Erika Peirce is one of many soldiers in the shadow army of scientists and researchers working behind the scenes, without glory or recognition, to protect the nation’s wheat crop from a tiny foe: the wheat stem sawfly.
An entomologist by training who now works for the U.S. Department of Agriculture (USDA) Rangeland Unit, Peirce has dedicated much of her career to studying the sawfly. It’s a native bug that started out benignly enough in wild grasslands. But in recent decades, it evolved to emerge about four weeks earlier each season, rendering it a major pest that devastates winter wheat — a crop that happens to be Colorado’s largest by acreage.
Sawfly do their damage in the larval stage. They feed inside developing wheat stems throughout the season, which makes the grain head smaller.
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“They'll actually move up and down the stem,” Peirce explained. “But at the very end of the season, they'll move all the way down.”
At that point, they take a final, large chomp out of the stalk at the ground level and then curl up in a hibernation chamber until emerging the next season as adults. That final cut at the base of the plant weakens the stalk, which will fall flat on the ground at the first heavy wind. Economists put the scale of the damage at $70 million in Colorado alone.
Peirce and the others who make up this shadow army – a network of entomologists, wheat breeders, agronomists and crop scientists spread across the Great Plains Breadbasket region - have spent years trying to solve the sawfly puzzle. But while they’ve had some wins, like developing strains of wheat with solid stems that are less hospitable to the sawfly larvae, the bug remains one of the biggest concerns for farmers trying to eke out a living growing winter wheat in Eastern Colorado.
Sometimes the scientific work is monotonous. To study the bugs, Peirce first has to find them – a painstaking process that involves cutting open individual wheat stems to look for the sawfly larvae embedded within. She estimates that she has personally cut open more than one million wheat stems in her time. And she admits that tedium may have contributed to a few personality quirks.
“I’m in the lab for several hours, and sometimes I get hungry,” she said. “So, I’ll eat the wheat stem sawfly larvae. I’ll make a little pile and then I’ll eat them as a snack.”
She says they taste like broccoli.
Peirce disclosed this fact in the middle of an experimental plot at the Central Great Plains Research Station run by the United States Department of Agriculture’s Agricultural Research Service in Akron, Colo., on the northeastern plains. It was the annual Wheat Field Day, when the members of shadow sawfly army get a chance to educate the local wheat community about their latest work.
And it turns out Peirce shares her taste for sawfly larvae with a parasitoid wasp, the bracon, that she says is at the heart of the shadow army’s recent breakthrough – their biggest one yet in the sawfly counteroffensive.
“Bracon is a small orange wasp that lays its eggs on the wheat stem sawfly larvae,” she said. “They have this kind of venom that arrests development and then it’ll eat that sawfly larvae.”
Each bracon female will lay about 100 eggs in her lifetime, permanently disarming 100 unlucky sawfly larvae.
“Yeah, they’re pretty efficient,” Peirce said.
Like the sawfly, bracon is native to the Great Plains region. But for decades, the parasitoid failed to make the same evolutionary leap toward earlier emergence. The unevolved parasitoid remained abundant among non-cultivated grasses, but there was no trace of them in wheat fields.
“In order to use the sawfly that's in the wheat, the parasitoids have to emerge about four weeks earlier than they normally do,” Peirce explained.
That's because the lifecycle of the parasitoid now has to catch up to that of its prey.
“It's the sawfly that has evolved to become a pest of wheat and move into the wheat,” said Peter Kleinman, a research lead with the USDA Agricultural Research Service, “And the parasitoid that is needing to evolve. That lag is really interesting.”
But in some pockets of the northern Great Plains – parts of Montana, North Dakota and Nebraska, for instance, local bracon populations have finally caught up with their prey: They are now emerging earlier and going after sawfly in the wheat fields.
In the places where they have evolved to that stage, the bracon populations are so efficient at parasitizing their prey that the specter of wheat stem sawfly has almost completely faded as a threat to the farmers in those regions.
“Where you see parasitoids in good numbers, the entomologists never get a call,” Kleinman said.
Alas, there’s been no such luck in Colorado — at least not until this recent breakthrough.
Last fall, Kleinman and his fellow USDA scientists started importing the evolved bracon wasps into Eastern Colorado. They transported the bugs from Sidney, Nebraska in 10 bales of straw, which they unrolled in the experimental plots at the research station in Akron.
Kleinman called the question behind the experiment "stupid simple:" would the parasitoids survive the trip?
“Initially the marker of success was simply a proof of concept in which we would see some parasitoids emerge,” Kleinman said.
And emerge they did. This spring, around 75,000 bracon hatched from the straw bales in the experimental plots – a wildly successful beneficial bug transfer. Kleinman was among the first to spot the bugs as they emerged in Akron this spring.
“We realized: ‘Hallelujah, we're on to something,’” he said.
The beauty of the "beneficial bug baler project," as Kleinman came to call the experiment, was its simplicity.
"It’s extremely low tech and it’s something that any farmer could do without even batting an eye,” he said. “you cut the twine, grab onto a little bit of the straw to get it started and then unroll. And it's like a roll of toilet paper at that point.”
Of course, there’s more to nurturing a new bug population than unrolling a straw bale in a field. The researchers also want to make sure the new ecosystem can sustain the parasitoids. The mature bracon subsists on nectar and needs a reliable food source near the wheat fields. The bracon larvae need a safe place to overwinter after the wheat is harvested.
“This isn't about just transporting bugs from one place to another. This is about putting them in conditions where they can thrive,” Kleinman said. “That requires a conservation approach.”
Assuming farmers can provide the right kind of ecosystem, Kleinman considers the bracon straw bales project a huge victory on the sawfly front.
“This is the first option that can be implemented today,” he said. “And I think that it is a game changer.”
But across Colorado, farmers plant about 2 million acres of wheat on thousands of independent farms. Getting bracon onto all of them, and ensuring a nurturing habitat would take a massive, coordinated effort.
Learning about the parasitoid bug baler project at the USDA field day event, farmer Jim Diamond was skeptical.
“On a big farm scale, over thousands of acres, that’s a lot of work and a lot of extra expense,” he said. “Logistically, it sounds very challenging.”
Diamond grows wheat on about 4,500 acres in Akron. Sawfly is responsible for about $100,000 worth of damage on his farm each year. Yet he’s still on the fence about whether he’ll try the bracon straw bales.
“Maybe on a small scale, yeah,” he said. “I mean, we got to do something.”
He said an incentive program could convince him to embrace the parasitoids more wholeheartedly.
At this point, there are no incentives for bracon straw bales and the associated conservation practices that ensure their survival, although Kleinman said an incentives package is likely coming within the next couple of years.
But some farmers have already warmed to the idea.
“We could be coming onto something that may be a potential cure - something that helps address this problem,” said Doug Schmale, who grows wheat in Western Nebraska and Eastern Colorado. “Because prior to that we had nothing that really did it well.”
Schmale thinks this bracon breakthrough could be the turning point.
“This parasitoid that could mitigate the problem is the first thing we've seen that doesn't come with an economic hit to us,” he said. “Best case scenario, (it) becomes a region wide background population of natural predators that will help suppress this sawfly infestation that we've been fighting.”
To that end, the USDA scientists plan to develop their experimental bracon population into a parasitoid nursery that supplies farmers across Colorado. But that won’t happen right away.
“Our hope over the next couple of years is that we can take this and scale this up to a level to where producers who are at the front end of the sawfly infestation can benefit from it,” Kleinman said.
