Cabbage loopers are the larvae of mottled grey-brown moths. The moths lay eggs on leaves, which hatch into green caterpillars that move by arching their bodies into a distinctive loop. Young larvae make small holes, while larger caterpillars chew large, ragged sections from leaves. Heavy infestations can substantially defoliate plants, interfere with head development and contaminate vegetables with caterpillar droppings.

How to Fix It
Hand-pick: Inspect leaves and growing points and remove caterpillars and eggs by hand.
Insect netting: Cover plants before moths arrive and seal the edges to prevent egg-laying.
Remove crop debris: Clean up infested plants and brassica debris after harvest.
Encourage beneficial insects: Parasitic wasps and other natural enemies can substantially reduce looper populations.
Bt: Bacillus thuringiensis var. kurstaki is effective against young loopers and has relatively little impact on most beneficial insects. Apply it to foliage the caterpillars are actively eating.
Spinosad: Spinosad can control heavier infestations when additional treatment is necessary. Avoid spraying flowers or when bees are actively foraging.