Root maggots are one of the most common garden pests in Alaska. They are major pests of most crucifer crops, which include radishes, turnips, cabbage, kale and others. Certain species of root maggots are a problem for onions and other alliums. If you have damage to crops in any of these families, it is important to learn what is causing it to avoid losing anymore of your crops to damage. Larvae feed on roots and can completely destroy the root system. The first sign of a problem is wilting of the plant on sunny days and yellowing or purpling of outer leaves. Later, plants collapse, wilt down, and die. On inspection of the root area you may find the legless white maggots feeding, the small brown, oblong pupae, or tunnels from maggot feeding. Root systems on older plants may be extensively damaged and the taproot may be destroyed. Feeding tunnels caused by the larvae predispose the plant to infection by softrot bacteria and to secondary infestation by springtails and thrips.
Basic understanding of root maggot life cycles can help prevent them in your growing area. They overwinter as pupae in the soil at a depth of one to five inches. As the soil warms in the spring, the root maggots complete pupal development and emerge as adults. Emergence continues for four to eight weeks as the soil slowly warms at different depths. Following emergence, adults mate and lay eggs at the bases of host plants at the soil surface, in cracks or under soil clods, or they may adhere to the plant stem. Egg laying begins two to seven days after the flies begin to emerge.
Early in the season the flies appear to have a preference for larger plants, usually faster growing varieties. More than 100 eggs can be laid on a single plant over a period of two days. Eggs hatch in three to 10 days. The tiny first-stage larvae migrate down and tunnel into root tissue as they feed. In middle to late summer, when the larvae are mature, they move out of the root tissue and into the surrounding the soil to pupate. Usually only one generation matures each year. Root maggot adults lay eggs in the spring. Delaying planting root maggot-susceptible crops until the summer may allow the crops to avoid infestation by root maggots.
Another method of control for your susceptible crops is to place a floating row cover on them. Floating row covers can be used to blanket new crucifer seedlings and young transplants to prevent adult flies from laying eggs early in the summer. Floating row covers allow some light and water to pass through the fabric and have the added benefit of protecting against frosts. These need to be placed on plants as early in the season as possible. Another method of control is the use of diatomaceous earth placed around the base of the seedlings will provide good control with no environmental hazards. Diatomaceous earth (a coarse sand-like particle) should be applied following each rain early in the season.