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Chap. 8, Crops and Varieties -No. 15, September 1993

Hessian Fly Management in Conservation Tillage Systems for the Inland Pacific Northwest

Keith Pike, Roger Veseth, Baird Miller, Roland Schirman, Larry Smith and Hugh Homan*


Hessian Fly is as Variable as the Weather!

The Hessian fly's life cycle is highly weather dependent. Rainfall patterns largely determine the level of fly infestation each year in dryland cropping regions in the Inland Northwest that receive less than 25 inches annual precipitation. Consequently, it does not reach an economic threshold every year in this production region. Extended periods of wet weather in the spring, late summer or fall can result in an increased number of flies emerging and/or generations produced. Another point to remember is that volunteer wheat and some native plants and weeds are alternate hosts of the Hessian fly. Managing for complete elimination of Hessian fly is impossible in favorable years since it can move in from surrounding fields and non-crop areas. Some significant infestations of Hessian fly have occurred in southeast Washington and north central Idaho over the past five years. The most recent infestation was in 1993, which was attributed to the wet spring weather and delayed planting.

Are Management Practices Compatible with Farm Conservation Plans?

Hessian fly is an exception to the general rule that insect pests of cereals in the Northwest are not influenced by choice of tillage and residue management. Conservation tillage systems leave more infested stubble and volunteer plants on or near the soil surface than under more intensive tillage practices. Because few fly larvae or puparia are buried to an adequate depth to prevent emergence, there can be a greater survival of the pest under conservation tillage. Fortunately, the potential for Hessian fly damage in the subsequent wheat crop depends more on management options for the following crops than it depends on tillage and residue management practices after an infested crop. Using an integrated approach, Hessian fly can be effectively managed under conservation tillage systems.

Publication Purpose and Other References

This PNW publication will present management considerations for minimizing crop losses from Hessian fly under conservation tillage systems. Other publications provide more details on the history, identification, and life cycles of Hessian fly. One in-depth, color publication which serves as an excellent reference is "Hessian Fly in Washington," Washington State University Bulletin XB0909. It is available through county offices of WSU Cooperative Extension or it can be ordered through the WSU Cooperative Extension Bulletins Office in Pullman, (509) 335-2857.

Life Cycle and Crop Injury

Under normal weather conditions, there are three or more major fly emergences each year in the Inland Northwest region, two in the spring and one in the fall. If late summer rains occur, there may be two emergences in the fall, instead of the usual one in this region. Two fall emergences occur more frequently in some areas, such as in Asotin County, WA, and Nez Perce County, ID, where climatic conditions favor the pest. The first spring emergence begins as temperatures reach a mean of 45-50F, commonly after April 1 (depending on the area and yearly variations). The second generation typically emerges in late May and June. Fall emergence normally takes place between mid-August and mid-October. Larval activity generally ceases about mid-October with the onset of cold weather.
Adults are fragile, mosquito-like flies that are weak fliers and live only 1-2 days. They prefer to lay eggs on the leaves of newly emerged and very young wheat plants in preference to older wheat plants and alternate hosts. Larvae hatch from the eggs within 3-10 days and migrate to the base of the leaf at the crown of the seedling or to the node in jointed wheat. There, the larvae feed for about 2 weeks and develop to the pupal stage, often referred to as the "flaxseed" because of its similarity in size, shape and color to the seed of flax. Adults emerge a short time later, depending on the weather and time of year. With favorable weather, the adults can emerge in as few as 15 days after the eggs are laid.
No single generation of Hessian fly ever completes its development uniformly. The emergence of at least some of each generation can be delayed 6 months to a year or more. This delayed emergence is a survival mechanism of the insect to maintain itself through unfavorable environmental conditions.
Hosts - Wheat is the preferred host of the Hessian fly. Winter wheat rarely sustains economic losses in the Inland Northwest. Since there are usually two spring generations of the fly, spring wheat (particularly that seeded late) has a greater exposure to infestation and typically sustains greater damage than winter wheat. Spring and winter barley, triticale and rye are generally considered resistant. Oats are not infested. Wild grasses such as quackgrass, western wheatgrass, ryegrasses, jointed goatgrass and timothy have been observed as alternate hosts for Hessian fly. Damage is relatively minor on these alternate host plants, but they help sustain the pest when wheat is not immediately available. An exception is in the Asotin Co., WA/Nez Perce Co. ID area where some significant yield losses have been reported in winter wheat and spring barley in years favorable to the fly.
Crop Injury - Hessian fly causes plant injury when the larvae feed on the juices in the stem tissue at the crown of young plants or just above the nodes on jointed wheat. Infested seedlings and tillers become stunted and the leaves become broader and darker green. Injury is more severe on newly emerged and young seedlings compared to older plants. Infestations during the seedling stages may lead to reduced stands that are open to greater weed problems.
Infested tillers, particularly in the younger plants, usually wither and die. If the tillers survive, their growth and yield will be reduced. Economic grain losses can be expected when 15 to 20% of the tillers become infested. Infestation as high as 70-80% have been reported in spring wheat under irrigation and some dryland cropping areas in the Inland Northwest.
Larval feeding on jointed wheat plants weakens the stem at the point of feeding and can result in lodging or stem breakage. Feeding can also interfere with the grain filling process, resulting in losses of grain yield and quality.

Management Options Under Conservation Tillage


For Hessian fly, as with most crop pests, there is no one management choice that will provide complete control. The most effective control will be through the use of an integrated management approach which takes into account all applicable management options. Growers need to balance control strategies with other yield limitations and management considerations. Management impacts on water conservation and erosion protection are just two examples.
Water Conservation - Management effects on water storage and resultant yield potential need to be considered. In this dryland production region with predominantly winter season precipitation, the amount of soil water stored overwinter is a primary yield-limiting factor. Consequently, the influence of the tillage and residue management practices on water storage must be considered when developing strategies for control of Hessian fly and other pests. Reduced water storage potential with intensive tillage and residue removal could limit yield more than fly damage.
Erosion Protection - Most farm conservation plans spell out the amounts of surface residue required to control wind and water erosion. Growers need to contact their local conservation district to make sure that the Hessian fly management practices they select will allow them to meet the goals of their farm conservation plans.
The following management options can help to minimize crop losses from Hessian fly in conservation tillage systems.

Fall Managing Options

  1. Crop Choice
    1. Grow Winter Wheat Instead of Spring Wheat - In the Northwest, winter wheat generally does not sustain economic yield losses from Hessian fly, but can serve as a host to sustain or build fly populations to attack spring wheat the following spring. During the vulnerable seedling stage, winter wheat is usually exposed to only one flight of flies. Although crop losses in winter wheat are generally low, damage may occur when it is planted early after an infested spring wheat crop and late summer rains have facilitated early fly emergence and growth of volunteer wheat.
Local exception -- Asotin County, WA and Nez Perce County, ID have climatic conditions which favor Hessian fly infestations of winter wheat. Therefore, growers in this area should consider other management options discussed later to minimize fly damage in winter wheat, as well as alternative crops.

Grow Other Less Susceptible Winter Cereals - Plant winter barley or triticale where the choice is practical. Winter barley and triticale are seldom infested and do not sustain noticeable yield losses.

Use Non-host Crops in the Rotations - Crop rotation is a key management tool for Hessian fly throughout the rotation. Winter rapeseed or Canola, winter pea and other non-host crops could be included in the fall where possible. Including non-cereal crops in the rotation can also effectively reduce the incidence of a number of other insect and disease pests in spring and winter wheat.

Control the "Green Bridge" of Volunteer and Host Weeds - Control volunteer wheat and grass weeds in fallow, after harvest and through planting of the next crop. Volunteer wheat and other host grasses which are allowed to grow for 2 to 3 weeks during fly emergence periods can become infested with Hessian fly and increase populations. Non-selective herbicides often replace some or all tillage operations for controlling volunteer plants and weeds under conservation tillage. It is important to spray early, before the Hessian fly emerges and "flaxseed" develop on these plants. Early control of volunteer, beginning in August if necessary, will reduce the number of host plants for Hessian fly production. Early control of volunteer grain and other host weeds can also reduce potential build-up of aphids and other insect and disease pests which can attack fall or spring cereal crops.

Delay Fall Seeding - Delayed fall seeding dates to avoid Hessian fly emergence (fly-free dates) is one of the most common management tools worldwide. If Hessian fly is a concern, delay seeding of winter wheat until after Hessian fly flights are reduced by cool fall temperatures, usually early to mid October. However, remember that delaying the winter wheat seeding date past the optimum seeding time also reduces yield potential and erosion protection benefits. Potential yield loss from a delayed seeding date must be weighed against the potential yield loss from Hessian fly and other pests if a "normal" or earlier seeding date is used.

Manage for a Healthy Productive Crop - Use optimal fertilizer rates, cultural practices and crop protection in order to insure a healthy vigorous crop which can tolerate a higher degree of infestation without noticeable yield losses.

Consider Insecticide at Planting - The first choices in managing Hessian fly should include the integration of crop rotation, seeding date, early control of volunteer wheat and weeds and other the management options listed above. Where important control options are not feasible, use of labeled insecticides at planting could be considered where Hessian fly potential is high. Granular and liquid insecticides are currently registered for in-furrow application at fall planting for control of Hessian fly. This insecticide application can also help to reduce the impacts of other crop pests, such as Russian wheat aphid and aphids carrying the Barley Yellow Dwarf virus. Contact your county extension office for more information. New seed-applied systemic insecticide treatments have been successful experimentally and should be available within 1-2 years.

Spring Management Options

When is Hessian fly potential high for spring wheat? The highest level of infestation in spring wheat usually occurs when a susceptible variety is planted after an infested spring or winter wheat crop, and wet, mild weather has delayed planting by 3 to 6 weeks. The delayed seeding date increases the potential for fly emergence when spring wheat is in the most susceptible seedling stage. Late summer rains can also permit increased fly production for attacking spring wheat the following spring.
A number of important spring management options are listed below.
  1. Crop Choice
    1. Plant Resistant Varieties - If spring wheat must be planted in spite of the possibility of Hessian fly damage, plant an adapted, Hessian fly-resistant variety. Wakanz is one soft white spring wheat variety that is resistant to Hessian fly. Other are being developed. Some hard red spring wheats have good resistance to the fly. Tables 1 and 2 show the results of some recent field evaluations of varietal resistance and tolerance to the fly. When considering resistance or tolerance in the choice of variety, a large number of other factors, such as yield potential, disease resistance, straw strength, date of maturity, and volunteer carryover must be considered in variety selection. Each factor must be weighed against others in making the final decision. Contact your county extension agent for additional information on performance of varieties in your area.
If a susceptible variety must be grown when Hessian fly is a concern, the spring wheat should ideally be planted after non-host crops, or after cereals which were not significantly infested with Hessian fly. Also consider other management tools described below.

Grow Less Susceptible Spring Cereals - Plant spring barley or triticale instead of spring wheat where feasible. These crops generally do not sustain significant yield losses from Hessian fly and have less than 1 percent infestation when infestations of susceptible spring wheats are up to 45 percent (Table 1). However, significantly higher levels of spring barley infestations have been reported around Nez Perce Co., ID and Asotin Co., WA because climatic conditions are

Table 1. Summary of Hessian fly resistance in spring cereal varieties based on a 1993 replicated, single field trial at Dayton, WA (K. Pike, B. Miller and R. Schirman,WSU).
Variety Percent Tillers Infested
Spring Wheat
Soft White
Edwall 16.8
Penawawa 45.6
Wakanz 0.0
Treasure 23.0
Sprite 45.2
Westbred Vanna 27.4
Calorwa 24.2
Hard White
Klasic 23.2
Hard Red
Wampum 15.5
Spillman 18.4
Butte 86 17.5
Westbred 926 0.0
Express 43.1
Spring Triticale
Juan 0.0
Victoria 1.5
Spring Barley*
* 14 spring barley varieties included in the Dayton study did not have more than 0.3 percent infested tillers
  1. more favorable for Hessian fly infestations. In these areas favorable to fly infestations, growers may find these options of less benefit.
  2. Use Non-host Crops in the Rotation - A change in crop rotation, particularly with the inclusion of non- host crops, should be a management consideration where possible since Hessian fly is primarily a potential problem for spring wheat under irrigation and in dryland cropping areas with about 18 to 25 inches or more annual precipitation. Rapeseed/Canola, peas, lentils, oats, triticale and other non-host crops could be grown as an alternative to or preceding spring wheat. Rotations with many of these crops also effectively reduce the incidence of a number of soilborne diseases of wheat.
  • Control the "Green Bridge" of Volunteer and Host Weeds - As in the fall, continue early control of volunteer wheat and grass weeds in fields to be planted in the spring, as well as those to be fallowed. Early control of volunteer and weeds with a non-selective herbicide before the fly emerges and/or flaxseed develop will reduce fly populations. Early control of volunteer cereals can also reduce the potential for Russian wheat aphids, aphids carrying Barley Yellow Dwarf, other insect pests and soilborne root diseases.
    Table 2. Summary of Hessian fly resistance in spring wheat varieties based on a 1991 replicated, single field trial south of Lewiston, ID (L. Smith, UI)
    Variety Percent Tillers
    Spring Wheat
    Soft White
    Centennial 30.0
    Wakanz 5.0
    Hard White
    Klasic 17.0
    Hard Red*
    Vandal 25.0
    Westbred 926** 0.0
  • * 11 experimental lines had infestations of 1% or less
    ** Westbred 906R also had no infested tillers in a 1990 study near Lewiston

    Plant Early in the Spring - If a susceptible spring wheat variety must be planted in spite of potential Hessian fly damage, plant as early as possible to reduce infestation potential during the early seedling stage, when the crop would suffer the greatest yield loss. However, adjustment of the seeding date to avoid fly infestation is difficult because the times of spring fly emergence change annually with weather conditions. Optimal seeding dates to reduce economic losses are in March or earlier in most areas.

    Manage for a Healthy Productive Crop - Use optimal fertilizer rates, cultural practices and crop protection in order to insure a healthy, vigorous spring crop which can tolerate a higher degree of infestation without noticeable yield losses.

    Consider Insecticide at Planting - The priority choices in managing Hessian fly in the spring should include the integration of crop rotation, fly resistant varieties, volunteer control, seeding date and other the management options listed above. When important control options are not feasible, use of labeled insecticides at planting could be considered where Hessian fly potential is high. Granular and liquid insecticides currently are registered for in-furrow application at spring planting. The insecticides could also help control Russian wheat aphids, aphids carrying Barley Yellow Dwarf, and other crop pests. Contact your county extension office for more information. New seed-applied systemic insecticides should be available within 1-2 years.

    Post-Harvest Residue Management

    After a wheat crop damaged by Hessian fly, a common question is whether the infested straw should be handled differently than normal to minimize carryover of the fly problem to future wheat crops. There are practices which can reduce the survival of the fly in infested residue and volunteer. However, Hessian fly can still be a problem for the subsequent winter or spring wheat crop because fly populations and generations are so highly dependent on rainfall patterns, and the pest is mobile, moving in from surrounding fields and alternate host plants. Consequently, crop rotation, varietal selection, early control of volunteer and host weeds, adjusting seeding dates and other practices mentioned in the above management options for establishing the next wheat crop are much more important to minimize Hessian fly damage than are tillage and residue management practices after an infected crop. Following is a brief summary of the effects of different tillage and residue management approaches after harvest of infested crops, and where they might apply.
    Intensive Tillage on Land Which is Not Highly Erodible - Moldboard plowing and rolling/packing of fields with infested crop residue after harvest will prevent fly emergence if puparia are buried to a depth of 3-4 inches in firm soil. However, this level of intensive tillage in the fall is not possible on highly erodible land, which includes much of the Inland Northwest cropland. Furthermore, subsequent fall or spring tillage, which returns some of the infested residue to or near the surface, permits emergence of Hessian fly from remaining viable puparia and can partially negate the control from the primary tillage.
    Stubble Burning not Recommended - Although stubble burning can kill the flaxseed stage in above-ground portions of the plant (especially stems infested after jointing began), burning has little effect on survival of those in the crown at or below the soil line, where a high percentage of the flaxseed usually occur. Stubble burning is not recommended because it provides inadequate Hessian fly control and can leave the land more vulnerable to erosion.
    Practices on Highly Erodible Land - On highly erodible land, growers need to utilize conservation tillage practices which at least result in the amount of crop residue required in their conservation plans. Use of as many control strategies listed in the preceding Hessian fly management options as feasible will provide effective control under conservation tillage systems.


    After harvest of a wheat crop infested with Hessian fly, growers should not significantly change tillage and residue management practices. Severe tillage to deeply bury infested residue should only be considered on cropland which is not highly erodible. Stubble burning is not recommended because of minimal reduction in fly survival and increased potential for soil erosion. Hessian fly can be managed within the tillage and residue management practices included in farm conservation plans. To control Hessian fly in conservation systems, growers should consider an integrated approach utilizing all feasible control options including: resistant spring wheat varieties, crop rotations with less susceptible or non-host crops, early control of volunteer wheat and host weeds, adjusted seeding dates and management for a healthy crop. In-furrow insecticides could be considered for reducing infestation potential during seedling establishment when other control options are not feasible.
    Pacific Northwest Conservation Tillage Handbook Series publications are produced jointly by University of Idaho Cooperative Extension System, Oregon State University Extension Service and Washington State University Cooperative Extension.

    *Keith Pike, WSU Entomologist, IAREC, Prosser, WA;

    Roger Veseth, WSU/UI Extension Conservation Tillage Specialist, Moscow, ID;

    Baird Miller, WSU Extension Agronomist, Pullman, WA;

    Roland Schirman, WSU Extension Agent, Dayton, WA;

    Larry Smith, UI Extension Agricultural Agent, Lewiston, ID;

    Hugh Homan, UI Extension Entomologist, Moscow, ID


    Contact us: Hans Kok, (208)885-5971 | Accessibility | Copyright | Policies | WebStats | STEEP Acknowledgement
    Hans Kok, WSU/UI Extension Conservation Tillage Specialist, UI Ag Science 231, PO Box 442339, Moscow, ID 83844 USA
    Redesigned by Leila Styer, CAHE Computer Resource Unit; Maintained by Debbie Marsh, Dept. of Crop & Soil Sciences, WSU