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 2002 after the fly-free dates that are provided in Table 3 for all counties of Illinois. Implementation of this cultural practice in 2002 could prevent economic losses in 2003. In addition, the use of fly-free dates reduces the selection pressure on the Hessian fly population to develop additional resistant biotypes. The Hessian fly has not caused significant problems in wheat in Illinois for many years, primarily because most of the commercial wheat varieties have had genes for resistance to this insect pest.
However, 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 population evolves, eventually rendering resistance genes in wheat ineffective. During 1995 to 1998, USDA-ARS entomologists from West Lafayette, Indiana, sampled wheat fields in Illinois and indicated that flies collected from southwestern Illinois were primarily Biotype L. This biotype is able to infest and damage wheat varieties that carry one or more of the four resistance genes available in soft winter wheat varieties. By using fly-free dates in concert with resistant wheat varieties, producers should be able to effectively manage Hessian flies and also enhance the longevity of resistant genes in wheat.--Mike Gray and Kevin Steffey