Root maggots are the larvae of flies that lay their white eggs in the soil at the base of host plants. Adults are gray and nondescript, and about the size of a housefly.
There are several different species that target different host plants. Onion maggots seek out anything in the onion family, including garlic and leeks; cabbage maggots look for any cabbage-family plant, including broccoli or turnips. Both onion and cabbage maggots are more common in the northern half of the United States.
The maggots are most active during the cool weather of spring and fall. Maggots disfigure crops with their tunneling, but the main problem is the entry points they create for rot diseases such as black rot. Infested plants lack vigor, may be stunted, and are prone to wilting during the heat of the day.
Check plants that show symptoms and immediately destroy any that are infested. Beneficial nematodes can be effective in controlling the larvae. Adults are attracted to the scent of decaying onions so remove and bury plant debris after harvest.
Photography by Whitney Cranshaw, Colorado State University, Bugwood.org