Today 
There are two culprits: the caterpillars of both the small and large cabbage white butterflies. The first is pale green with a body up to 2.5cm long, the second is bigger at 4cm and is yellow with black markings. The adult butterflies lay eggs at different times: the small from February to April and again in late-summer, the large in April and May. This means the caterpillars can be busily munching away from early spring through to autumn.
Solution
Organic
Cover plants with horticultural fleece to prevent butterflies reaching the leaves. Check plants regularly and pick off any caterpillars that do make it through. Spray young caterpillars as they hatch with the biological control Steinernema carpcapse.
Chemical
With heavy infestations, use an insecticide based on pyrethrum or bifenthrin (only use once a season on crops).
