Imported Cabbageworm on Vegetables

Imported cabbageworm with frass. Photo: Whitney Cranshaw, Colorado State University, Bugwood.org

Updated: February 20, 2023

Appearance

Eggs: Tiny, white to yellowish colored, bullet-shaped, ribbed eggs usually laid singly, on end.
Larvae: Velvety green caterpillars with a single yellow stripe down the center of the back, and yellow spots or a broken yellow line along each side. About 1 1/4" long in final instar stage.
Pupae: Chrysalis about 1" long with one dorsal and two side "fins." Color varies, but usually matches background: yellow, gray, green, brownish are common.
Adults: Butterfly with 2" wingspan. Wings are white on top, usually with black tips and 2 or 3 black spots, and yellowish underneath.

Overwinter as pupa or chrysalis in plant debris and emerge as butterflies in spring.
Butterflies are very active, flitting from host plant to weedy flowers, sipping nectar and laying eggs on the undersides of outer leaves of host plants.
Larvae feed on leaves and sometimes bore into broccoli or cabbage heads.
Pupae are attached to food sources or nearby objects by silken threads.
Two to four, or even more, generations are produced per year.

Cole crops (a.k.a. crucifers or brassicas), including cabbage, broccoli, Brussel sprouts, cauliflower, collards, kale, turnip, and radish, etc., and many related weeds.

Larval feeding causes damage; adult butterflies feed only on nectar, causing no damage.
Larval feeding initially produces small, smooth feeding holes in leaves, which soon enlarge until the entire leaf is consumed, leaving only the midrib.
Entire plant may be defoliated. Damaged young plants may be unable to produce heads.
Larvae sometimes bore into heads.
Large amounts of dark green fecal pellets will be scattered on or beneath the plants.

Be alert for small white butterflies flitting among cole crops.
Watch for irregular feeding holes that enlarge. Leaves or entire plants may be skeletonized.
Well-camouflaged larvae can be spied resting on leaf midribs.
Look for dark green fecal pellets under areas of feeding activity.
To determine if larvae have bored into heads, soak harvested heads in salty water. Larvae will float to the surface.

Remove plant debris after the growing season to eliminate overwintering sites.
Floating row cover prevents egg-laying.
Fast-maturing cultivars sustain less damage.
Handpick larvae.
Bacillus thuringiensis (Bt), an "organic" microbial insecticide, is highly effective on younger caterpillars.
Spinosad, neem, and pyrethrum are effective "organic" insecticides. Brassica leaves are slick, so add a spreader-sticker to the sprayer tank.
Many different diseases, predators, and parasitoids reduce larvae numbers naturally. (Pollen-rich flowers attract predators.)
Avoid broad-spectrum insecticides, like Sevin, that harm beneficial insects, and birds.
Larvae infected with granulosis virus or nuclear polyhedrosis virus can be collected, soaked in water, and made into a microbial insecticide spray to infect healthy larvae.
Turnip, rutabaga, and kale are less favored by cabbageworms.