Use a lightweight row cover in early spring as a barrier to egg-laying butterflies. Spray with Bacillus thuringiensis (Bt), especially when catepillars are small. Be sure to direct spray to undersides of leaves.
Photograph by Whitney Cranshaw, Colorado State University, Bugwood.org
Found throughout the U.S., the cabbageworm is the larva of a common white butterfly with three to four black spots on its wings. The damage done by these caterpillars is similar to that of the cabbage looper -- the pests chew large, ragged holes in the leaves of cabbages, broccoli, Brussels sprouts, and cauliflower, and may bore into the heads, leaving trails of dark green frass (droppings). There are several generations per year.