The maggots of cabbage root fly eat the roots of cabbages and other brassicas, as well as tunnelling into the roots of swedes, turnips and radish.
Recent transplants can be given some protection by placing a brassica collar around the base of the stem. These can be bought from garden centres or they can be made from circles or squares, about 8-15cm (3¼-6in) across, using carpet underlay, roofing felt or cardboard. The collar prevents the female fly placing eggs in the soil surface close to a host plant. Eggs deposited on the collar often dry up and fail to hatch.
Plants can also be protected by growing them under the cover of horticultural fleece, or an insect-proof mesh such as Ultra-Fine Enviromesh. Horticultural fleece may be preferred for seedbeds as it will warm the soil.
Crop rotation must be practised, otherwise cabbage root flies will emerge from overwintered pupae in the soil under the fleece cover if host plants are grown in the same piece of ground in successive years.
Nemasys Grow Your Own is a mixture of pathogenic nematode species that is sold as a biological pest control for use against cabbage root fly larvae and other pests, including the larvae of carrot fly, onion fly, leatherjackets, chafer grubs, sciarid flies, caterpillars, gooseberry sawfly, thrips and codling moth.
None of the pesticides currently available to amateur gardeners are suitable for use against cabbage root fly.
There are three generations during the summer but it is the first generation in late spring-early summer that is often the most damaging.
Adult cabbage root flies resemble house flies in size and appearance.
The larvae are white, legless and headless maggots that are up to 9mm long. They feed on the roots and can kill seedling and recently transplanted brassicas. Later generations are less damaging to cabbages and other leafy brassicas, as older plants have larger root systems and are better able to tolerate the damage. Host plants where the root is the edible part, such as radish, turnip and swede, are damaged by any of the generations.
When fully fed, the larvae go into a brown pupal stage in the soil, either emerging as adult flies a few weeks later or remaining in that state overwinter.