Cabbage root fly
All Brassicas and related vegetable species - turnips, swede and radish are attacked. Affected plants grow slowly and tend to wilt on sunny days. Brassica seedlings are vulnerable especially shortly after transplanting. Soil borne white legless maggots feed on the finer roots until just the stump remains. Maggots also bore into the swollen roots of turnip, swede and radish.
Cultural measures: Prevent damage immediately after transplanting young Brassicas by covering developing plants with horticultural fleece to exclude adult flies from laying eggs in the soil at the roots of brassicas.
Discs of cardboard approximately 10cm diameter can be placed on the soil around the stems of the transplants. The female flies will deposit her eggs on the discs, and not in the soil. The eggs will dry out before they have a chance to hatch.
There is currently no chemical control for this problem.