Cabbage and onion maggots

Quick facts

There are two important root maggot species that can be found in Minnesota:

Onion maggot (Delia antiqua), found on root vegetables such as onion, garlic, carrot and radish.
Cabbage maggot (D. radicum), found on cabbage, broccoli, cauliflower, Brussels sprouts, radishes and turnips.
The adult flies for both species are very similar in appearance.
Root maggots can occur in any year but are more common during cool, wet springs.
Infested plants can appear discolored, wilted or stunted.
Damage can be severe enough to kill these vegetables.

How to identify root maggots

Adults are 1/4 inch long, dark gray with dark colored stripes. They resemble small house flies.

The legless maggots are ¼ inch in length and yellowish white in color. Their shape is generally cylindrical, tapering towards the head.

Life cycle of root maggots

Root maggots spend the winter as pupae in the soil. In spring, the maggots transform into adult flies. Adult females lay about 50-200 small, white eggs at the base of plant stems and in nearby cracks in the soil.

The eggs hatch within a week into small, legless, whitish maggots. These maggots move into the soil and feed on the roots, root hairs and germinating seeds of crucifers or onions. After feeding for three to four weeks, maggots convert into pupae in plant roots or the surrounding soil. There are several generations per year.

Dark gray adult onion maggot fly — Onion maggot adult

A cylindrical, yellowish maggot in a yellowing onion root — An onion maggot

Damage caused by root maggots

Root maggots have several generations in a year, but most of their damage is limited to early spring plantings.
Seedlings and transplants suffer more damage from root maggots during a wet, cold spring.
The maggots feed on roots and bulbs, creating tunnels.
Plants first begin to wilt and can become stunted and yellowed.
Heavily damaged plants can ultimately die.

Green onion with a tunnel near its roots — Onion maggot damage

Brownish dried cabbage leaves — Wilting cabbage due to cabbage maggot infestation

How to protect your plants from root maggots

Once you notice damage from root maggots it's too late to treat them. Protect your vegetables by preventing or removing conditions that favor root maggots.

If you regularly have experienced root maggot problems then you probably will see them again.

Keep your garden clean

Do not use animal manure or green manure in your garden in spring. Rotting and decaying organic matter attracts root maggots and can lead to plant damage. When possible, wait until June 1st to plant varieties that can be attacked by root maggots.

Remove target plants in the fall, including their roots, and destroy them. This will kill any pupae that might be left.

White row cover covering the young plants

Use a physical barrier

Row covers are an effective option to prevent adult flies from getting near the plants to lay eggs.

Choose a barrier that allows both sunlight and rain to get to the plants.
Make sure to set up the barrier in your garden by the time adult flies are laying eggs, usually early to mid-May.
Keep the barrier in place until the end of the month when the flies are finished laying eggs.
Floating row covers may not be practical in large gardens.
Row covers can be purchased at lawn and garden supply stores and online.

Do not place row covers if onions or other root vegetables were planted in the same area the previous year. Root maggots live through the winter as pupae in the soil near their target plants. Placing a row cover will trap adults that hatch from the pupae and it will no longer protect the plants from the flies.

Practice crop rotation to minimize this issue: plant susceptible crops in different areas of your garden or alternate seasons when you grow them.

Using pesticides

There is no pesticide available as a pre-plant treatment for cabbage and onion maggots.

Jeffrey Hahn, Extension entomologist and Suzanne Wold-Burkness, College of Food, Agriculture and Natural Resource Sciences

Reviewed in 2018