Soft wheat growers across much of the country have gradually inched planting dates ahead of long-established Hessian fly-free dates. Giving Hessian fly-resistant varieties a few extra days of warm growing weather establishes the crop to take up more nitrogen and reduce nutrient losses.
But now comes confirmation that these tiny, fast-reproducing flies have adapted their way around single-gene plant resistance. Of 21 resistant genes evaluated under a U.S. Department of Agriculture study, only five would provide effective protection of wheat from Hessian fly in the Southeast – H12, H18, H24, H25 and H26.
None of the released genes, from 20 different locations, were effective at all locations. That's bad news for the Mid-Atlantic as well.
Sue Cambron, a USDA Ag Research Service scientist, evaluated flies from Alabama, Georgia, North Carolina, South Carolina and Louisiana. The study didn't include all of the 33 named resistance genes. But even those genetic lines considered the most effective are allowing wheat to become susceptible to the fly larvae.
Even some newer genes that haven't been deployed in commercial cultivars weren't too effective. The genes recognize avirulent Hessian flies and activate a defense response that kills the fly larvae attacking the plant. However, she explains, this leads to fly strains that can overcome resistant wheat, much like insects becoming resistant to pesticides.
The breakdown of resistance may be hastened by fly crossbreeding with those from other cereal crops such as rye, adds Brandi Schemerhorn, a USDA-ARS entomologist at Purdue University. And, she suspects a certain number of flies in any population survive to overcome any wheat resistance gene. Then the survivors breed and eventually establish a population that renders the gene ineffective.
"What we have to do is slow down that adaptation or virulence," she adds. Stacking genes in wheat cultivars may be the best solution. Only a few resistant genes haven't been deployed. Combining two of them would be the best option.
The scientists agree that releasing wheat lines with only one resistance gene is no longer a good idea. Schemerhorn is working to combine two of the unreleased genes – H24 and H26 – for testing.