The 2011 wheat crop will be one of the most valuable on record when it’s harvested later this spring.

Protecting the crop from Hessian flies is an ongoing battle for growers in the upper Southeast and the fight is going to get tougher.

Many of the varieties that contain genes that make wheat resistant to Hessian flies and/or Hessian fly damage are not working in the Southeast. The U.S. Department of Agriculture contends care should be taken to ensure resistance genes not currently used in commercial wheat varieties be used prudently.

Wheat resistance genes recognize a virulent Hessian fly and activate a defensive response that kills the fly larvae attacking the plant. However, this leads to strains of the fly that can overcome resistant wheat, much like insects becoming resistant to pesticides.

In the Southeast there are multiple generations of Hessian fly during the growing season, which increases the odds of that these flies can overcome wheat’s resistance.

USDA analysis of wheat lines carrying resistance genes from dozens of locations throughout the Southeast showed that some give little or no resistance to the Hessian fly. Others, even those considered the most effective, are allowing wheat to become susceptible to the fly larvae, which feed on and kill the plants.

While the study did not include all of the 33 named resistance genes, it did show that only five of the 21 genes evaluated would provide effective resistance to flies in the Southeast, and none was effective in all the Southeast locations.

In the Southeast, it was particularly troublesome that some of the newer genes that haven't been deployed in cultivars weren't too effective in protecting wheat from Hessian flies.

USDA researcher Brandi Schemerhorn says it's possible that some of the genes were introduced to flies unintentionally in plots where wheat cultivars with those genes were being tested for suitability to Southeast climates. The resistance genes also could have come from other plants, such as rye, and the flies may already have started to overcome those genes.

Pay attention to recommendations

North Carolina State University Small Grains Specialist Randy Weisz says the USDA study indicates growers in the upper Southeast should pay careful attention to the “Wheat Variety Performance & Recommendations” SmartGrains newsletter that is published every July by North Carolina State University.

“We rate varieties every year for resistance to the specific Hessian fly populations present in North Carolina, and publish those ratings in this newsletter to help growers keep up-to-date on which varieties are the best,” Weisz says.

The newsletter is available from North Carolina county Extension offices and on line at: www.smallgrains.ncsu.edu/_SmartGrains/No28VarietySelection2010.pdf.

Weisz adds, “Variety resistance is the best defense against Hessian fly. There is a really good visual demonstration of how powerful variety resistance is in Photo 10-15 in the new production guide. The photo can be seen on line at: http://www.smallgrains.ncsu.edu/_Pubs/PG/Insects.pdf.

For the 2011 crop, he says the best management for Hessian flies in late winter and the spring is to check fields prior to top-dress time. If a wheat field looks healthy and is thickly tillered, Hessian fly will almost certainly not be an issue.

Hessian fly infestations are easy to identify. Fields with fly damage are thin, poorly tillered, and have plants that appear sick or that have died.

Pulling up a sickly plant, and peeling back the leaves will usually reveal Hessian fly pupae that look like small brown grains of rice, or seeds.

Fields with evidence of Hessian fly damage might benefit from a spring application of a long-residual pyrethroid. However, the application must be made close to the time the spring generation of adults emerge from the pupae, usually in mid-March.

Because the success of this rescue control measure is so dependent on correct timing, it is not always effective.

Growers with a serious Hessian fly problem in the spring, should look over the recommendations given below, to help prevent a future reoccurrence.

For Virginia-Carolina growers, prior to planting wheat this fall, some factors to consider for optimum Hessian fly management are:

Rotation

Because the Hessian fly life cycle is largely dependent on the presence of wheat stubble, rotations which prevent new wheat from being planted into or near a previous wheat crop’s stubble will be the most effective way to prevent infestations. Growers should avoid planting wheat into last season’s wheat stubble. Continuous no-tillage, wheat-double-cropped-soybeans may result in severe problems and should be avoided in Hessian fly problem areas.

Use distance to advantage

Additionally, since the Hessian fly is a weak flier, putting distance between the location of new wheat plantings and the previous season’s wheat fields can be a successful method of preventing new infestations. Although Hessian fly can become serious under other situations, most serious infestations occur when wheat is early-planted into wheat stubble or into fields next to wheat stubble.

Disking wheat stubble after harvest effectively kills Hessian fly. Planting soybeans no-till into wheat stubble enhances Hessian fly survival by preserving the site where puparia spend the summer. Burning wheat straw will reduce puparia, but many puparia are found below the soil surface. Therefore burning is not as effective as disking.

Choosing cover crops

Serious Hessian fly infestations have occurred in areas where wheat for grain was planted near early-planted wheat for cover or for dove hunting purposes. In cropping systems where cover crops are used, such as in strip-till cotton or peanut production, the use of other small grains besides wheat will reduce Hessian fly populations.

Oats, rye, and triticale are not favorable for Hessian fly reproduction and do not serve as a nursery and are preferred over wheat for cover cropping in areas where wheat for grain is also produced

Delayed planting

Because Hessian fly adults are killed by freezing temperatures, a traditional method for preventing Hessian fly infestation is to delay planting until after the first freeze (often called the fly free date).

This concept has not worked well in North Carolina because an early freeze is not a dependable event. Often a “killing freeze” may not occur until December in many areas of North Carolina, after most growers need to have wheat planted for agronomic purposes.

Variety Selection

Correct varietal selection is probably the most inexpensive and effective method of Hessian fly management. Use the “Wheat Variety Performance & Recommendations” SmartGrains newsletter to keep up to date.

Insecticides

Systemic insecticidal seed treatments provide about 19 days of protection from Hessian fly. If after planting, cold weather sets in within this time frame, these seed treatments can be very effective. Another insecticidal control measure is to apply a long residual pyrethroid to the developing wheat plants at or before the three-leaf stage.

rroberson@farmpress.com