The wheat stem sawfly has long been one of the most vexing and costly pests on the Prairies. But researchers say a subtle shift in harvest technique could tip the scales back in farmers’ favour by preserving a crucial natural ally: the tiny parasitic wasp Bracon cephi.
This enemy-of-my-enemy strategy is the focus of a new research paper published in the Canadian Journal of Plant Science. The lead author of that paper is Brian Beres, a senior research scientist with Agriculture and Agri-Food Canada in Lethbridge, who with his collaborators makes a compelling case for battling sawfly by simply raising the cutting height at harvest.
The problem with sawfly
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Sawflies are exceptionally hard to control. Adults lay eggs inside wheat stems in the spring, where larvae hatch and tunnel up and down, damaging plants from the inside out.
As harvest approaches, the larvae notch the base of the stem and cocoon themselves for winter. These weakened stems break easily, often making it impossible for combines to recover the grain.
The result is a one-two punch: roughly half the yield loss comes from larval feeding, and the other half from lodging caused by the notching — with total losses that can exceed 30 per cent.
Adding to the challenge is that chemical controls are largely ineffective. The problem is, there’s no way to know whether a female sawfly laid her eggs before the insecticide took her out.
“A farmer might feel good seeing a bunch of dead sawflies on the ground,” Beres says, “but that doesn’t help if they already laid their eggs. That’s what makes them so resilient — and why chemicals, to date, haven’t been very effective.”
In fact, most insecticides will only hurt the very organism that could help control sawfly numbers. That’s why Beres and his team advocate a whole-system, integrated approach.
A systems approach: G × E × M
Researchers frame their sawfly management strategy within the well-established GxExM model — shorthand for genotype by environment by management — a systems-based approach common in genetics and agronomy.
In this case, genotype refers to choosing appropriate varieties. Management includes the timing and height of harvest, while the environment reflects current sawfly pressure and risk factors like drought.
“You’re not going to eliminate sawflies,” Beres says. “The key is to rope-a-dope them. Keep the pressure low and buy time for the parasitoid to build up.”
Know your enemy — and its enemy
Enter Bracon cephi, a parasitoid wasp and the main natural enemy of sawfly on the Prairies. The wasp finds the larva of the sawfly in the stem, stings it, paralyzing the larva and then lays an egg on or near it so that when the wasp egg hatches, it can feed on the paralyzed sawfly larva.
One of Beres’ key collaborators was Scott Meers, a former provincial entomologist with Alberta Agriculture. He now runs Mayland Consulting, an independent consulting firm. Meers studied the sawfly–parasitoid dynamic during his master’s research at Montana State University (MSU) and later returned to the topic as part of this integrated study.
“The instant it stings the sawfly, the feeding stops,” said Meers. “So it’s really quite an important natural control of wheat stem sawfly.”
Bracon cephi typically completes two generations each season. The first parasitizes sawfly larvae in early summer. The second is poised to emerge in late summer or fall, just as farmers head into harvest.
At that point in the season, the researchers found that more than 80 per cent of the second generation’s pupae were located in the lower third of the stem.
That revelation turned out to be somewhat of a eureka moment. The randomness of the first generation gave no clues as to what to expect with the second generation. Meers says the intent was just to see where they overwintered. There was no expectation they would be so concentrated in that portion of the stem.
“I was surprised by how strong the relationship was to the lower part of the stem. So, that was nice,” he says.
According to Beres, ‘nice’ might be putting it mildly. He was more emphatic about the implications. The finding dramatically increased the relevance of the work, especially for farmers.
“It meant we had a chance to conserve a big chunk of the beneficial population by doing something as simple as cutting higher,” he says.
Why timing matters
When wheat matures earlier — often due to hot, dry conditions — the sawfly larvae reach the base of the stem sooner, notching and retreating to safety. That creates a situation where the second generation of Bracon cephi may emerge too late to reach them. If the sawfly has already notched the stem and moved into its overwintering chamber near or below the soil surface, it’s no longer accessible to parasitism.
“In early-ripening years, the sawfly larvae beat the parasitoids to the bottom of the stem,” Meers explained. “But if you cut high, you still save the ones that are already overwintering.”
Early harvest often forces farmers into a tough conundrum. When stems are already lodged, cutting low may be the only way to recover the crop and limit yield loss. But cutting low to save grain can also wipe out existing beneficial wasps that are in the cocoon stage inside wheat stems.
“You’re trying to recover your grain, minimize your yield losses, but at the same time you’re also wiping out one of your control strategies,” Beres says.
A boom-and-bust battle
Like many predator-prey relationships, sawfly and Bracon cephi populations tend to fluctuate over time. Parasitism drives sawfly numbers down, but when sawfly pressure drops, so too does the food supply for the parasitoids — setting the stage for another outbreak.
Weather, especially moisture conditions, adds another layer to that natural cycle.
Dry conditions give sawflies an edge, while wet years allow Bracon cephi populations to rebound. But even then, the benefits of biological control come with a lag. It can take several years before the wasps catch up to a growing sawfly population.
“In really simple terms, in dry years, sawfly tend to increase because the parasitism is less successful, but in wet years sawfly is driven down by parasitism,” Meers says.
The more of those natural enemies farmers preserve at harvest, the faster that balance can return.
Harvest strategy – the data is clear
Beres tested several harvest methods, and the results showed a clear threshold.
When swathing or straight cutting, maintaining a stubble height of at least 15 cm (about six inches) increased Bracon cephi emergence by 40 to 60 per cent.
Stripper headers, which clip the heads and leave most of the stem intact, performed even better. While Beres said they offer the best preservation of Bracon cephi, he emphasized they’re not essential.
“The good news from our study is you don’t necessarily need one,” he says. “You just need to make it your goal not to cut below six inches. Anything higher is a bonus.”
Cutting high is most effective when stems are still standing at harvest — something that becomes more difficult when sawfly cutting causes severe lodging. Solid-stemmed wheat varieties can help delay that collapse, and so can windrowing ahead of harvest.
Straight-cutting remains the most common harvest method, especially with modern combines on large farms. But Meers and Beres emphasize the importance of scouting fields and adjusting cutting heights to avoid losses and protect natural enemies.
Windrowing, meanwhile, helps preserve stem height by getting the crop down before wind causes lodging — offering another way to conserve parasitoid habitat.
Chopping straw does reduce Bracon cephi emergence slightly, but it doesn’t negate the benefits of higher cutting.
“We really studied that intensively,” Beres says, noting straw chopping is important for residue management in no-till systems. “We went through the field with a combine, chopped the straw, covered it up with mesh and then put an emergence cage over top, so we knew exactly what natural enemies were emerging in spring.”
The solid stem debate
Solid-stem wheat varieties are widely promoted as the front-line defence against sawfly. The idea is that larvae have a harder time developing in denser tissue (pith). But solid-stem genetics can come with tradeoffs. In the absence of sawfly, solid-stem plants typically yield about 15 per cent less.
Some early solid-stem varieties were also notoriously inconsistent — especially in shaded or lush canopies, where lack of light suppressed stem solidness. While that kind of growth is normally desirable, it could cause “solid-stem” varieties to express only partial pith development.
“The problem with conventional genetics was that they were highly influenced by sunlight,” Beres says. “In a lush canopy, you get shading, which inhibits pith expression, so a variety that’s supposed to be solid might not be in all conditions. This required reducing both nitrogen and seeding rates.”
There has been progress. Some durum varieties now show strong, consistent pith expression even under high seeding rates and optimal nitrogen levels. And a new CWRS variety, AAC Oakman, appears promising. But Beres cautions against relying on genetics alone.
—> WATCH: Solid-stem wheat variety resistant to wheat stem sawfly
“Using that as your lone strategy and not altering anything else is a disaster,” he says. “You have to integrate everything if you want to make a difference.”
Even when solid-stem varieties perform as intended, farmers still face practical trade-offs that influence decision-making.
Highly solid stems can interfere with Bracon cephi’s ability to locate sawfly larvae. They can also be harder on equipment and slow harvest operations.
“I’ve had growers tell me they won’t grow solid-stem varieties because they slow down ground speeds and create greater wear-and-tear on the threshing case of the combine, which in today’s scale of farming and equipment costs it isn’t easily dismissed,” Beres says.
He shares the story of one farmer who went all-in on a high-yielding, hollow-stemmed variety called Superb during a heavy sawfly outbreak. The rationale was that the variety’s higher yields would partially offset sawfly losses, and he’d get the crop off faster.
“It’s not the most sustainable approach, but if it improves ROI, it’s kind of hard to argue against — other than to remind him he is also farming sawflies, and to suggest a higher seeding rate, since we know that helps reduce infestations in hollow-stem varieties.”
Putting theory into practice
Even though it’s just a simple tweak at harvest, Beres admits it might take time before the practice is widely adopted. Integrating biological control into a harvest routine can feel abstract — especially when the threat isn’t obvious.
“They don’t see that tiny red wasp flying around,” he says. “They probably aren’t even fully aware of their sawfly issue until they get a wind event.”
He recalls working with a farmer near Warner, Alta., who was growing a field of McKenzie wheat. Things were looking great, but he took his family on a weekend camping trip. A strong wind blew through, and half the field was lying flat by the time he got home.
“Before that, I’m not sure he even knew. If he’d harvested earlier, he might’ve saved the crop — and the natural enemies working on his behalf.”
However, sawfly populations can surge quickly. A field with 10 per cent cutting one year can see 50 per cent or more the next. So even modest infestations deserve attention.
“We’re not trying to eradicate sawflies,” Beres said. “We’re trying to manage the system in a way that makes it harder for them to thrive and easier for their natural enemies to do their job.”
A tale of two theses
The roots of the study go back more than two decades. In the early 2000s, sawfly was wreaking havoc across southern Alberta. Beres had just begun his PhD on harvest practices and natural enemy conservation when Meers, then a master’s student at MSU, was working on a complementary study — mapping the location of parasitoids inside wheat stems.
Though they never worked in the field together, the two researchers unknowingly laid the groundwork for a future collaboration. Meers had no plans to publish his thesis due to the demands of his new role as Alberta Agriculture’s entomologist. Beres, who was intending to publish, was sitting on his manuscript, waiting for his own time to free up.
Then MSU reached out. They recognized the symmetry of the two theses and saw the potential to publish a merged study. But then it was MSU’s turn to sit on the research for several more years.
Eventually, when his time freed up, Beres got back in touch with MSU and offered to complete the work. The study was published this spring in the Canadian Journal of Plant Science, after more than a decade in limbo.
And not only was there good serendipity in terms of the complementary research, but the field tests took place during a period when the sawfly problem was raging.
“It was a once-in-a-generation thing,” Beres says. “An entomologist’s dream — and a golden opportunity on the agronomy side.”
