Hessian fly comeback

You always wait to plant wheat until the fly-free date, but you’ve never seen Hessian fly damage. It’s been a rare thing as of late, especially in MFA’s trade territory, but that’s no reason for complacency.

“We have been successful so far, but we also have to keep researching to stay one step ahead of it,” said Brandon J. Schemerhorn, an USDA Agricultural Research Service entomologist. “We have a lot more tools to fight it today, but so does the fly.”

In the Southeast, where infestations are more common, there have been unusually severe outbreaks in recent years. In 2006, the West Lafayette, Ind., area saw its first Hessian fly infestation in more than a decade. Over the last 2 years, the flies showed up in Missouri for the first time ever, indicating their westward expansion.

“By 2000, they’d adapted enough to overcome the four primary wheat resistance genes,” said Schemerhorn. “It took them just 14 years to overcome that defense. Now we use those four genes as a standard to judge the virulence of Hessian flies as they grow ever stronger.”

Where have they been hiding between appearances? Schemerhorn said there is evidence that tall fescue grass is providing safe harbor for Hessian flies until they can find wheat fields.

Hessian flies don’t just suck the liquid life out of wheat plants for their own nourishment. They also inject toxic substances that appear to create a more hospitable feeding environment for them and hasten their access to certain nutrients. It’s a cunning approach for a simple, gnat-sized fly. But considering that this insect has had thousands of years to perfect its assault on wheat, it’s no surprise it’s been so tough to swat.

That’s about to change. Thanks to new genomics technology in the hands of ARS researchers, the fly’s tactics are starting to be revealed genetically.

At the center of the research is the insect’s most finely tuned weapon—its saliva. The fly’s salivary glands, acting like tiny poison factories, churn out noxious compounds that cripple wheat plants.

After dissecting thousands of Hessian fly salivary glands and analyzing their DNA, researchers have identified roughly 2,000 Hessian fly genes that provide some role in producing salivary proteins.

Schemerhorn said that researchers have only recently acquired enough genetic markers on the fly’s genome to start searching for fly genes that correlate to the resistance genes in wheat plants. It is assumed that for every resistance gene there’s a fly gene that overcomes it.

The West Lafayette researchers have 52,000 markers for Hessian fly genes, with enough duplication and overlap to more than cover the entire Hessian fly genome. Schemerhorn and colleagues are working toward sequencing the entire genome. Tools like this allow a gene-for-gene approach to match the fly’s strategy for attack.

Schemerhorn is also doing something never done before: She’s sampling all Hessian fly populations across the United States to determine the extent of their genetic variability. The more variable the genome, the less likely it is that the limited number of genes in resistant wheat plants—like the standard complex of four resistance genes currently relied on—will work.

Compiled from ARS reports by Don Comis and Erin K. Peabody.

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