Tony Dickerson answers your questions
QCan you tell me how to treat cabbage root fly? Mr M Gardner, Lymington, Hampshire
ACabbage root fly caterpillars eat the roots of cabbages and other brassicas, causing the plants to make poor growth and eventually wilt and die. They may also tunnel into the roots of related vegetables, such as radish, swede and turnip.
The adult flies resemble house flies, while the maggots are white and about 9mm (⅓in) long. There are up to three generations over summer, but it’s the first, in late spring or early summer, that are the most damaging.
The simplest way to avoid damage is to plant out transplants later in summer. This isn’t practical with brassicas such as Brussels sprouts that need a long summer growing season to establish good-sized plants before winter, but works well with calabrese or lateseason cabbages.
No insecticides are available for controlling cabbage root fly. The usual means of control is to put brassica collars around the stem base of the plants. These are available from garden centres or can be made from pieces of carpet underlay or similar material about 15cm (6in) across. The collar prevents the fly laying eggs close to the transplant.
An alternative approach is to protect plants with horticultural fleece or insect-proof mesh. Whichever method you use, it’s vital to practise crop rotation, otherwise cabbage root flies will emerge from overwintered pupae in the soil underneath the cover.