"Several types of worms attack cabbage, collards and their relatives. All of them can be controlled with Bt (Bacillus thuringiensis), a natural bacteria that paralyzes the digestive system of the worms. It has no effect on other animals or humans. The worms have to eat some of the Bt, and it usually takes several days for them to die.
Another option is to keep the plants covered with floating row cover fabric to prevent the adult moths from laying their eggs on the plants in the first place. "