By Mike Pumphrey

Since starting at Washington State University in 2010, I’ve seen Hessian fly infestations that caused significant losses in both production fields and test plots.  The first time I wrote about Hessian flies in Wheat Life, was in April 2013 due to concern at that time.  Fast forward six years. Now, when I visit spring wheat fields across Eastern Washington, the problem is worse, with infestations heavier and more widespread. Making matters worse, damage in winter wheat is being reported more often.

Hessian fly damage is not new to the area.  Damaging infestations have been seen west of the Cascades for more than 90 years, since before the 1930s, certainly. In the late 1970’s to early 1980’s, Hessian fly was recognized as a widespread pest throughout the Columbia Basin in Eastern Oregon and Washington, and in northern Idaho prompting research, breeding and management efforts.

Hessian fly pressure is not the same year after year, location after location, or even farmer to farmer because of many factors that influence infestations. Hatching is strongly dependent on suitable temperatures and is stimulated by rainfall events. Factors like date of planting may have a major impact depending on how crop development aligns with Hessian fly hatching.  This is particularly true for winter wheat. A practice used to manage fall infestations across North America is to delay planting too early, while flies are active.

Crop rotation history and crop residue are also factors influencing the level of local Hessian fly survival and infestations.  Wheat infested in prior crop years harbors the over-wintering and over-summering puparia (flax seed-looking stage) and more residue generally means more infestation potential.  Volunteer wheat seedlings are also a means of reproduction and survival, and attention to green bridge management is recommended.

Several parasitic wasp species have been found that target Hessian fly in the PNW and parasitism is thought to be a factor in fly population dynamics.  As a result of all these variables, Hessian fly impact on dryland wheat production regions in the inland PNW is variable.

I am often asked about the efficacy of insecticides for Hessian fly management.  Research is lacking, but available scientific literature concludes that aerial applications targeting Hessian fly females during their egg laying is problematic and does not reliably generate a return on investment.  The inconsistent hatching of fly populations, the long window of fly hatching in the spring and difficulty in accurately timing insecticide application are all factors.

The prevailing scientific opinion from those U.S. regions with Hessian fly is that seed treatment insecticides may help protect winter wheat infestation of Hessian fly more than spring planted wheat infestations.  Our experience planting spring wheat variety trials around the state is that susceptible varieties suffer significant yield losses due to Hessian fly in many locations each year. This is despite seed treatment insecticides that are applied to all variety trial entries.

Planting resistant wheat varieties is the most effective control measure. Hessian fly resistance in spring wheat has been valued at between $45 to $104 per acre by WSU and Oregon State University scientists. By the early 1990’s, Wakanz soft white spring wheat and Westbred 926 hard red spring wheat were identified among the first Hessian fly resistant varieties and recommended for spring wheat production acres.

Today, Hessian fly resistance is one of the main traits to consider when selecting spring wheat varieties. Most popular cultivars are resistant. The WSU Extension Cereal Variety Testing Program coordinates testing of new public and private varieties that enter WSU field trials.  Results with Hessian fly resistance rankings can be found by using the Variety Selection Tool ( https://varietyselection.cahnrs.wsu.edu/ ).

Leading soft white spring wheat varieties like Louise, Seahawk, Ryan, Whit, Diva, and WB6121 are resistant to Hessian fly.  Resistance to Hessian fly is lacking in the two commercially available spring club wheat varieties, JD and Melba.  While the hope of a club premium is tempting, spring club wheat fields have been affected by Hessian fly the past few years.  Leading hard red spring wheat varieties including Glee, Alum, WB9668, SY Selway, Kelse, and Chet are resistant to Hessian fly.

Winter wheat production fields with significant Hessian fly pressure have been reported in Walla Walla and Columbia counties the past few years, as well as under irrigation in central Washington.  It’s likely an unusually warm spring allowed multiple cycles of infestations compared to historical data that indicates a single cycle is more common.

With funding from the Washington Grain Commission and collaboration with the University of Idaho’s entomologist Nilsa Bosque-Perez, we have made strides in understanding Hessian fly resistance in winter wheat varieties.  For the first time ever, we screened public and private soft white winter wheat varieties, representing almost all PNW acres, and found that 39 out of 40 of them were Hessian fly susceptible. Only one new club wheat variety, ARS-Castella, had moderate resistance to Hessian fly.

Another recent advancement in our scientific knowledge has been in our understanding of which resistance genes are effective against our local populations of Hessian fly.  We collected all of the available wheat lines with different Hessian fly resistance genes and tested them against our local Hessian fly.  Twenty-nine different germplasm lines were available containing 29 known resistance genes. Of those 29, germplasm with resistance genes H5H13H15H22H26, and H32 were identified as having consistent resistance to our Hessian fly populations sampled in the PNW.

Hessian fly populations are notoriously good at defeating wheat resistance genes.  What this means in the field is that by planting a variety or varieties with the same resistance genes repeatedly, we are putting strong pressure on Hessian fly populations to mutate or re-shuffle their genetic composition to defeat the resistance.  This has happened repeatedly across the U.S. and world.  Now that we know which genes are effective in this region, we are working to combine resistance genes while breeding new varieties and developing new varieties with different resistance genes.

With winter wheat Hessian fly infestations being observed more the question arises: “Should we be breeding winter wheat varieties with resistance to Hessian fly?”  I believe the answer is yes. But very carefully.  In most of our wheat-based cropping systems, winter wheat has served as a refuge crop where lack of resistance in winter wheat varieties has meant that the rare Hessian flies with tolerance to our resistance genes are able to mate with vulnerable Hessian flies and produce offspring that are susceptible to the genes we use in spring wheat.

This phenomenon greatly prolongs the effectiveness of the resistance genes we use in spring wheat. Refuge acres/areas are required in Bt– trait crops like corn and cotton and are also used for Orange Wheat Blossom Midge-resistant wheat varieties in Canada.

If winter wheat varieties contain the same genes as spring wheat varieties in the region, those genes are more likely be defeated more rapidly as a result. WSU winter wheat breeder Arron Carter has started breeding for resistance by using germplasm from the southeastern U.S. with Hessian fly resistance.  The WSU spring wheat breeding program is focused on using our existing resistance genes, while bringing in genes that have not been used in the PNW or in the southeastern U.S.

Mike Pumphrey holds the O.A. Vogel Endowed Chair of Spring Wheat Breeding and Genetics at Washington State University.