Remember the Fly-Free Dates When Planting Wheat This Fall

August 27, 1999
As the season for growing corn and soybeans is winding down and these crops are maturing, growers who plant wheat are getting ready to plant the 1999­2000 crop. As is always the case, we want to spread the word about planting wheat after the fly-free date. Although these dates originally were generated as management guidelines for the Hessian fly, a serious pest of wheat in the past, other pest-management benefits also are derived from planting wheat after the fly-free date. Wheat planted after the fly-free date is much less likely to be injured by 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, transmitted by aphids and mites, respectively. Finally, wheat planted on or after the fly-free date will probably suffer less from soilborne mosaic virus.

The Hessian fly has not caused significant problems in wheat 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. Roger Ratcliffe, Research Entomologist, USDA-ARS, West Lafayette, Indiana, and his associates have been monitoring populations of Hessian flies in Indiana and Illinois for years. Their objective is to keep track of specific biotypes of the insects collected. As Roger stated in an article printed in 1998, "Biotypes of the Hessian fly become prevalent in the field in response to exposure to resistant wheat varieties. Over a period of 6 to 8 years of continual exposure to wheat varieties carrying specific genes for resistance, fly populations change so that new virulence genes become prevalent and render the resistance ineffective." Roger reported that all Hessian fly populations collected from southwestern Illinois during 1995­1998 were largely biotype L (84 to 100 percent). Biotype L has the ability to infest and injure wheat varie-ties that carry one or more of the four resistance genes that presently are available in soft winter wheat varieties. Most resistant wheat varieties presently available to Illinois wheat growers are largely ineffective in controlling the Hessian fly. Roger and his colleagues have developed a new soft red winter wheat variety that is resistant to Hessian fly biotype L. If it is available, consider planting it this fall.

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. We encourage all wheat growers to plant wheat in 1999 after the fly-free dates that are provided in Table 1 for all counties of Illinois. Implementation of this cultural practice in 1999 could prevent economic losses in 2000.--Kevin Steffey

Author: Kevin Steffey