Plant Diagnostic Database

Root Maggots  (Delia spp.)











Type: Food preferences:
Cabbage Maggot
Cabbage, broccoli, cauliflower, kohlrabi, radishes, and rutabagas, along with a variety of other plants.
Onion Maggot
Onion and related bulb plants.
Seed-Corn Maggot
Bean, Field corn, and peas.
The maggots are cream to white in color and about 10 mm or 3/8 inch long when mature. The fly is gray and resembles a house fly, but is only 5mm or 3/16 inch long.
Life Cycle:
Root maggots spend the winter in a resting stage called a puparium, an elongate brown structure with rounded ends. It is buried from 1 to 5 inches in the soil. In early spring, the adult root maggot, a fly, emerges from the puparium and rises to the soil surface; it lays very small, white, oblong eggs on or just below the soil surface near the base of the host plants. Maggots hatch from the eggs in three to seven days, then migrate through the soil and feed on underground plant parts. The insect causes damage only during the maggot stage, which lasts from three to five weeks. Mature maggots leave the plant and pupate in the soil nearby. In two to four weeks the adult fly emerges. Several generations occur in a growing season.

Use transplants. Older plants may outgrow moderate cabbage maggot populations if well irrigated. Cabbage maggots do not develop at temperatures below 43° F. Plant before or after peak adult maggot flight in the spring (300 degree days at base 43° F. after soils thaw). A floating row cover or cheesecloth tent, which has no gaps through which flies can enter, may be placed over the seed furrow or transplants at planting time. The tent base should extend at least 6 inches on each side of the stems. Standard "backdoor" screening constructed with scrap wood framing has also shown excellent results as a fly barrier in WSU experiments. Washington State University scientists tested other non-chemical techniques on experimental plots. Two popular treatments, use of garlic sprays or wood ashes, had little value.

Do not add uncomposted manure or plant residues, such as fresh grass clippings, to soils. Root maggot adults are attracted to raw organic matter to lay their eggs. There is some indication that well-composted materials may reduce maggot problems. Till under infested plants immediately after harvest. 

Numerous parasites and predators attack cabbage maggots, but usually do not reduce populations quickly enough to avoid damage in the short run. Among these parasites are the Rove beetle (Aleochara spp.), an 1/8 inch, black to reddish-brown beetle, and wasps in the Braconidae, Ichneumonidae, also the wasp, Trybiographa rapae. Among the predators are a number of the ground beetles.


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