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Hessian Flies and Wheat

September 8, 2000
Wheat growers are beginning to prepare for the 2001 crop, even as harvest of corn and soybeans gets under way. Although the recommendation of planting wheat after the (Hessian) fly-free date seems repetitive, it is a time-proven tactic for reducing the potential for infestations by this historically threatening pest. Planting wheat after the fly-free date in a given county also reduces the likelihood of the occurrence of Septoria leaf spot, which is favored by the excessive fall growth usually associated with early planting. In addition, wheat planted after the fly-free date is less susceptible to the barley yellow dwarf and wheat streak mosaic viruses vectored by aphids and mites, respectively. Finally, wheat planted on or after the fly-free date probably will suffer less from soilborne mosaic virus.

The Hessian fly has not caused significant problems in wheat in Illinois for years, primarily because most of the commercial wheat varieties have had genes for resistance to the pest. However, many biotypes of Hessian fly that overcome individual genes for resistance have developed over the years. These biotypes develop in response to selection pressure by exposure to wheat varieties that carry specific genes for resistance. The Hessian fly populations change, rendering resistance genes in wheat ineffective.

We have not yet received an annual report from Roger Ratcliffe, research entomologist, USDA-ARS, West Lafayette, Indiana, who, with his associates, samples wheat fields in both Indiana and Illinois to determine the prevalence of different biotypes of Hessian flies in wheat. However, Roger has reported that Hessian fly populations collected from southwestern Illinois during 1995­1998 were largely biotype L (84% to 100%). Biotype L has the ability to infest and injure wheat varieties that carry one or more of the four resistance genes available in soft winter-wheat varieties. We will offer updates of information when it becomes available.

Hessian fly maggots at the base of a wheat stem.

Hessian fly adults emerge in late summer and early fall; the fly-free dates typically occur after peak emergence of the fly. By planting wheat after the fly-free date, the egg-laying females are not able to find a suitable host, so they die without laying a full complement of eggs. If the Hessian fly female finds wheat that has been planted early enough, she will lay her eggs. The destructive maggots will hatch and feed in the fall and then overwinter in puparia at the bases of the plants. Infested plants become weakened in the fall and fail to tiller.

We encourage all wheat growers to plant wheat in 2000 after the fly-free dates that are provided in Table 1 for all counties of Illinois. Implementation of this cultural practice in 2000 could prevent economic losses in 2001.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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