early stage differently. It says that larval feeding first makes small, smooth holes in the leaves, which soon enlarge until the whole leaf is eaten, leaving only the midrib. An entire young plant may be defoliated.

Droppings are the second clue. The Wisconsin page describes a copious quantity of greenish-brown frass, and it says that frass can contaminate heads and foliage. The Maryland page says that large amounts of dark green fecal pellets will be scattered on or beneath the plants. Look under the outer leaves, because that is where the pellets collect. A dark spot pattern under a feeding area is a stronger sign of larvae than a single hole.

Heads need a separate check. The Maryland page says that larvae sometimes bore into broccoli or cabbage heads. Its method is to soak the harvested head in salty water. Larvae will float to the surface. The Wisconsin page also notes that larval damage to developing buds on young cabbages can cause the heads to abort.

Scout every week and watch the eggs

Scouting is the step that makes the rest of the plan work. The Wisconsin page says to scout fields weekly throughout the season, and it says to check plants carefully even when no feeding damage is apparent. The reason is that the eggs are small and laid singly. The page says they hatch within a week, and that the larvae may take several days to a week after you see the egg to appear. So a plant with clean leaves today can have larvae next week.

The Wisconsin page says to examine the lower leaves, because larvae are often found there. It also says that feeding damage and droppings are signs of activity, but that larval counts are a better guide to how serious the infestation is. Keep a simple record of the life stage you see and the share of plants that are infested. That record shows whether the population is rising or falling from one week to the next.

The Wisconsin page gives treatment thresholds as a percentage of infested plants, and it says those thresholds are well established for commercial situations. The thresholds change with crop stage, so a seedbed, a cupping plant and a plant at harvest each have a different number. The page does not give a home-garden threshold, so this guide does not offer one. The scouting habit is useful for any size of planting even without a number to compare against.

Block the butterflies with row cover

Physical barriers are the first defense that both pages describe. The Wisconsin page says fine mesh netting, usually called row cover, can keep the adult butterflies from reaching the crop to lay eggs. It adds that this approach is often most effective in home gardens and other small-scale plantings. The Maryland page gives the same advice in shorter form: floating row cover prevents egg-laying.

Because row cover works by stopping the egg-laying step, it is most useful before eggs appear. Once larvae are already on a plant, the cover does not remove them. If you put cover on a young crop, lift it and check the plants for eggs and larvae on the same weekly schedule as the scouting walk. Neither page gives a removal date for row cover, so follow the cover manufacturer’s instructions for securing it and check your crop’s needs at each stage.

The Wisconsin page also recommends a clean start. Starting your own plants from seed indoors is low risk for imported cabbageworms. If you buy transplants from a greenhouse or another supplier, check them before planting so that you do not carry larvae or eggs into the garden. This step costs little and removes one common route into the bed.

Handpick larvae on a small planting

For a small planting, the Wisconsin page says hand-picking, squishing, or knocking caterpillars into a container of soapy water can be a practical control. The Maryland page lists handpicking as a control in its own right. Both pages stress that the caterpillars blend in well with the greenish foliage. That means you have to look under the leaves and along the midribs rather than scanning the top of the plant.

The Wisconsin page adds a useful point: while you inspect the plants, look for the tiny, pale, individually laid eggs, which can also be crushed by hand. The Maryland page says the butterflies lay their eggs on the undersides of the outer leaves, so that is where to look first. Crushing eggs and larvae at the same time means one walk through the bed does two jobs.

Handpicking is most realistic when the planting is small. The Wisconsin page frames it as a small-scale setting option, and it is not presented as a practical control for a large commercial field. If you find larvae every week on the same plants, that is a sign that the cover or cleanup steps need attention as well.

Use Bt and other sprays with the label in hand

Both pages name Bacillus thuringiensis as a useful option against these caterpillars. The Wisconsin page says that Bacillus thuringiensis var kurstaki can be very effective at controlling small imported cabbageworms. The Maryland page calls Bt an organic microbial insecticide that is highly effective on younger caterpillars. Both pages therefore point to the same timing: Bt works best when the larvae are still small, so spray early in the infestation rather than after the leaves are already full of holes.

The Wisconsin page lists other home-garden sprays, including insecticidal soaps, horticultural oils such as neem oil, and pyrethrins. The Maryland page lists spinosad, neem and pyrethrum as effective organic insecticides. It also notes that brassica leaves are slick, so a spreader-sticker in the spray tank can help the product stay on the leaf. The Wisconsin page says that good plant coverage matters for these products to work. Read the label of any product you buy, follow the rate and the interval it gives, and check whether it is listed for vegetables before you use it on a crop you plan to eat.

Both pages warn against broad-spectrum insecticides. The Maryland page names Sevin as an example of a product that harms beneficial insects and birds. The Wisconsin page says that the use of broad-spectrum pesticides can harm natural enemies of the caterpillar and hinder their activity. A narrow product used at the right stage usually leaves more of the natural controls in place.

Let natural enemies and cleanup do part of the work

Natural enemies are part of the plan, not an extra. For background on how plants respond to leaf-feeding insects, see how plants defend against insect herbivores. The Wisconsin page says that many predators and parasites attack imported cabbageworms, and that these natural enemies are often quite effective at preventing large buildups. The Maryland page says that many diseases, predators and parasitoids reduce larvae numbers naturally, and it notes that pollen-rich flowers attract predators. Leaving a few flowering plants near the cole crops can therefore help, as long as you avoid broad-spectrum sprays on those flowers.

Cleanup also affects next season. The Wisconsin page says that imported cabbageworm overwinters as pupae on plant debris. The Maryland page says the same thing, describing pupae attached to food sources or nearby objects by silken threads. The Wisconsin page recommends good cleanup in the fall, or plowing in spring, to reduce the number of overwintering adults. The Maryland page recommends removing plant debris after the growing season to eliminate overwintering sites. Both steps reduce the population that starts the next year.

Varieties matter too. The Maryland page says fast-maturing cultivars sustain less damage. It also says that turnip, rutabaga and kale are less favored by cabbageworms. Those notes do not replace scouting, but they can shape what you plant in a bed that has a history of trouble.

Plan around the generations

The number of generations per season changes when the peak damage happens. The Wisconsin page says that there are usually three generations per season in Wisconsin, and that the second generation, which emerges in mid-July, is the most damaging to cole crops grown there. The Maryland page gives a wider range and says that two to four, or even more, generations are produced per year. The two statements are not in conflict, because the number depends on the climate and the length of the season.

A practical plan is to start the scouting walk when the first butterflies appear in spring and keep it going until the crop is harvested. The Wisconsin page says that the adults emerge in spring and begin laying eggs.

What we could not verify

Both pages are useful, but they leave some gaps. The draft does not fill them with guesses.

What the sources do not say

The Wisconsin page gives treatment thresholds for commercial fields only. Neither page gives a home-garden threshold for when to spray.
Neither page names a specific brand, rate or interval that a home gardener should use. Both name classes of products and ask the reader to follow the label.
Neither page compares row cover with Bt or with handpicking in numbers. The draft does not rank the methods.
Neither page says how long row cover should stay on a crop, or when it should come off for pollination or harvest.
The Maryland page does not say how reliable the salt-water check is for finding larvae in a head. It gives the method only.
The generation counts are regional. The Wisconsin count is for Wisconsin, and the Maryland range is for the Maryland area. Your own season may differ.

Frequently asked questions

Are cabbage worms and cabbage loopers the same pest?

No. The Wisconsin page lists imported cabbageworm, cabbage loopers and diamondback moth larvae as three separate caterpillar pests of cole crops. This guide covers only the imported cabbageworm, which both pages describe in detail.

Do the white butterflies damage the plants?

The Maryland page says that adult butterflies feed only on nectar and cause no damage. The damage comes from the larvae that hatch from the eggs the butterflies lay.

Is handpicking enough on its own?

The sources present handpicking as a practical control for a small planting, not as a complete plan for a large one. The Wisconsin page lists it alongside row cover, cleanup, natural enemies and sprays, and it recommends using several methods together.

How can I tell whether larvae have bored into a cabbage or broccoli head?

The Maryland page says to soak the harvested head in salty water. Larvae will float to the surface. The method is described on the page without a concentration, so check a trusted source for the exact salt amount if you plan to use it.

Does row cover alone solve the problem?

Neither page says that row cover alone is enough. The Wisconsin page lists it as one of several non-chemical options, and it recommends scouting throughout the season even when cover is in place.

Sources

UW-Madison Division of Extension, Wisconsin Horticulture, “Imported Cabbageworm” (XHT1032, published 1/30/2026): https://hort.extension.wisc.edu/articles/imported-cabbageworm/ (opened 2026-10-29)
University of Maryland Extension, “Imported Cabbageworm on Vegetables” (updated July 22, 2024): https://extension.umd.edu/resource/imported-cabbageworm-vegetables (opened 2026-10-29)