Cabbage root fly. Never had it so no idea what you can do about it sorry.

Sounds like cabbage white butterfly Caterpillars sometimes called cutworms have a look at the link below

http://www.bbc.co.uk/gardening/advice/pests_and_diseases/identifier.shtml?cabbage_white_butterflies

Unfortunately the best thing you can do is remove the plants and destroy them - do not compost or add to your green recycling bin as there will be grubs in them. Turn the soil in the area over so that the birds can get at any remaining grubs and plant new seedlings in a different area.

If its cabbage root fly you can buy or make little collars to sit round the stems at soil level that prevent the fly from laying its eggs there.
Cabbage white caterpillars are usually found on the leaves rather than the stems.