K. Van Wychen Bennett, E. C. Burkness and W. D. Hutchison
Department of Entomology, University of Minnesota
The seed corn maggot, Delia platura, is an occassional pest of many vegetable crops including snap, kidney, and lima beans, corn, turnips, peas, cabbage, and cucurbits. They cause the most damage in spring to newly emerging seedlings, especially if germination is retarded due to wet, cold conditions.
Biology & Life Cycle
Seed corn maggot is an exotic pest from Europe that has been in the United States since the mid-1800's and has spread throughout most of the nation. The seed corn maggot overwinters as pupae in the soil. In early spring, the adults emerge, and large swarms of adult flies can be seen flying over freshly plowed fields. The flies mate within 2-3 days after emerging and lay eggs in soil with abundant decaying organic matter and/or on seeds or plantlets within these fields. The eggs hatch in 2-4 days in temperatures as low as 50 F (10 C). The larvae or maggots develop over a large temperature range: 52 F-92 F (11-33 C). The maggots are yellowish-white, about ¼ inch in length, legless, and very tough-skinned with head-ends that are wedge-shaped. The maggots complete their entire development within the soil by burrowing into seeds or feeding on cotyledons emerging from seeds. The pupae are brown, oval-shaped capsules 1/5 inch in length. The adults, which resemble small houseflies, are dark gray, with wings that overlap their bodies when at rest. Generally, seed corn maggots complete their life cycle within three weeks and have three generations in Minnesota. The first generation causes the most crop damage.
Seedlings are more susceptible to seed corn maggots during a wet, cold spring in which seed germination is slowed. However, the damage caused by the seed corn maggots is sometimes difficult to distinguish from other problems. For example, poor seedling emergence during a wet, cold spring could indicate infestation of a fungal root pathogen such as Pythium. In addition, wireworms (the larvae of the adult click beetle) also invade seeds of various vegetable crops. Wireworms are hard, dark brown, wirelike worms ½-1½ inches in length and tend to be more problematic in fields formerly planted in hay or some other grass crop. Besides burrowing into seeds and destroying the germ, seed corn maggots also feed on cotyledons and the first true leaves. The first true leaves may have holes or the maggots may completely eat the leaves, resulting in "snakehead" seedlings. In some cases, the feeding damage completely destroys the growing point. Lab experiments indicate that 5 maggots per snap bean seed cause significant damage.