There are three vegetable pests with the name ‘root fly’. These are the cabbage root fly, the carrot fly and the onion fly.
The cabbage root fly resembles a house fly. Its larvae are peg-shaped, white maggots. Females lay their eggs under the soil surface near cruciferous plants including cabbages, radishes and cauliflowers. Their larvae feed on roots and when fully developed move into the soil and pupate in chestnut brow and barrel-shaped puparia.
Symptoms of cabbage root fly infection include edible roots with maggot tunnels and plants growing slowly, before wilting and dying.
The Onion fly is another housefly-like creature in the adult stage and a peg-shaped maggot in the larval stage. The adults appear in May before laying their eggs on or near onions. Maggots emerge from the eggs and feed on onions developing below the ground (see below). Onion flys may also attack shallots and leeks. Symptoms of onion fly infection include yellowing and wilted leaves, damaged bulbs and dead seedlings.
The carrot fly attacks not only carrots but also parsnips, celeriac and celery. Symptoms of infestation include carrots spoiled by rusty brown tunnels. Thin, creamy-yellow maggots may also be visible in the roots. the adult fly is more delicate than the onion fly or cabbage root fly, it is black with yellowish legs.