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Chapter 8 - Crops and Varieties, No. 9, Fall 1988

Effective Hessian Fly Control: Plant Resistance

Roger Veseth

Few insect pests of cereals in the Northwest are impacted by tillage systems. One exception is the Hessian fly. The incidence of Hessian fly and associated crop loss has been found to increase where wheat, particularly spring wheat, is directly seeded into wheat stubble or seeded under reduced tillage. Plant resistance, however, one of the most effective pest control options, will soon be available. In light of the increasing trend toward conservation tillage in this region, the development of Hessian fly resistant spring wheats is a good example of how integrated research efforts can help predict and solve obstacles to conservation tillage before they become significant problems.

A new Hessian fly resistant soft white spring wheat variety, named Wakanz, will be available for registered seed production in 1989. The development of Wakanz and several other promising soft white and hard red spring wheat lines with Hessian fly resistance has been a cooperative research effort between Keith Pike, Washington State University entomologist at the Irrigated Agriculture Research and Extension Center at Presser, and Cal Konzak, WSU plant breeder at Pullman. Their genetic selection and resistance evaluation program has also involved USDA-ARS entomologist Jim Hatchett in Kansas and seed increases in New Zealand. The research by Pike and Konzak has been a part of the STEEP (Solutions to Environmental and Economic Problems) research program on conservation farming in Washington, Oregon and Idaho.

Hessian Fly Background

History and Distribution

Pike points out that the Hessian fly, a world-wide pest of wheat, was first identified in the U, S. over 200 years ago in 1778. It was found on Long Island, New York following the encampment of Hessian soldiers during the Revolutionary War, hence the name Hessian fly. Within 120 years, it had spread to most eastern and mid-western wheat regions. By 1933, the fly occurred all along the west side of the Cascade Range. Not until the 1960's was it found east of the Cascades. In the Inland Northwest, extensive field surveys by Pike have shown that Hessian fly infestations have been largely restricted to the irrigated Columbia Basin fields and dryland wheat fields located close to irrigated areas. Occasionally, he has found Hessian fly infestations in dryland wheat in extreme eastern areas receiving about 20 or more inches of annual precipitation. Consequently, most of the region's dryer wheat cropland (about 80 % in Washington) is relatively free of the pest.

Life Cycle

The Hessian fly has four life cycle stages: adult, egg, larva (maggot) and pupa. The adult mosquito-like fly lives only about 4 days, laying eggs on the leaves. They prefer to lay eggs on newly emerged and very young wheat plants in preference to older wheat plants and alternate hosts. Larva hatch from the eggs within 3 to 10 days and migrate down the leaf to the crown of the seedling or to the node in jointed wheat. Once the migration stops, the larvae feed for about 2 weeks and develop to the pupa stage, which is commonly called a "flaxseed" because of the similarity in size, shape and color. Adults emerge a short time later, depending on the weather and the time of year.

There are usually three major fly emergences each year in Washington — two in the spring and one in the fall. The first spring emergence begins as temperatures reach a mean of 45 to 50F, commonly after April 1 (depending on the area and seasonal variations). The second generation typically emerges between May and June. The main fall emergence normally takes place between late August and mid-October. Adults lay eggs on volunteer and seeded wheat in the fall, or other alternate hosts, if wheat is not available. Larval activity generally ceases about mid October with the onset of cold weather.

Pike points out that no single generation of Hessian fly ever completes its development uniformly, with the emergence of at least some of each generation delayed. There are always some larvae of the first spring generation that oversummer and emerge in the fall or continue to overwinter in the stubble and emerge the next spring. Similarly, there are larvae of the second spring generation that do not emerge in the fall and may overwinter until the next spring. He explains that delayed emergence is a survival mechanism of the insect to maintain itself through unfavorable environmental conditions.

Plant Hosts

Wheat is the preferred host of the Hessian fly. Since there are usually two spring generations of the fly, spring wheat has a greater exposure to infestation and typically sustains the most economic damage, The Hessian fly can also develop on other grain crops such as barley and rye, but causes little economic damage. Oats is not infested. Occasionally the pest is found on wild grasses such as quackgrass, western wheatgrass, ryegrasses, jointed goatgrass and timothy. Damage is relatively minor, but these host plants can help sustain the pest when wheat is not immediately available.

Plant Injury

Hessian fly causes injury to wheat when the larvae feed on the juices of the stem tissue at the crown of young plants or just above the nodes on jointed wheat, Pike points out that plant injury is generally greater on newly emerged and young seedlings compared to older plants. Infested tillers are stunted by the feeding, and the leaves become somewhat broader and darker green.

Stunted tillers, particularly in the younger plants, usually wither and die. If the infested tillers survive, their growth and yield will be reduced. Pike estimates that significant grain losses can be expected when 20 percent or more of the tillers become infested. Serious infestations may lead to thin stands that yield poorly and are open to greater weed problems than healthy stands. In 1987, Pike found a 70 to 80 percent infestation in an irrigated spring wheat field in Benton County, WA. The field was not harvested because of the extremely low yield, Injury in jointed wheat plants is a weakening of the stem at the point of feeding, which can cause lodging or stem breakage. Feeding can also interfere with the nutrient supply to the head during kernel formation, resulting in losses of grain yield and quality.

Tillage Impact

How does the tillage system affect the survival and potential for Hessian fly in the subsequent wheat crop? Pike and Konzak point out that tillage operations which bury infested wheat stubble or live volunteer plants prevent emergence of the fly. Larvae and puparia exposed to the environment from their protected plant or stubble "home" by tillage also rarely survive.

In contrast, conservation tillage systems may improve the chances of Hessian fly survival into the next crop. When herbicides are used to control infested volunteer wheat before direct seeding of wheat without tillage, the infested stubble from the previous wheat crop and volunteer plants are left largely intact, allowing the Hessian fly to complete its life cycje. Under reduced tillage, Hessian fly survival will depend on the proportion of infested straw and volunteer plants which remain intact on the surface.

Factors Affecting Tillage Importance

Other management factors influence the impact of the tillage system selection on Hessian fly potential. The researchers stress that one of the most important factors is crop rotation. If the previous crop was not a wheat crop, the choice of tillage system will not influence the Hessian fly potential. Crop rotation is also important for reducing the potential of several soilborne diseases of wheat.

Seeding date can also influence the potential for Hessian fly infestation of winter wheat in the most susceptible early seedling stage. Delaying the seeding date until after the major fall fly activity period (August to mid-October) will reduce the potential for infestation and thus reduce the impact of the tillage system. Delaying the winter wheat seeding date past the optimum seeding time, however, also reduces yield potential. Economic losses in winter wheat because of the Hessian fly have not been common in the Inland Northwest.

Hessian fly distribution has been sporadic from year to year and from area to area in the Inland Northwest. Before changing residue management strategies, producers should be fairly certain that they have a significant Hessian fly problem, as indicated by a heavily infested previous wheat crop or volunteer wheat, or consistent losses under irrigated wheat. This is particularly important where there is a high potential for soil loss by water or wind erosion under more intensive tillage. Other options, such as crop rotation, delayed fall seeding date and systemic insecticides should be considered in those situations.

In dryland production areas, stored soil water content is an important yield-limiting factor. Consequently, the influence of the tillage system on water storage must also be considered. Reduced water storage potential with intensive tillage could limit yield more than Hessian fly damage with conservation tillage.

Plant Resistance

Plant resistance to Hessian fly is the most effective and economical means for preventing production losses from the pest, Consequently, this has been the main research focus for Hessian fly control by Pike and Konzak in the STEEP program.

Waid, a spring durum developed for irrigated production, is currently the only available Northwest cereal variety which has Hessian fly resistance. Through efforts of Pike and Konzak, in cooperation with researchers in Kansas and New Zealand, Wakanz will be the first Hessian fly-resistant cereal variety widely adapted to the Pacific Northwest. With the release for registered seed production scheduled in the spring of 1989, certified Wakanz seed should be available in 1990.

Konzak points out that Wakanz has consistently been among the highest yielding soft white spring wheat varieties developed at Washington State University to date. It is the first variety developed for conservation tillage production in the Northwest, Wakanz will also be an excellent variety under conventional tillage.

The genetic resistance carried by Wakanz originated from resistant soft red wheat in the Midwest. Wakanz has adult-plant type resistance for stripe rust and moderate to good resistance to leaf and stem rust. Milling and baking qualities have been good as well.

Two sister lines to Wakanz are being developed as potential backups to the variety. They have a somewhat higher type of stripe rust resistance than Wakanz and have equal yield potential. Several other soft white and hard red spring wheat lines, which carry one of two different resistance genes for Hessian fly, are being evaluated. The researchers plan to continue to incorporate Hessian fly resistance into all of the new spring wheat varieties developed for Washington.


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Hans Kok, WSU/UI Extension Conservation Tillage Specialist, UI Ag Science 231, PO Box 442339, Moscow, ID 83844 USA
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