A native bug is flattening Colorado’s wheat fields. Farmers are trying to keep ahead of it

One glance around the Northrup dining room will clue you in to the family business. A bouquet of dried wheat stems sits at the center of the table. Even the china dinnerware set on display in the built-in hutch is embellished with a gold-plated wheat pattern. It’s a family heirloom that goes back generations.

Nate Northrup grew up in this dining room. He is now the 4th generation of his family growing winter wheat on about 3,000 dry acres in New Raymer, Colorado, a small town in far-eastern Weld County. It’s just him and his father working the land with a rotation of wheat and other crops like millet and milo, in between fallow years.

Dryland farming has never been easy. But in recent years, Northrup has been battling a new challenge that would have baffled earlier generations: the wheat stem sawfly. It’s a pest that infests wheat stems at the base, flattening fields – usually just before the harvest.

Northrup described a slow progression of sawflies infiltrating his wheat fields, starting in 2010.

“It used to be just a few swaths around the edges,” he said. “And then, the following year, it would just be entire fields, just laying on the ground.”

Last fall, Colorado farmers planted more than 2 million acres of winter wheat for the 2022 harvest. But persistent drought is hurting Colorado’s crop, and the sawfly infestation only worsens things.

The mature bugs emerge in the spring and lay their eggs in young wheat stems. As the wheat grows, so do the sawfly larvae, eating their way down to the bottom of the plant. Just as the wheat ripens and becomes ready for harvest, the larvae ripen and get ready to hibernate. It makes itself an overwintering chamber just above the root and, in the process, takes a final big bite at the base of the wheat stem, weakening it beyond repair. The first wind or sprinkling of rain topples the weakened stalks flat on the ground.

Dr. Erika Peirce is a postdoctoral researcher at Colorado State University who specializes in integrated pest management of the wheat stem sawfly, which, she is quick to point out, is not actually a fly.

“Contrary to the name. It’s actually a wasp,” she explained. She says sawfly may be a new pest, but the bug is not new to Colorado. “It was initially discovered in non-cultivated grasses – the grasses on the side of the road – in 1874 in Colorado. It only became a pest of winter wheat in Colorado in 2010.”
Peirce says the sawfly’s transformation from benign native insect to threatening pest happened because of a change in its lifecycle. Initially, adult sawfly timed their emergence to align with the growth of the non-cultivated grasses that were its native host. She explained that winter wheat develops earlier in the season than those native grasses.

“The sawfly, in order to use winter wheat, has to mature and emerge about 3 to 4 weeks earlier than they normally would for their native hosts,” she said.

And eventually, they started doing just that.

“The sawfly over the years slowly started emerging earlier and earlier,” Peirce explained. Eventually, the bugs synced up with winter wheat. Peirce is still studying why this shift took place. “We’re looking into climate and landscape variables that might impact sawfly,” she said.

Whatever the science behind this shift in sawfly behavior, Nate Northrup says it’s devastating to his fields and not just for the crop that gets flattened — a field leveled by sawfly also devastates the next year’s harvest.

  Nate Northrup investigating a stalk of wheat for sawfly larvae
Nate Northrup investigating a stalk of wheat for sawfly larvae

Many dryland growers like Northrup don’t till their fields, a practice that keeps moisture in the ground and helps maintain soil health. For those no-till farmers, an outcome almost as important as the wheat crop itself is the crop residue — the plant stubble left over after harvest.

Residue prevents soil erosion – a major concern on these wind-swept plains. It also keeps the snow settled in place during the winter – locking in precious moisture.

Sawfly damage obliterates that crop residue, which threatens future growing seasons. “Out here with limited rainfall, the wheat residue is really vital to the crop rotation,” Northrup explained.

According to Colorado Wheat, a local industry group, sawflies infested nearly one million acres on Colorado’s Eastern Plains last year, causing more than $31 million in damage. Numbers from the 2022 harvest are still trickling in, but with wheat prices surging — the group expects that number to rise to $41 million — all thanks to wheat stem sawfly.

Many common pest control methods do not work against sawfly, making it a tricky adversary. “Since it lives inside the stem, contact pesticides don’t work on it because they’re kind of hidden and protected inside the stem,” Dr. Peirce explained. Systemic pesticides – those taken up inside the wheat plant – have also proven ineffective.

Esten Mason is the lead wheat breeder at Colorado State University. He says the best defense is better wheat breeding. “The only way that we can solve this problem right now is through genetic resistance — new varieties that are resistant,” Mason explained.

Specifically, Mason and his team are trying to breed a better wheat stem. Traditional wheat stems are hollow, like a drinking straw. Mason and his team have been developing new strains with solid and semi-solid stems that are stronger and denser. The solid stems don’t prevent the infestation, but they do provide a natural defense against the sawfly larva’s damage.

“The sawfly can still lay an egg in there, but that developing larva suffocates and just dies basically,” Mason explained.

The downside to the new varieties is that more material in the stem means less material for the grain. The solid stem varieties produce a lower yield than traditional wheat. But a lower yield is a lot better than no yield.

“At the end of the season, [the solid] wheat would be standing in the field, whereas [the hollow] one would be falling over,” Mason said.

And that wheat standing in the field makes all the difference to no-till dryland farmers like Nate Northrup. After years of increasing sawfly damage, he started experimenting with solid stem varieties on his farm a few years ago.

  A healthy wheat field
A field sown with a 50/50 mix of traditional and solid stem wheat.

This year he tested out a new approach on one of his fields: a 50/50 mix of solid stem wheat and a traditional hollow variety. There’s an elegant logic to this strategy — the traditional variety will provide Northrup with a higher yield.

The solid stems provide structure for the field – they can prop up adjacent hollow stems that have started to lean. And they’ll remain standing in the form of residue after the harvesting combine passes through.

Before the harvest, Northrup was encouraged by the results of this particular experiment. But he still expected a lackluster overall yield compared to recent averages.

“We’ve had phenomenal years, and we’ve had pretty bleak, pretty tough years,” he said. “So I guess you take the good with the bad, and the bad years make you appreciate the good.”

He knows it won’t be a phenomenal year. But with the solid stem wheat, things are looking good for a strong crop residue when the harvest wraps up. And that means he’ll have a shot at a better season next year.

  Nate Northrup standing in his wheat field
Nate Northrup standing in his wheat field

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