|Cabbage Root Fly|
The cabbage root fly, Delia radicum, is found all over Europe. They are 0.5-1cm long, grey, and resemble common houseflies. As the weather warms up in spring the flies hatch from over-wintering pupae in the soil.
The flies feed on nectar and lay small white eggs (1mm in diameter) near the surface of soil next to brassica plants. After about 6 days eggs hatch producing white maggots which eat the finer roots of brassica plants and which may tunnel into their main stem. Larvae feed for about 3 weeks and when fully fed may be around 0.9-1cm in length. Fully-grown larvae form reddish brown pupae in the soil. These hatch into the adult flies after another 20 days. Around three generations of eggs may be laid each year between mid-spring and early autumn. The generations overlap resulting in the continuous presence of flies.
The larvae of the cabbage root fly attack cauliflowers, broccoli, brussels sprouts, spring and autumn cabbage, savoy cabbage and kale. They may also burrow into radish, swede and turnip. Affected plants tend to grow more slowly and may wilt on sunny days. Adult plants survive attack quite well but young transplants are vulnerable and may be killed.
To prevent attack by cabbage root flies
Place squares of cardboard, roofing felt, or carpet around the stems of newly transplanted brassica seedlings. Make a cut from the edge of the square (10cm diameter) into the centre. Then place the square around the transplant such that soil around the young brassica plant is covered. Flies lay eggs on the squares instead of the soil. Exposed eggs dry up and die.
Encourage populations of predatory beetles as these eat cabbage root fly larvae.