The larvae (maggots) of the cabbage root fly eat the roots of all brassicas – not just cabbages. This reduces the strength of the plant, causes them to wilt and, in the case of small plants, usually eventually die.
Adult cabbage root flies look like house flies. Cabbage root flies affect the brassica family of cabbages, Brussels sprouts, cauliflower and broccoli. They can also tunnel into the roots of swedes, turnips and radish.
The leaves of affected plants are edged blue and often wilt. The white maggots can be very damaging as they eat the roots and stem base. This then leads to wilting and the plants won't recover when watered.
Treatment and control
There are no chemical controls for cabbage root fly.
Place brassica discs or collars around stems when transplanting seedlings to prevent the adult female fly laying her eggs at the base of the stem.
Plants can be protected from attack by covering plants with horticultural fleece, or an insect-proof mesh.