try using netting or and spray with old dishwater

thankyou x

Look for the caterpillar eggs on the underside of leaves and squash them before they hatch. If you're too late for that, pick off the caterpillars and dispose of them.

Only use dish water if your washing up liquid is eco friendly like Ecover otherwise it will not be doing the cabbage or the soil any good. You can make or buy garlic spray which deters the insects from getting near your cabbages.