No. 22 Article 3/September 2, 2004

Hessian Flies and Fly-Free Dates

It's the time of year for your annual reminder of Hessian fly-free dates. Table 2 lists by county average dates for seeding wheat for highest yield, otherwise known as "fly-free dates." Hessian fly adults emerge in late summer and early fall. Typically, fly-free dates are after peak emergence. When wheat is planted after the fly-free date, Hessian fly females are unable to find host plants in which to lay their eggs and die without laying their full complement of eggs. When wheat is planted early enough, females will lay their eggs.

Hessian fly adult.

Hatch occurs in the fall. Maggots feed in the grooves of the leaves and work their way down behind the sheath. As the maggots feed, they rasp the stem of the plant. They pupate in late fall and spend the winter as puparia at the base of the wheat plants. The puparia is dark brown and resembles a flax seed. Plants infested with Hessian fly larvae become weakened in the fall and fail to tiller. Weak and stunted plants may also die during the winter.

Hessian fly larvae on wheat stem. (Courtesy of Kevin Black, Growmark.)

Hessian fly larva and pupa on wheat stem. (Courtesy of Kevin Black, Growmark.)

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 1995 to 1998 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.

We encourage all wheat growers to plant wheat in 2004 after the fly-free dates. Implementing this cultural control practice in 2004 could help prevent economic losses in 2005.--Kelly Cook

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