The adult is a white, day-flying butterfly with 3 to 4 black spots on its wings. The larvae is a 1 1/4" green caterpillar with one yellow stripe. Bullet-shaped eggs are laid singly on leaves. Several generation occur per year. Larvae overwinter in the ground and emerge in late spring. I never gave them time to make it to the ground. >:|
The larvae chews foliage and produces soft green excrement. A sure sign that you have the pest. The female butterfly flies around from plant to plant, depositing her eggs at the base of leaves. In a week, tiny caterpillars hatch and begin to chomp leaves. It only takes a couple nights for the worms to devour your plants. They get big fast and defoliate your plants completely in a week. They eventually bore their way into the main head of the cabbage, turning it to mush.
Affected plants include cabbage, broccoli (I had this pest this past year. Fun to knock them off the plant and squash them with a rock. I'm demented sometimes.), brussels sprouts, cauliflowre, chinese cabbage, collards, kale, kohlrabi, radish, rutabagas, and turnips.
Bt is your best control. Also good is net barriers placed on plants early in the season. Handpicking is fun if you don't have many plants.
Rye flour can be sprinkled on damp leaves. Worms eat the coating and bloat up and die. That should be a pretty site!
Fall cleanup should be a rule.
Intercrop with catnip and tansy to keep them off your important crops. Use pantyhose cover to cover the heads as soon as they start to form. The nylon stretches and keeps out the butterflies.
Plant resistant varieties like Mammoth Red rock, Savoy, and Savoy Chieftain cabbages.
Attract birds and toads as well as parasitic wasps, lacewings, lady bugs, and ground beetles.
Another thing you could try is putting cornmeal around the cabbage family plants. This also bloats the worm.
In fall, winter and early spring, till your ground so that grubs and larvae are pulled out of the ground where birds can see them. Attract birds with feeders in winter and keep them fed so that they stick around. It's necessary to till the ground at least 2 times but preferably 3. If you don't usually till, then dig a shovel/fork into the dirt to locate the worms.
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