Cabbage root fly

The adult cabbage root flies look like, and are the same size as, common houseflies.

The larvae look like typical maggots – white, legless, headless and up to 9mm (1/3in) long.

There are up to three generations per year, during summer. The first generation, from late spring to early summer, is usually the most damaging.

When fully fed, the maggots stop eating and turn into brown pupae in the soil. Depending on the generation, they either emerge as adult flies a few weeks later, or remain as pupae in the soil over winter. These then emerge as adult flies the following spring.

• All brassicas – including swede and turnip
• Radish

Because cabbage root fly maggots eat the roots of brassicas, the plants cannot absorb water. They grow slowly, making poor and stunted growth. Once significant root damage has been done, the plants wilt and then die.

Seedlings and young plants are killed fairly quickly, whereas more established plants, with more extensive root systems, will take longer before they wilt and die. Large, well-established plants may be able to grow through an attack and still produce a good or reasonable crop.

The maggots also tunnel through brassica root crops, such as swede and turnip, and radish. Extensive tunnelling ruins the roots and makes them inedible.

Sometimes the maggots will even tunnel into individual Brussels sprout buttons.

Once the symptoms of wilting are noticed, it is usually too late to do anything about it. So, it is important to check brassica plants regularly, looking for signs of wilting and then do something about it as quickly as possible.

Prevention is better than cure!

Careful hoeing around the plants, so as not to damage them or their roots, may kill the maggots or expose them to foraging birds and other animals. This is more important in the autumn or after the brassica plants have died to kill or expose the pupae.

Don’t grow brassicas or radishes in the same soil year after year. Grow something else in that place for at least one year.

Unfortunately, there are currently no insecticides approved for home gardeners to use to treat cabbage root fly – or any other soil-borne insect pests.

You can protect young plants and new transplants by placing a brassica collar around the base of the stem. These prevent the female flies laying their eggs in or on the soil surface close to the brassica plant – it is essential for the emerging maggots to immediately find roots to feed on.

Totally covering plants with insect-proof, fine-mesh netting or horticultural fleece, and growing the plants under it, provides a physical barrier to prevent the female flies getting to the plants to lay their eggs.

• Brassica collars
• Horticultural fleece
• Hoes