Cabbage Root Fly
Pest · Commonly affects: Cabbage, Cauliflower, Calabrese, Brussels sprouts, Kale, Kohlrabi, Swede, Turnip
Cabbage root fly is a common brassica pest whose larvae feed unseen around roots. A plant may look well one day and wilt badly the next, especially during dry weather. It is most serious on newly planted cabbages, cauliflowers and sprouts, but a simple collar and fine mesh cover can make a substantial difference in home vegetable plots.
How to identify it
Adults are grey flies, about 5-7 mm long, resembling small houseflies but rarely noticed. The damaging larvae are white, legless maggots about 7-10 mm long, found at or within roots. Watch for young brassicas that turn bluish-green, wilt in sunshine, grow poorly or fail to firm into the soil. Gently lifting a struggling plant may reveal brown, tunnelled roots and pale maggots nearby.
Damage
Maggots tunnel through roots and the stem base, restricting water uptake and causing wilting, stunting or death. Young transplants can collapse completely. Older plants may survive but grow slowly, form smaller heads or become more vulnerable to drought and wind. Root wounds may also allow rot to develop, particularly in wet ground.
Prevention
Fit close-fitting square collars, 12-15 cm across, around each transplant stem so flies cannot lay eggs at the base. Cover the bed with fine mesh or horticultural fleece immediately after planting, sealing every edge. Raise sturdy plants in modules before transplanting, firm them well and water them in. Rotate brassicas around the plot, clear crop remains promptly, and avoid planting brassicas in the same area year after year.
How to manage it
Lift and remove plants that are beyond recovery, along with any visible maggots, before they complete development. For plants with only light damage, firm soil around the stem, water in dry weather and add a little soil to encourage extra roots. Check collars remain flat and replace damaged covers. At season's end, remove all brassica roots and debris rather than leaving potential pupae in the bed.
Cabbage Root Fly: frequently asked questions
How do I identify cabbage root fly?
Adults are grey flies, about 5-7 mm long, resembling small houseflies but rarely noticed.
How do I prevent cabbage root fly?
Fit close-fitting square collars, 12-15 cm across, around each transplant stem so flies cannot lay eggs at the base.
How do I get rid of cabbage root fly?
Lift and remove plants that are beyond recovery, along with any visible maggots, before they complete development.
General guidance for UK home gardens and allotments, focused on prevention and non-chemical control. For a confirmed diagnosis, the RHS or a local garden centre can help. Page updated 19 Sep 2026.