Adult butterflies do not cause any damage to plants. Large white larvae (yellow and black patterned caterpillars) will destroy small brassica plants and severely damage larger plants. Large infestations can often defoliate whole brassica crops if left untreated. Small white larvae (green caterpillars) feed singly and will often burrow deep into head forming brassicas causing holes and contaminating leaves with excrement.
You can use netting to prevent egg laying by adults if you frequently have trouble with cabbage white caterpillars in spring. 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. Check any exposed outer leaves regularly for cabbage white butterfly eggs and remove these by hand.
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.
Planting nasturtiums close to a netted brassica crop will attract cabbage butterflies away from the crop to lay their eggs. Caterpillars can then be managed and controlled. Netting should be raised above the brassica leaves to prevent butterflies laying their eggs through the holes. The mesh size also needs to be small enough to prevent butterflies from crawling through.