GreenlyCabbage root fly: how to recognise and treat it
Pest
Symptoms
- Brassicas grow poorly, wilt and may die, especially recent transplants in early summer.
- White, legless maggots up to 9 mm long feed on the roots and stem base; the tunnels let rots in.
- Swede, turnip and radish roots are tunnelled and ruined.
- Occasionally the maggots tunnel into Brussels sprout buttons.
Organic treatment
- Pull up and remove plants that are already infested.
- Water on a biological control of insect-killing nematodes sold against root-fly larvae; it can also harm other soil insects.
- Encourage ground beetles, rove beetles, birds and hedgehogs, which eat the maggots.
Chemical treatment
- No insecticide is a reliable cure — once plants are infested the maggots cannot be dependably controlled with pesticides, so rely on collars, covers and rotation.
Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.
Prevention
- Fit a collar 8–15 cm across (carpet underlay, roofing felt or card) snugly round each transplant stem so flies cannot lay eggs at its base.
- Grow under insect-proof mesh or fleece kept on all season, and rotate the bed so flies do not emerge under the cover from pupae in the soil.
- Plant transplants rather than sowing direct, and check plants often from spring.
- Do not overload the bed with manure — the flies prefer to lay eggs in rich soil.
Plants it affects
Sources
Figures checked against these sources.