Cabbageworms are the larvae of the familiar white cabbage butterfly. The butterflies lay eggs on brassica leaves, which hatch into velvety green caterpillars that chew increasingly large, irregular holes through the foliage. Heavy feeding can strip young plants, reduce growth and contaminate leaves and developing heads with caterpillar droppings. Several generations can occur during the growing season.

How to Fix It
Hand-pick: Check leaves regularly and remove caterpillars and eggs by hand.
Insect netting: Cover plants before butterflies begin laying eggs and keep the edges securely sealed.
Encourage beneficial insects: Parasitic wasps and other predators naturally attack cabbageworms. Avoid broad-spectrum insecticides that kill these beneficial insects.
Bt: Bacillus thuringiensis var. kurstaki (Bt) is a biological insecticide that works well against young caterpillars. It must be eaten, so apply it thoroughly to foliage where caterpillars are feeding.
Spinosad: Spinosad is a biologically derived insecticide that can control cabbageworms when infestations are too heavy for hand-picking.