MANHATTAN - For nearly seven decades, Kansas wheat producers have been advised of the "Hessian Fly-Free Date," a calendar date which, by planting afterward, was theoretically intended to prevent problems with Hessian Fly infestations in wheat. Kansas State University Extension Entomology publishes a map with "Fly-Free Dates," which is available online here.

 
As it turns out, the Hessian fly-free date may not be as ironclad as experts once believed. Jeff Whitworth, K-State Extension entomologist, says planting wheat after the fly-free date is a good management tool for wheat pests, but research conducted in 2008 shows that Hessian flies often persist well after the fly-free date.
                                            
There is much to be learned about Hessian fly, which has become a more prevalent problem in recent years. A generation ago, popular wheat varieties contained resistance to the insect. As Hessian fly problems waned, however, the emphasis on breeding resistance into varieties faded too.

Whitworth and his colleague, Holly Davis, are in the midst of a multiple-year research grant from the Kansas Wheat Commission, in which not only are they hoping to learn much more about Hessian fly, including control methods, wheat treatments and other best management practices. One  component of the research is a collaboration with USDA's Agricultural Research Service to find new varieties and cultivars that have resistance to Hessian fly, with the intent to release new, Hessian-fly-resistant varieties to Kansas wheat farmers. 
 

Hessian fly is a gnat-like insect feeds on nearly every wheat field in Kansas in the fall, and often causes yield losses up to 50% or more. Whitworth says Hessian fly is becoming more pervasive in areas of Kansas in which it has seldom been a problem.

"In 2009, we actually had problems in far western Kansas, actually totaling some fields, which we'd never had before. We had infestations in south central and north central Kansas, too," Whitworth says.

Whitworth adds that evolving tillage practices exacerbate the Hessian fly problem. "As we go to more no-till or reduced till, there is more volunteer wheat throughout the state.

Growers do a good job for the most part at controlling volunteer, but sometimes there are extenuating circumstances, such as too much rainfall, that prevent volunteer control. This represents an ideal green bridge in the summer," he says.

Because the use of insecticides is fruitless, few producers, are willing to destroy wheat fields heavily infested with the insect. Thus, there are only two options to control Hessian fly: the aforementioned Fly-Free Date, or using wheat seed-applied insecticide treatments prior to planting. Both of these have their limitations - the Fly-Free Date because of the insects' persistence into late fall, and the seed treatments because they work only up to three to four weeks after planting.

Thus, Whitworth's work, a multi-faceted approach which focuses on finding resistant varieties and cultivars to use as base genetic material; testing different seed treatments for efficacy against Hessian fly and wheat aphids; determining the effect of temperature and moisture on Hessian fly activity and developing a rescue treatment in the case of severe Hessian fly infestations. 

Until more work is completed, Whitworth's recommendations to thwart Hessian fly are simple: "We need to have volunteer wheat removed at least two weeks before wheat planted in the fall germinates. If you plant wheat back into an area with a known Hessian fly infestation, it is a good idea to use an insecticide seed treatment. That will give you 21-28 days protection against Hessian fly. This is especially important if you're planting back into wheat stubble," he says.