If the plants are still young and leafy, you might want to use something containing B.t. (Bacillus thuringiensis kurstakii) which is a natural bacteria that kills moth and butterfly larvae--ie. caterpillars--and nothing else. If they are starting to head up, you might need to use something containing spinosad, which is also a natural insecticide, and probably the only thing that can get inside the head without poisoning you! Spinosad is toxic to bees, so spray after sunset, when the bees are safe in the hive. Best of luck!