Introduced from Europe, the cabbage maggot (Delia radicum, formerly Hylemya brassicae) is a serious pest of vegetable gardens in the northern United States and Canada. The fly primarily feeds on crucifers such as cabbage, cauliflower, broccoli, brussels sprouts, collards, kale, kohlrabi, radish, and turnip, but can also attack beets and celery.
BIOLOGY: Cabbage maggot pupae, which are tan-colored and about the size of a grain of wheat, overwinter in the soil at a depth of one to five inches. As the soil warms in spring, the adult flies, which look like smaller versions of common houseflies, emerge and feed on flower pollen and nectar. Emergence is in April in mild climates and as late as June in colder regions. The adults live for five to six weeks. The females lay white, one-millimeter-long, oval-shaped eggs on plants or in cracks in the soil close to plants’ stems. The eggs hatch in 3 to 10 days, and the white, legless larvae burrow into the soil and feed on small, tender rootlets. Eventually, they tunnel into the roots and lower stems. The larvae become full grown in three to four weeks. They pupate in either the root burrows or the soil for two to three weeks before the second generation of adults emerges. There can be two to four broods, although the first one is often the most destructive.
SYMPTOMS: Initial feeding causes cabbage leaves to develop a sickly grayish-blue color. Other crucifers look stunted with off-colored leaves. They wilt during the hottest part of the day. Eventually the leaves turn yellow and die. Damage to radishes, turnips, and other large-rooted crops may go unnoticed until the plants are lifted, revealing disfigured roots riddled with brown, slimy tunnels. The injury caused by the larvae's feeding creates entryways for soil-borne pathogens such as black rot and blackleg.
CONTROL: Anything that prevents the fly from laying eggs near the plant is a benefit. Remove infested plants. Cover seedlings or transplants with a floating fabric row cover and bury the edges to keep the flies out. Protect them for about five to seven weeks. Alternatively, place 6- to 10-inch-square shields of tar paper or old carpeting, each with a small hole in the center, around the stems of transplants. Press the shield down to the ground to prevent the adults from crawling underneath it. Because cabbage maggot damage is most serious in cool, wet soil, delaying planting until soil has warmed and dried will also provide control.
At the end of the season, remove the crop residue and deeply incorporate it to speed decomposition and eliminate overwintering sites. Fall tilling will reduce the survival of pupae.