Cabbage Root Fly
On Crops: Broccoli, Brussels sprouts, cabbage, cauliflower, mustard, radish, swede, turnips and oil seed rape
Worldwide, especially in cool northern climates
Adult cabbage root flies are dark grey in colour and closely resemble common houseflies. There are always two generations of adults each year, occurring in early May and mid August. However, if conditions permit, a third generation can occur. The adults lay tiny white bullet-shaped eggs around and on the main stem, at the soil line. Eggs hatch after 3-7days into cream-coloured legless maggots.
When eggs hatch, cabbage root fly larvae tunnel into roots. Plants wilt and shrivel when feeding is heavy. When you pull up wilting plants, rice-size white maggots can be seen feeding on the roots.
Netting or horticultural fleece is effective in preventing egg laying by adults. However, rather than covering the plants, it is sufficient to have vertical row cover “fences” on either side of the row, because adults locate host plants by flying low, from side to side. When only a few plants need protection, cut a round of cardboard, cloth or heavy paper, slit it to the middle, and install it as a skirt around the plant’s main stem (a 'cabbage collar'). This simple method protects seedlings from egg laying by adult cabbage root flies. Ground beetles are important natural predators, and they are encouraged by mulch. Cabbage root flies are most problematic in late summer, when populations are high enough to damage autumnal crops of cabbage, cauliflower, and closely related crops.
Remove badly affected plants and wash their roots in warm water to remove maggots. Allow them to drown or feed them to chickens.
Regularly hoe and dig around the base of overwintering host plants and also where brassica plants have been growing. This exposes the cabbage root fly pupae to insectivorous birds and predatory beetles.