Try Resistant Varieties
Cabbage varieties resistant to cabbageworm include Mammoth Red Rock, Savoy, and Savoy Chieftain.
Encourage Predators
Animal Predators -Bluebirds, chickadees, and English sparrows relish eating the butterfly of the imported cabbage worm. Birds rarely eat the larvae or worms because of their toxic body fluids.
Insect Predators and Parasites Found in the Garden - Lacewings, lady bugs, ground beetles, parasitic wasps, and yellow jackets all prey upon imported cabbage worms. The butterfly milkweed (Asclepias tuberosa) hosts a parasite of cabbage worms, but a substance in the plant's leaves may harm some garden crops.
Natural Diseases - If you find cabbage worms that have turned chalky white and appear nearly dead they have probably been infected with nuclear polyhedrosis virus (NPV).