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Fly-Free Date Reminder

September 5, 2003

As growers prepare to plant wheat for 2004, we once again send out the reminder to plant after the fly-free date. In late summer and early fall, Hessian fly adults begin to emerge from wheat stubble. Fly-free dates typically occur after the peak emergence of Hessian fly adults. When wheat is planted after the fly-free date, there is not a suitable host for females to lay eggs on. These females die without laying their full complement of eggs. Eggs that are laid on wheat hatch and larvae feed between leaf sheaths and stems until they pupate in midautumn. Infested plants become weakened and fail to tiller. Plants may also lodge during grain fill.

The Hessian fly has not caused significant problems in Illinois for many years because of the availability of commercial wheat varieties that have genes for resistance to this insect. Unfortunately, biotypes of the Hessian fly have overcome some of these genes for resistance over the past years. The biotypes develop in response to selection pressure by exposure to wheat varieties that carry specific resistance genes. 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 in-fest and damage wheat varieties that carry one or more of the four resistance genes available in soft winter wheat varieties.

We encourage wheat growers to plant wheat in 2003 after the fly-free dates that are provided in Table 3 to prevent economic losses in 2004. By using fly-free dates with resistant wheat varieties, producers reduce selection pressure on the Hessian fly to develop additional resistant biotypes and should be able to effectively manage the Hessian fly.--Kelly Cook

Author: Kelly Cook

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

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