Cabbage caterpillars

Caterpillars are the larval stage of various butterflies and moths. There are several species of caterpillar that feed on cabbages, other brassicas and other plants including turnip, swede, horseradish and nasturtiums. Large cabbage whitebutterfly caterpillars are yellow and black with obvious hairs on their bodies (see picture above). Those of small cabbage white butterfly are pale green and covered in short, velvet-like hairs. Cabbage moth caterpillars are yellowish green or brownish green, with no obvious hairs on their bodies.

Symptoms
Holes are eaten in the outer leaves of all brassicas and damage may also be seen on the inner leaves of cabbages when the heart is cut through. Caterpillars and their excrement are often found on the plants.
There are three common caterpillar culprits:
Large cabbage white butterfly (Pieris brassicae)
Small cabbage white butterfly (Pieris rapae)
Cabbage moth (Mamestra brassicae)

All three may be present at the same time. Caterpillars of cabbage moth and small white butterfly are more damaging as they bore into the hearts of cabbages, whereas the yellow and black caterpillars of the large cabbage white stay mostly on outer leaves.