As the growing season for corn and soybeans comes to a close, the growing season for wheat begins. Wheat growers are preparing for the 2002 crop, getting ready to make decisions about planting for optimum yield and protecting the crop from pests. 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. Wheat planted after the fly-free date also 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 many years, primarily because most of the commercial wheat varieties have had genes for resistance to the 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 populations change, rendering resistance genes in wheat ineffective.
We have no recent data regarding the biotypes of Hessian fly present in Illinois wheat fields. As you may recall, USDA-ARS entomologists from West Lafayette, Indiana, have sampled wheat fields in Illinois in the past and have reported that Hessian fly populations collected from southwestern Illinois during 19951998 were largely biotype L. 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. For more information about this situation, you can read an interpretive summary from Dr. Roger Ratcliffe's research at http://www.nps.ars.usda.gov/publications/ publications.htm?lognum=0000101621.
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 2001 after the fly-free dates that are provided in Table 1 for all counties of Illinois. Implementation of this cultural practice in 2001 could prevent economic losses in 2002. Some seed treatments are labeled for control of Hessian flies in wheat (refer to the article "Wheat Seed Treatments for Fall 2001" in this issue of the Bulletin), but the economics of their use against unknown populations of Hessian flies have not been thoroughly explored in the Midwest.--Kevin Steffey