Root maggots are the larvae of flies. Adults of most species resemble houseflies; the onion maggot fly is further distinguished by its odd humpback. The maggots are generally fat, whitish grubs growing to 1/3" long and tapering to a pointy head. They emerge from eggs laid at the base of host plants, and eat their way downward through the root system, destroying entire plants or their cultivated roots. Vulnerable garden crops include onions, cabbage, broccoli, cauliflower, brussels sprouts, turnips, radishes and rutabagas. Cabbage and related crops will exhibit damage by sudden yellowing, wilting and dying, particularly while at the seedling stage. Root crops may wilt somewhat, but the worst damage will be underground. Scars and tunnels and the often concomitant rot and disease mar the root and render it inedible.
The most effective method for controlling root maggots is to prevent the adult flies from laying their eggs in the first place. Exclude them from vulnerable crops by spreading a barrier of agricultural fleece. Alternately, place 4"-wide tarpaper collars around each seedling at transplant time; make collars by cutting holes in the center of the paper just large enough for the seedling stem, slit the collar to fit it around the plant, and then slide the collar into place. Discourage root maggots by sprinkling wood ashes, diatomaceous earth, or rock phosphate around the base of their favorite plants. A traditional control involves preparing a "lime drench" at the rate of 1 cup of lime mixed with 1 quart of water and allowed to sit overnight; pour off the clear water and use it to soak the soil around the base of vulnerable plants. A mulch of oak leaves is said to produce similar results.