Cabbage root fly

The maggots of cabbage root fly eat the roots of cabbages and other brassicas, they can also tunnel into the roots of swedes, turnips and radish.

Cabbage root fly.

Quick facts

Common name Cabbage root fly
Latin name Delia radicum
Plants affected Cabbages, cauliflower, Brussels sprout and other brassicas, oriental greens, swede, turnip, radish
Main symptoms Plants grow slowly, wilt and die as their roots are eaten. Edible roots are also tunnelled by the maggots
Caused by Larvae of a fly
Timing April to September

What is cabbage root fly?

Adult cabbage root flies resemble house flies. The larvae are white, legless and headless maggots that feed on the roots and can kill seedling and recently transplanted brassicas.

Symptoms

Vegetables affected by cabbage root fly can show the following:

  • Cabbages and other brassicas make poor growth and may wilt and die, especially recent transplants in early summer
  • Root crops, such as swede, turnip and radish are ruined when white maggots, up to 9mm long, tunnel into the edible parts of the roots
  • Occasionally the maggots will tunnel into individual buttons on Brussels sprout plants

Control

Non-chemical control

  • 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, cardboard or similar materials. The collar prevents the female fly placing eggs on the soil surface close to a host plant. Eggs deposited on the collar often dry up and fail to hatch
  • Plants can 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 can emerge from overwintered pupae in the soil under the fleece or mesh cover, if host plants are grown in the same piece of ground in successive years
  • A mixture of pathogenic nematode species (Fruit and Vegetable Protection) 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, leatherjacketsfungus gnats, caterpillars, gooseberry sawfly, thrips and codling moth

Chemical control

None of the pesticides currently available to home gardeners are suitable for use against cabbage root fly.

Biology

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.

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