Adult moths do not cause any plant damage. The caterpillars feed within the outer leaves of brassica plants and often burrow deep inside the developing head.
You can use netting tunnels to prevent egg laying by adults if you frequently have trouble with cabbage moths. Plant lots of flowers and blooming herbs around your cabbage patch to provide a strong supply of nectar for beneficial insects, such as garden wasps, parasitic wasps and insect eating birds. Cabbage moths overwinter as pupae in the soil, so regularly turning over the soil during the winter months will expose the pupae to insectivorous birds.
Check plants regularly, especially for the appearance of holes in the leaves and caterpillar excrement. Remove caterpillars by hand or use a hose daily to knock them off the plants and expose them to ground-dwelling predatory beetles and insectivorous birds. As a last resort, if a spray is required then organic pyrethrum-based products are available from garden centres. These need to be applied following label instructions, safety warnings and harvest intervals.
Netting should be raised above the brassica leaves to prevent moths from laying their eggs through the holes. The mesh size also needs to be small enough to prevent moths from crawling through.