Researchers launch all-out assault on sawflies damaging wheat crops

Sadie Harley

Scientific Editor

Andrew Zinin

Chief Editor

Colorado wheat—the foundation for flour used in bread, noodles and pastries—has been besieged by the wheat stem sawfly, a native insect that recently developed a preference for wheat over native grasses. This destructive agricultural pest has spread across the state's Eastern Plains, devastating fields where it takes hold and causing $70 million in direct and indirect losses a year to one of Colorado's top agricultural commodities.

Colorado State University researchers are working with farmers to develop promising—and sometimes unexpected—natural solutions, and they are starting to pay off. Research tactics range from developing wheat varieties that crush the pest to importing its natural enemy.

"Colorado is among the top wheat-producing states in the country, and this insect is destroying not only the livelihoods of Colorado growers but global wheat production," said Punya Nachappa, an associate professor in the Department of Agricultural Biology who is leading several projects to stop the sawfly's spread. "Pesticides are not effective against the sawfly, and the spread has only gotten worse. Some fields are 100% flat."

While the sawfly's range expanded in Colorado from 2025 to 2026, the overall infestation rate for the state declined in 2026. Nachappa points to extreme drought and several control methods developed by CSU as possible contributors.

CSU alumnus Alan Linnebur and his brothers grow wheat in Adams, Arapahoe, Washington and Weld counties. They partner with the Colorado Wheat Research Foundation to produce, certify and distribute locally adapted wheat varieties developed by CSU. Since the infestation began 16 years ago, roughly two-thirds of their wheat has suffered wheat stem sawfly damage, amounting to thousands of acres annually.

Linnebur is working closely with Nachappa and her team to find solutions.

"Overall, the team is making progress in understanding the insect and developing a multifaceted approach," he said. "I think they may find some unexpected solutions to this very difficult and costly problem."

What is wheat stem sawfly?

Don't be fooled by the wheat stem sawfly's name; it's actually a primitive wasp.

Female sawflies lay eggs inside wheat stems. Once the eggs hatch, larvae feed on the soft center of the stem, burrowing down toward the soil surface. As larvae build pupation chambers for winter, they saw through the stems, razing wheat at its base and making the grain impossible to harvest.

"It's a very challenging insect because the larva is hidden inside the stem of the wheat plant for most of its life cycle, so growers don't know that they're even there, and the plant seemingly looks healthy," Nachappa said.

To detect sawfly infestation, stems must be manually split open to reveal the larvae inside. Nachappa's team dissects 10,000 stems from 100 wheat fields around the state every year to map the pest's range and track its spread.

New wheat varieties mitigate damage

Colorado winter wheat is worth up to $350 million a year, but its value extends beyond the commodity itself. The residue left behind retains soil moisture and improves the yield of other crops planted after wheat is harvested.

"Wheat is the foundation here for our crop rotation because it provides the stem density and residue to help protect the soil from erosion and increase the amount of effective precipitation, acting as a natural mulch, shade and snow fence," Linnebur said. "Because the sawfly cuts the wheat stem at the base, the stem falls to the ground and is not able to be harvested, and it also severely reduces the effectiveness of the straw as natural mulch, erosion protection and as a snow fence."

CSU's Wheat Breeding and Genetics Program has developed new varieties of wheat with semisolid stems that crush the larvae as they grow. These varieties effectively mitigate wheat damage, but because the plants invest more energy in stem hardening, they don't produce as much grain as hollow-stem varieties.

"Our farm is a certified seed producer, and there is a broad demand for the semisolid wheat varieties that mitigate wheat stem sawfly damage," Linnebur said. "These wheat varieties tend to yield less, which indicates that farmers find the risk of sawfly damage to be great enough to plant varieties of wheat that will yield less."

About 85% of Colorado wheat fields are planted with varieties developed at CSU—each bred to thrive in the state's harsh growing conditions. One of these varieties—called Amplify—was the most widely purchased variety in Colorado last year because sawfly is the top problem impacting the state's wheat production.

While semisolid varieties are a successful advancement, Nachappa and her team are hard at work on alternative solutions that won't have a yield penalty or could be used in tandem with semisolid varieties for greater effectiveness.

Wasp vs. wasp

Nachappa, an entomologist by training, is exploring whether the sawfly's natural enemies can keep it in check.

Bracon, a type of parasitoid wasp, lays eggs on sawfly larvae, and the Bracon larva that emerges feeds on the sawfly larva.

While parasitoid wasps are abundant in Nebraska, they are rare in Colorado. After analysis to ensure a genetic match between parasitoid wasps across state lines, Nachappa and collaborators at the University of Nebraska and U.S. Department of Agriculture baled hay containing parasitoids and literally rolled them out in several Colorado fields. They have patented this novel technique they call "Beneficial Bug Baler."

Research in Nebraska found that the parasitism rate must reach 50% to improve yield in sawfly-infested fields, and some sites where Nachappa's team released Bracon are approaching that threshold. The team is also working to boost parasitoid populations in Colorado by planting food to sustain adult Bracon—natural grasses, cowpea and Kernza.

Unrelated to the research, homegrown Bracon recently have increased in Colorado wheat fields as well, a promising development for controlling sawfly. Nachappa can't explain the sudden resurgence, but her team is trying to get to the bottom of it.

"We're thrilled about it because for the first time, sawfly numbers are going down while parasitoid numbers are increasing," she said.

Her team is investigating whether the newfound parasitoids are genetically different from previous populations in a project led by undergraduate researcher Casper Harvey.

"I love wasps. I especially love parasitoids because the interactions between host and parasite are so interesting to me," said Harvey, a junior in entomology who even has a wasp tattoo.

Unexpected natural solution

Every year for the past 13 years, the CSU wheat entomology team has undertaken the laborious process of splitting 10,000 wheat stems from 100 Colorado fields to monitor sawflies and now parasitoids too. In 2023, they discovered an unexpected helper in their quest to rid wheat fields of sawflies.

A fungus had infected and killed some of the sawfly larvae. Surprisingly, when they molecularly analyzed the fungus, it turned out to be a notorious wheat pathogen called Fusarium. However, this type of Fusarium did not harm the wheat and infected the larvae instead.

"These fungi can infect a wide range of insects, but we are only beginning to understand their association with sawflies," Nachappa said. "What's exciting is that we're finding these naturally occurring pathogens in more locations, suggesting they could play an important role in suppressing sawfly populations."

Nachappa's team has identified six species of beneficial Fusarium, and her students are evaluating three of them in the lab and greenhouse. Results are promising: The fungi kill up to 90% of sawfly larvae, and not only is exposed wheat unharmed, but one of the strains seems to improve plant growth.

"It's like the plant has its own bodyguard waiting for the sawfly to get in," said Ph.D. student Venkatesh Padimi, who is leading the evaluation.

Next, they're experimenting with using the fungi to inoculate wheat stubble in fields.

Nachappa said biocontrol methods, like harnessing fungi already found in the landscape, are the best, most sustainable option for growers.

"We're exploiting natural interactions," Padimi said. "It's not chemicals. It's safe."

Bug repellent

Because the sawfly is inside the wheat stem for most of its life, insecticides aren't effective against it, but what if there were a way to repel sawflies before they lay their eggs inside wheat? Nachappa's team is pursuing this new avenue of discovery based on a farmer's observation.

Jim Mertens, a grower and Colorado Wheat Research Foundation board member, noticed that some hollow-stem cultivars were less impacted by sawfly. He pointed this out to Nachappa, and her team analyzed the volatile organic compounds, or chemicals, emitted by different wheat varieties. They discovered chemical emissions that repel sawflies and identified compounds that attract parasitoids.

"It was probably the most comprehensive study of wheat volatiles to date," said Mj Riches, a postdoctoral fellow on Nachappa's team who has a background in atmospheric science. "We now have critical chemical information that companies could use to develop lures or repellents."

Searching for natural resistance

Because wheat stem sawfly is such a destructive pest, growers and Nachappa's team are trying everything to stop it, including screening for resistant varieties. Adam Osterholzer, a research associate who started with the team when he was an undergraduate student, hauls experimental wheat plants with novel genetics to fields to test their immunity.

"Sawfly larvae emerge by the millions in the spring," he said. "I've had it so bad where I literally have to turn the truck windshield wipers on because there are so many sawflies flying around."

Osterholzer annually surveys every wheat-producing county in eastern Colorado and maps sawfly infestation for growers and Colorado Wheat.

"I have family out east who deal with sawflies," he said, "so any solution we come up with here directly helps them, and that's important to me."

Provided by
Colorado State University

Citation: Researchers launch all-out assault on sawflies damaging wheat crops (2026, November 10) retrieved 10 November 2026 from https://phys.org/news/2026-11-assault-sawflies-wheat-crops.html