Diamondback moths lay tiny eggs on brassica leaves. The small green caterpillars are the damaging stage and often wriggle vigorously or drop from the plant on a silk thread when disturbed. Young larvae scrape leaf tissue, creating pale translucent “windowpanes”; as the damaged tissue dries and falls away, small holes appear. Larger populations can damage growing points and developing heads. Several generations can occur in a single season, allowing populations to increase rapidly.

How to Fix It
Hand-pick: Check leaf undersides and growing points regularly and remove caterpillars when you find them.
Insect netting: Cover plants before moths arrive to prevent egg-laying.
Remove crop debris: Clean up brassica plants and debris after harvest so they don't continue supporting larvae.
Encourage beneficial insects: Predatory and parasitic insects provide important natural control, so avoid unnecessary broad-spectrum insecticides.
Bt: Bacillus thuringiensis var. kurstaki is most effective against young larvae. Thorough coverage is important because the caterpillar must eat the treated foliage.
Spinosad: Spinosad can be used when additional control is needed. Use it carefully around flowering plants because it can harm bees while the spray is wet.