Cabbage root fly

Cabbage root fly (Delia radicum), known variously as the cabbage fly, root fly or turnip fly, is a pest of crops. The larvae of the cabbage root fly are sometimes known as the cabbage maggot or root maggot. The adult flies are approximately 1cm (½in) long and are grey in colour but otherwise resemble the common house fly. Pupae overwinter in the soil close to brassica roots.

Symptoms
Growth is stunted, more-so if infected as seedlings. Roots turn black and rot and leaves are discoloured and wilted.

Treatment
Either spray with a chemical treatment or dig up all affected plants. Chemical treatment will not cure plants and a lower yield should be expected. There is no known organic treatment.

Prevention
Flies feed on pollen and nectar, particularly that of cow parsley. When cow parsley flowers the first generation of root fly are at large and preventative protection should already be in place. Putting collars around plant stems will stop the female root fly laying her eggs close enough to the plants for the maggots to reach. Alternatively; using a fine mesh over plants and beds will stop flies getting close to crops as well as providing an excellent barrier for many other crops.