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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.
The 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; large swarms of adult flies can be seen in spring 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 (see picture, left). In some cases, the feeding damage completely destroys the growing point. Lab experiments indicate that 5 maggots per snap bean seed cause significant damage.
Once seed corn maggot damage is noticed, it is too late to apply control procedures. Thus, economic thresholds are not useful and all management options are preventative. However, growers may monitor seed corn maggot population fluctuations to determine fly-free periods using the following procedure. In early April, put out yellow pails filled with soapy water along the field edges at intervals of 100 feet. Many insects are attracted to yellow and are trapped when they fall into the water. Empty pails every 4-6 days and refill with soapy water. Keep records of the captured seed corn maggot flies; these records should indicate whether populations are increasing or decreasing. In addition, you can estimate peak emergence by accumulating degree-days after the ground has thawed. The daily formula to use is: (maximum temperature + minimum temperature/2) - 39 F. Peak emergence of the first three generations will occur, respectively, when 354, 1080, and 1800 degree days have accumulated.
The available chemical options are preventative; no insecticides are labeled for use once outbreak has occurred. One control option is to plant seeds pretreated with an insecticide. Another option is to apply a broadcast insecticide soil treatment that is incorporated into the soil prior to planting. However, applying soil insecticides may harm natural enemies of maggots such as beetles and other ground insects that eat maggot eggs. To ensure proper use of insecticides, refer to the most recent edition of the Midwest Vegetable Production Guide.
Since adult seed corn maggot flies are attracted to rotting organic matter and freshly plowed fields, avoid plowing animal manure, weeds, green manure, or other cover crops in spring. If there is a winter cover crop, plow it as early in spring as possible and thoroughly cover with floating row covers. Plowing crops in fall is better than plowing crops in spring because the maggot flies are more attracted to live, green organic matter incorporated into the soil. Conservation tillage seems to result in lower maggot populations because the organic matter stays on the surface of the soil rather than being incorporated into the soil. In addition, the type of cover crop that is incorporated into the soil also influences maggot population levels. Maggot populations are generally higher after a legume (i.e., beans, peas) is incorporated into the soil than when a grass (i.e., corn, rye, wheat) is incorporated.
In addition, handle seeds carefully to avoid cracking the seed coat; a cracked seed coat provides entry points for maggots and fungus. If the spring is cold and wet or if a particular field is low lying or has poor drainage, delaying planting for several days until the soil warms and dries decreases the likelihood of maggot problems. Shallow planting of seeds in well-prepared seedbeds also enhances quick germination, making the seeds less susceptible to seed corn maggot infestation. Planting seed during fly-free periods as determined under the scouting method noted above will also decrease the likelihood of an infestation.
Naturally occurring fungi may attack and decrease seed corn maggot larval populations. Predacious ground beetles also eat seed corn maggot eggs, larvae, and pupae. Since beetles are susceptible to any broadcast soil insecticides, they should be used sparingly.
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