The cabbage maggot may seriously injure cabbage, cauliflower, turnip, radish and related crucifer crops. Early-planted crucifers or seedbeds of late ones are more likely to be attacked. The young maggot begins feeding on the tender rootlets and then rasps out a channel in the main root of the plant. An early indication of attack to the cabbage plant is the plant wilting during the heat of the day. The plants may also take on a bluish cast. The plant either dies in a few days or persists in a sickly condition for some time. In cases where the plant dies quickly, there usually are a large number of maggots that riddle the root, making way for decay organisms to enter and take over quickly.

Description:

Cabbage maggot adults are true flies, slightly smaller than the common housefly. The flies themselves, which are gray and long-legged, are seldom seen by the home gardener. The larvae are white and legless, tapered towards the head, and have a pair of black mouth hooks which curve downward for rasping. The puparia are reddish or tan capsules resembling grains of wheat in the soil near the plant.

	Cabbage maggots feeding on roots (Clemson University Extension)	Cabbage maggot damage to roots (R. A. Casagrande)

Life Cycle:

The adult flies emerge from the soil where they overwintered as pupae about the time the first crucifer plants are set in the garden. The females either tuck the eggs down between the plant stem and the soil of transplants or lay eggs directly on the soil in a recently planted field so that the young plant is attacked at emergence. Eggs hatch in 4 to 10 days, and in about three weeks the maggots are fully grown. The pupal stage lasts 12-18 days before the adults emerge. There may be four generations of cabbage maggots in a season. The first occurs in late April through May, the second in late June to mid-July, the third in mid-August and the fourth in the fall.

Management:

The first generation is the most important to control because plants are small and very susceptible to damage. Natural predators usually provide a good degree of control later in the season as well. Radishes in home gardens can be grown in successive plantings, seeding at weekly intervals--in this way some will avoid damage. Infested radishes should be pulled and destroyed (not composted) if cabbage maggots are present.

The best control method consists of preventing the flies from laying eggs. Barriers (row covers) or netting of fine screening can be used for this purpose. In order to be effective, there must be no gaps or tears in the material, the material must be placed over the crop before or immediately upon crop emergence or transplanting, and no hosts of the pests should have been grown on that site the previous year. It is also important to bury the edges of the netting so that flies cannot get under the edges, and to allow the cover to be a little loose so the plants have room to grow. Shields constructed of tar paper, weed fabric or old carpeting may also be used. The shield will serve as an egg-laying barrier to adult flies. The shields are cut into 15 cm (6 inch) squares or 25.5 cm (10 inch) circles with a small hole in the center, and a slit is cut to the center. These are placed around the base of the transplant. Press them against the soil so that adult flies cannot crawl underneath.

If an insecticide is needed, be sure the crop you wish to treat is listed on the label before treatment. Follow the label instructions carefully.

Adapted from the Cornell Cooperative Extension, 1999