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 can be 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-pesticide control

• Recent transplants can be given some protection by placing a brassica collar around the base of the stem. These can be purchased 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 an insect-proof mesh. Horticultural fleece may be preferred for seedbeds as it will also help warm the soil. This can also help reduce other brassica problems such as cabbage caterpillars. It should be kept in place throughout the growing season. 
• Crop rotation should 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 insects, including the larvae of carrot fly, onion fly, leatherjackets, fungus gnats, caterpillars, gooseberry sawfly, thrips and codling moth
• Encourage predators and other natural enemies in the garden such as birds, hedgehogs and ground beetles. Some ground and rove beetles are efficient predators of root feeding maggots 

Pesticide control

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

Biology

There are three generations of cabbage root fly 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.