No. 4 Article 1/April 20, 2007

Planting Corn Following Wheat: Wireworms May Present Challenges in Some Fields

Because of the prolonged hard freezes in many areas of Illinois this spring, some producers are electing to till existing wheat fields and plant corn. The chances of increased wireworm damage in corn following wheat are very real. For a more in-depth discussion of wireworm life history and scouting procedures (bait stations) that can be used to detect the potential severity of infestations, please go to this University of Illinois Extension website.

Because of the surging interest in planting Bt corn (especially the stacked hybrids), some producers who elect to plant Bt corn following a plowed-down wheat crop may be surprised to see stands that have been thinned by wireworms. Although Bt seed is treated with a low rate of a neonicotinoid insecticide (Poncho 250 or Cruiser Extreme 250), there are concerns that these systemic seed treatments may not afford the desired level of protection against economic infestations of wireworms. Because efficacy data collected from intense wireworm infestations are not plentiful, producers are urged to consider the full spectrum of control choices for this insect pest. An in-furrow application of a granular soil insecticide has been considered the traditional management tactic for heavy infestations of wireworms. For more detailed product information concerning treatment options for wireworms, please consult chapter 1 ("Insect Pest Management for Field and Forage Crops," Table 3, page 10) in the 2007 Illinois Agricultural Pest Management Handbook. The handbook is also available online.

Wireworm larva, collected 1 inch below the soil line in southern Illinois, April 12, 2007. Photo courtesy of Ron Hines.

Another insect to be alert for in tilled wheat fields planted to corn is the seedcorn maggot, which thrives in conditions of high levels of decaying plant material. In 1991, a severe hailstorm in late April destroyed many wheat fields in Greene County, Illinois. Producers who planted corn into these hail-damaged wheat fields were advised to use insecticidal treatments to reduce the threat of seedcorn maggot infestations. Those who didn't use seed treatments were reported to have suffered more significant stand losses. Insecticidal seed treatments are expected to provide greater control against maggots compared with other secondary soil insects; however, under intense infestations, greater damage to seed may occur, resulting in stand losses.

Seedcorn maggots.

Seedcorn maggots survive the winter in previously infested fields. They pass the winter as pupae or as maggots in manure or in association with the roots of clover. The flies (adult stage) are grayish brown and approximately 1/5 of an inch long. In early May, the flies can be observed seeking out suitable fields in which to deposit their eggs. Once laid, the eggs begin to hatch at temperatures as cool as 50°F. Seedcorn maggots can complete an entire life cycle in 3 weeks, and as many as five generations per year are frequently passed in the Corn Belt.--Mike Gray

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