Many of the genes that allow wheat to ward off Hessian flies are no longer effective in the southeastern U.S., say USDA and Purdue University scientists. Researchers say that care should be taken to ensure that any new resistance genes that haven’t already been used in commercial wheat lines are used prudently.
An 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, a major wheat pest that can cause millions of dollars in damage to crops each year. Other varieties, even those considered to be the most effective, are allowing wheat to become more susceptible to the fly larvae.
More tools to defend. When pressured, resistance genes recognize avirulent Hessian flies and activate a defense response that kills the fly larvae that are attacking the plant. However, over time this leads to strains of the fly that can overcome resistant wheat, much like insects becoming resistant to pesticides in other crops.
"The number of genes available to protect wheat is limited. There really aren’t that many," says Richard Shukle, a research scientist with the USDA–Agricultural Research Service (ARS) Crop Production and Pest Control Research Unit. Shukle is also a Purdue University adjunct associate professor of entomology. "In the Southeast, having multiple generations of Hessian fly each year enhances the ability of these flies to overcome wheat’s resistance."
|Over time, Hessian flies have become more resiliant against resistant wheat cultivars.
Photo: Tom Campbell
USDA–ARS research support scientist Sue Cambron took Hessian flies from 20 locations in Alabama, Georgia, Louisiana, North Carolina and South Carolina and used them to infest 21 varieties of wheat that each contained different resistance genes, most of which have been deployed in commercial wheat and a few that haven’t yet. 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 that none were effective in all the Southeastern locations.
"Even some newer genes that haven’t been deployed in cultivars weren’t too effective," Cambron says.
That’s because flies have likely already interacted with, and adapted to, those genes, says Brandi Schemerhorn, a USDA–ARS entomologist and Purdue University assistant professor of entomology. She says it’s possible that some of the genes were unintentionally introduced to flies in plots where wheat cultivars with those genes were being tested for suitability for Southeastern 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.
Diligence is key. Schemerhorn says she suspects a certain number of flies in any population have the ability to overcome any wheat resistance gene. When a resistance gene kills off some of the flies, the survivors breed and eventually establish a population that renders the gene ineffective.
"We’re creating a system in which the fly is becoming more virulent," Schemerhorn says. "What we have to do is slow down that adaptation."
Shukle and Schemerhorn suggest stacking genes in wheat cultivars. There are only a few genes that haven’t been deployed, and combining two of those might be the best option.
"With a small number of identified resistance genes, we can’t afford to release wheat lines with only one resistance gene," Shukle says. "If you deploy two different resistance genes, it’s unlikely that a population of flies could overcome both of them."
Schemerhorn is working to combine two of the unreleased genes for testing with Hessian fly populations.