How to Get Rid of Cabbage Worms on Broccoli, Kale, and Cabbage

What Cabbage Worms Are and How to Stop Them

Cabbage worms are the caterpillars of the imported cabbageworm, the white butterfly with a 2-inch wingspan that drifts over broccoli, kale and cabbage beds from spring to fall. To get rid of them, cover the bed with lightweight row cover at planting so the butterfly cannot lay eggs, pick off any caterpillars you find, and spray Bacillus thuringiensis (Bt) while the larvae are still small. University of Maryland Extension lists exactly those three measures and adds spinosad as a second spray.

The caterpillar is velvety green with a single thin yellow stripe down the center of its back, and it grows to about 1 1/4 inches. Its color matches a brassica leaf so closely that most gardeners see the damage first. Feeding starts as small holes, which widen until only the midrib of the leaf is left. Dark green pellets of frass collect on the leaves and under the plant.

The butterfly is easy to recognize. The wings are white on top with black tips and yellowish underneath. University of Wisconsin Extension notes that females have 2 black dots on each forewing and the smaller males have one. The butterflies do no feeding damage themselves, but each one seen over the bed means eggs are being laid.

The Cabbage Worm Life Cycle

The insect passes the winter as a chrysalis attached to plant debris, and butterflies emerge from it in spring. In Illinois they first appear in mid-April and remain a problem until mid-September, according to University of Illinois Extension. A female lives about three weeks and lays 300 to 400 eggs, as reported by the University of Florida.

Each egg is laid alone, standing on end, on a leaf or other above-ground part of the plant. It is bullet-shaped, ribbed, white to yellowish, and about 1 millimeter long. Wisconsin’s fact sheet says the eggs hatch within a week.

The caterpillar passes through five growth stages. UC IPM puts the feeding period at 2 to 3 weeks, after which the larva attaches itself to a stem or a nearby object with a few strands of silk and becomes a chrysalis about 1 inch long. The chrysalis usually takes the color of its background, so it may be green, yellow, gray or brownish. In summer the University of Florida gives the pupal stage as about 11 days.

The University of Florida puts the complete cycle at three to six weeks, depending on weather. That short turnaround is why the pest comes in waves. The University of Florida reports three generations a year in New England, three to five in California and six to eight in the South. In Wisconsin there are usually three, and the second, whose butterflies emerge in mid-July, does the most damage.

Cabbage Worm, Looper or Diamondback?

Three green caterpillars feed on the same crops, and they are often present together. Clemson and Utah State University Extension describe them this way:

Caterpillar	Full size	How to tell
Imported cabbageworm	About 1 1/4 inches	Velvety green, thin yellow stripe on the back
Cabbage looper	1.25 to 1.5 inches	Arches its body into a loop as it moves
Diamondback moth larva	0.5 inch	Wiggles hard when disturbed, leaves window-pane patches

Size matters because damage scales with it. Clemson’s fact sheet rates one cabbage looper as doing roughly the damage of 5 diamondback caterpillars, and an imported cabbageworm as doing about two thirds the damage of a looper. Row cover, hand-picking and Bt work on all three, so a rough identification is enough. If the holes in your leaves do not match any caterpillar, compare them against the leaf damage identification chart.

Cabbage Worms on Broccoli

Broccoli tolerates leaf feeding well while it is young and poorly once the head forms. The larvae burrow into the heads of broccoli and cabbage, where they and their frass end up in the part you eat.

Wisconsin’s scouting thresholds reflect that shift. From transplanting until the first flower buds appear, broccoli and cauliflower can wait for treatment until 50% of plants are infested. Once heads begin to develop, the threshold drops to 10%. For cabbage the figures are 30% from transplant to cupping, 20% from cupping to early heading, and 10% from early heading to harvest. Seedlings in a seed bed warrant control at 10%.

In a home bed of 6 or 8 plants, that translates to a simple rule. Tolerate a few holes in the outer leaves early on, check every plant once a week, and act on any caterpillar you find once a head is forming. Check plants even when no damage shows, because eggs present today hatch into small caterpillars within a week.

Cabbage Worms on Kale

On kale the leaf is the harvest, so holes and frass matter from the start. The same applies to collards and other leafy brassicas. The University of Florida lists broccoli, Brussels sprouts, cabbage, cauliflower, collards, horseradish, kale and kohlrabi among the hosts.

Kale has one advantage: it is picked before it flowers and never needs bees. Crops that do not need insect pollination can stay under a cover until harvest, so a kale bed can be protected for its whole season. On uncovered plants, look over both surfaces of the leaves once a week, rub off the 1 millimeter eggs, and pick caterpillars from along the midribs, where they tend to feed.

Row Cover Goes On at Planting

Floating row cover prevents egg-laying, which makes it the one control that works before any damage is done. Utah State University Extension advises placing covers over the plants, or over metal hoops, just after planting or transplanting and burying the edges securely in the soil. Before closing the cover, look over each transplant for eggs or small larvae that came with it.

Choose the lightweight grade. University of Maryland Extension describes it as 0.45 ounce per square yard with 90% to 95% light transmittance, which suits a cover that stays on for weeks. Our guide to row covers for pest protection compares the grades and covers hoops and anchoring.

Hand-Picking and Bt

Hand-picking is practical in a small planting. Check weekly, pull caterpillars off and crush them or knock them into a container, and wipe off eggs as you go.

When there are too many to pick, Bt is the standard spray. The variety sold for caterpillars is Bacillus thuringiensis var. kurstaki. Utah State University Extension explains that it must be eaten by the caterpillar and works best when caterpillars are young and actively feeding. It has a short residual and is non-toxic to mammals and to beneficial parasitoids and predators. In practice, spray when you first see small larvae, cover the leaf surfaces where they feed, and expect to repeat the spray as later generations hatch. The 1 1/4 inch caterpillars are easier to remove by hand.

Spinosad is the usual second choice, and UC IPM rates both Bt and spinosad as very effective against this pest. Follow the product label for rates, timing around bees, and the interval before harvest. The same pick-then-Bt approach applies to other garden caterpillars, including tomato hornworms.

Natural Enemies and Fall Cleanup

Several insects kill cabbage worms without help. The best known is Cotesia glomerata, a small parasitic wasp whose larvae develop inside the caterpillar. The University of Florida describes dead caterpillars surrounded by clusters of 20 to 30 wasp cocoons. Leave those where they are, since each cluster releases more wasps. Utah State University Extension also lists shield bugs, ambush bugs and many birds as predators.

Wisconsin’s fact sheet counts encouraging natural enemies among its non-chemical controls, and Bt fits with that because it spares them. Our guide to companion herbs for pest control covers plantings that go with this approach.

Cleanup breaks the cycle over winter. The chrysalis overwinters on crop debris, so removing old brassica stalks and leaves at the end of the season, and tilling where you till, reduces the number of butterflies that emerge in the first spring generation. With three or more generations following in most regions, a smaller first flight means fewer caterpillars all season.