There are many types of caterpillars which are usually the larval (caterpillar of a grub) stage of moths such as cabbage moth (Plutella xylostella) and small cabbage white butterfly (Pieris rapae).
Cabbage white butterfly is white with distinct black spots on the wings and is around 40mm across. The cabbage moth is greyish and small and is around 10mm across.
The moths and butterflies lay their eggs on the underside of leaves. The larvae of caterpillars hatch from the eggs and then feed on the leaves or fruit, so it’s the caterpillar of the cabbage white butterfly or cabbage moth which does the damage.
The blue-green smooth textured caterpillar is that of the cabbage white butterfly, while the caterpillar which is green-brown is that of the cabbage moth. They both start eating the outer leaves before moving to the inner heart of the cabbage.
It is a good idea to try and either pick off the caterpillars or spray them when they are still on the outside of the leaf, before they tunnel into the heart. Plants favoured by cabbage moth and cabbage white butterfly include cabbage, broccoli, Brussel sprouts, Chinese cabbage, celery, beetroot, rocket and watercress.
Large holes in outer leaves. Bluey-green frass on the inside of the leaf or in the heart of the cabbage, which is actually their droppings.