Wireworms are a possibility, but cabbage root maggots are more likely. These are little white maggots. You should find wireworms if they are culprit. I don't know of any chemical treatment that is available to a home gardener for either pest. Rotation with corn or grasses may help.

Here is a link that might be useful: wireworm, cabbage maggot

Peter, you can try beneficial nematodes against wireworms and other soil-borne pests. It has worked well for me in the past.