There are two species of fly maggots that can be pests in gardens, cabbage maggot and onion maggot. Cabbage maggots attacks crucifers, including broccoli, radishes, cabbage, cauliflower, and turnips, while onion maggots infest onions. Root maggots can occur any year but are more common during cool, wet springs.
Root maggots spend the winter as pupae in the soil. They emerge in spring as adult flies. These flies are dark gray, have stripes on their thorax and resemble small house flies. The females lay eggs at the base of plants 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 of crucifers or onions for about four weeks. Infested plants can appear discolored, wilted, or stunted. Depending on the age of the plants and how many maggots are attacking them, damage can be severe enough to kill these vegetables.
You must be proactive to protect your vegetables. Once you find damaged plants, it's too late to treat them. In the past, you could apply diazinon granules into the soil as you were planting crucifers or onions. However, diazinon has been phased out and is no longer available to the general public. It was hoped that another insecticide would be available with labeling for soil treatment in home gardens but this has not happened. So currently there is no insecticide available as a preplant treatment for cabbage and onion maggots.
Without insecticides, your only real option is to use a floating row cover, or similar barrier, to protect your plants. Floating row covers are readily available through gardening catalogs and stores. This barrier should allow sunlight and rain in and should reach the ground. You should have the barrier set up in your garden by the time adults flies are laying eggs which is 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.
You should base your need for a barrier on how much root maggot damage you've had in the past. If you have experienced problems on a regular basis, then assume you will see them again this year. If however, you have not encountered root maggots often in the past or not at all, you can assume you don't need to do anything this season.
Published in Yard & Garden Line News, May 1, 2004