Disappointing that cabbage root fly has been found at the plots.
Recent transplants can be given some protection by placing a brassica collar around the base of the stem. These can be purchased from garden centres or they can be made from circles or squares, about 8-15cm (3¼-6in) across, using carpet underlay, roofing felt, cardboard or similar materials. The collar prevents the female fly placing eggs on the soil surface close to a host plant. Eggs deposited on the collar often dry up and fail to hatch
Plants can be protected by growing them under the cover of an insect-proof mesh. Horticultural fleece may be preferred for seedbeds as it will also help warm the soil
Crop rotation should be practised. Otherwise cabbage root flies can emerge from overwintered pupae in the soil under the fleece or mesh cover, if host plants are grown in the same piece of ground in successive years