For years the Hessian fly has been a persistent, but typically localized pest for High Plains wheat growers, but
infestation reports have been increasing for the past three to four years, a Kansas State University scientist said.

The adult Hessian fly is a small, gnat-like insect that typically goes unnoticed by growers, said Jeff Whitworth, entomologist with K- State Research and Extension.

The flies typically emerge on warm fall days, often after rain. They mate and the females lay eggs. Adults are relatively short-lived, but manage to deposit their eggs during this time, usually on the upper surface of wheat leaves.

The eggs are tiny, but can be seen without magnification. The eggs are orange, however, so are often mistaken for leaf rust. They hatch in three to 10 days.

The Hessian fly was first reported in the United States in 1779 from an area near some Hessian soldiers' campsite -- hence its name, said Kansas State University entomologist Jeff Whitworth. The tiny pest was in Kansas by 1871 and has been reported there every year since then. "Hessian flies were a serious problem in the first half of the century. Then they were of little concern, except for scattered and localized areas of economic losses, until three to four years ago," Whitworth said. "Since then, reports of Hessian fly populations and consequent feeding damage have been increasing throughout the state."

Because there are no remedial or rescue insecticide treatments available if Hessian fly infestations are discovered nor is there an established treatment threshold or economic injury level, Whitworth encourages wheat growers to adopt one or more management strategies that have proved successful over the years:

• Use wheat varieties that are resistant to Hessian flies. This may be the best method to avoid Hessian fly damage, but there may be other reasons for not using a resistant variety. For example, available resistant varieties may not have the same yield potential as do some of the more susceptible ones, or they may be more susceptible to common diseases.
   
• Plant after the "fly-free" date. This date is published annually in the KSU Wheat Insect Management Guide, available at county and district Extension offices. It is the date after which newly planted wheat has an above-average chance of not being infested with Hessian flies. The relatively mild fall weather the past three to four years, however, has lessened the effectiveness of using this date as a planting guide.
   
• Destroy infested wheat stubble and volunteer wheat. Leaving infested stubble favors Hessian fly survival as that is where they "oversummer." Burying the stubble prior to adult emergence reduces the number of flies available to lay eggs. Destroying volunteer plants eliminates a potential food source to the flies, if no domestic wheat is available. This needs to be done on an area- or community-wide basis, however, as these insects have the capability of flying to other fields to lay eggs.
   
• Use insecticide-treated seed. Based on K-State entomology studiess, systemic insecticide seed treatments provided good control of Hessian fly larvae for up to 30 days. This may protect plants through the egg- laying period in fall or allow growers to plant during their usual planting time with an extra 30-day period in which cold weather may stop adult emergence and larval feeding naturally. Either way, the impact of Hessian fly infestations is reduced.

"This (these methods) does not ensure wheat will not be infested in the spring," said Whitworth, "but if fall populations and consequent damage are reduced, it should lessen the impact in the spring."

For more information about Hessian flies, interested persons can contact their county or district Extension agent and ask for Extension fact sheets, MF-1004 and MF-1076.

Hessian fly life cycle can occur twice yearly

Hessian flies, those tiny insects that can plague a wheat field and rob yields, generally emerge on warm September or October days, said Kansas State University entomologist Jeff Whitworth. They often emerge after rain and mate, and the females lay eggs.

Adults are relatively short-lived, but deposit their eggs during this time, usually on the upper surface of wheat leaves. The eggs are small, but can be seen without magnification. Because the eggs are orange, they are often mistaken for leaf rust. They hatch within three to 10 days. depending on temperature.

The larvae, sometimes called maggots, move down the plant soon after hatching and generally start feeding just above the crown between leaf sheaths and stem. Feeding may continue for eight to 30 days. Larval development is usually finished before cold weather. The mature larvae form hardened, mahogany-colored capsules or puparia
called "flaxseeds," in which they overwinter.

"This is the fall brood, and this process repeats itself during the spring," Whitworth said.

Adults can emerge from overwintering in flaxseed as early as March, but usually not until April, and the cycle starts again with flaxseed for oversummering in wheat stubble. Additional or supplemental broods are possible in either fall or spring, depending on the weather.

Fall infestations often go unnoticed by growers, the entomologist said. Infested plants tend to have stunted shoots or tillers or plant death. Outward signs of fall infestations are undeveloped central shoots with a broad, thickened and darker green leaf.

Whitworth urges growers to confirm an infestation by examining several plants exhibiting this characteristic. Once a grower can find infested plants, identification becomes easier.

Spring infestations may also go unnoticed until harvest, he said. Hessian fly larvae often feed at the base of the plant, causing that tissue to stop growing while surrounding tissue continues to grow. This creates a pocket for the maggot but also weakens the stem, which may result in partially filled heads or stem breakage at the point of
feeding. This, in turn, leads to lodging.

"Often, this is the first time growers notice Hessian fly damage," the scientist said.

K State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K State campus in Manhattan.

Mary Lou Peter-Blecha, K-State Research and Extension