Root maggots in home gardens
There are two important root maggot species that can be present in Minnesota gardens; the onion maggot, Delia antiqua, and the cabbage maggot, D. radicum. Because the adult flies are very similar in appearance, they are most easily distinguished by the crops they attack. Onion maggots are an early season pest of root vegetables such as onion, garlic, carrot, and radish, whereas cabbage maggots are mainly a pest of cabbage, broccoli, cauliflower, Brussels sprouts, radishes, and turnips.
Adults are 1/4 inch long, dark gray with dark colored stripes on their thorax. They resemble small house flies. The legless maggots are ¼ inch in length, and yellowish white in color. They are generally cylindrical in shape, tapering towards the head.
Jeff Hahn, University of Minnesota
Onion maggot adult
Adults emerge from overwintering pupae in the early spring. Adult females begin depositing 50-200 small, white eggs in or near stems of host plants right at the soil line. Eggs hatch in a few days and the larvae burrow down into the soil to feed on small roots, root hairs, and germinating seeds. After feeding for three to four weeks, maggots begin to pupate in plant roots or the surrounding soil. There are several generations per year.
Seedlings and transplants are more susceptible to root maggots during a wet, cold spring. Although they have several generations in a year, most of their damage is limited to the early spring plantings. The maggots feed on the roots and the bulbs (in the case of onions), creating numerous tunnels. Plants first begin to wilt and can eventually become stunted and yellowed. Heavily infested plants can ultimately die.
Department of Entomology, University of Minnesota
Onion maggot damage
Manitoba Agriculture, Food and Rural Initiatives
Wilting cabbage due to cabbage maggot infestation
Most root maggot adults are attracted to rotting organic matter; avoid incorporating animal manure or green manure in spring. When possible, delay planting susceptible plants until the threat of root maggots is reduced, which is generally after June 1st.
It is important to remove host plants in the fall, including their roots, and destroy them which kill the overwintering pupae. This practice also helps to expose over-wintering pupae to predators and cause them to dry out.
University of Minnesota
Row covers are an effective option for controlling adult flies. Row covers prevent adult flies from getting near the plants to lay eggs. Row covers can be purchased at many local lawn and garden supply stores and online at suppliers like Burpee and Gardens Alive. It is important to make sure the fabric you choose allows both sunlight and moisture to get to the plants. In addition, row covers must be set up in your garden by the time adults flies are laying eggs which is usually early to mid-May.
Don't use row covers if onions or other root vegetables or cucurbits were planted in the same area the previous year. This is because root maggots overwinter in the soil as pupae near their host plants. When the adults emerge the following summer, they may end being trapped under row covers instead of being kept out. Practice rotation to minimize this issue by planting susceptible crops in different areas of your garden (if possible) or alternate seasons when you grow them.
Once damage is noticed it is too late to treat root maggots. Although it would be most effective to treat root maggots preventatively, there currently are no pre-plant granular insecticides available for home gardeners.