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Sawfly population fluctuates state-wide as efforts to combat them continue

 

Last updated 7/29/2020 at 11:28am



The Havre area has seen a high population of sawflies this year, along with many other areas in Montana where the wheat cutting insect hasn’t been a serious problem for years.

See more on this story in the Hi-Line Farm and Ranch special edition next Wednesday.

David Weaver, a professor of integrated pest management and insect physiology and sawfly researcher at Montana State University, said this year has seen some serious shifts in the insect’s population with areas where they have been a persistent problem seeing decreases, likely as a result of cool weather.

He said cool weather is good for sawflies looking to breed, but it’s much better for the parasitoid wasps that use them as hosts for their own breeding and are one of the primary controls for the sawfly population.

“Cooler, wetter, weather helps the sawflies, but it helps their enemies more,” he said.

However, he said this isn’t the only change happening this year.

“The flip side of that is there are some places where we hadn’t seen them for quite a while, where those predators were working on them,” he said, “They’re spiking now, so there’s a more wide-spread problem and a lot of places are seeing outbreaks for the first time in a while.”

The wheat stem sawfly is an insect which lays its eggs inside the stems of wheat. After the larvae hatch, they travel down the stems and eat the stem lining from the inside out, often weakening the plants enough that they break off and lodge.

Hill County Extension Agent Tom Allen said this is a problem for wheat producers for two main reasons.

First, because when the wheat is cut down it makes it impossible to the combine harvesters to collect it effectively, and second, even when it is collected the act of cutting it interrupts the wheat’s life cyle diminishing its ultimate usefulness.

Allen said these insects can cost the industry millions of dollars a year, and devastated it in the 1940’s.

“There’s been guys who’ve had practically their entire fields knocked down,” Allen said, “They’ll still harvest some, but if you’re harvesting 30 percent, that’s pretty major.”

The primary method of combatting sawflies is the development of solid-stem varieties of wheat, which develop denser pith making it much harder for the insects to hatch and feed, killing many early in their life cycles.

“That’s what the go-to has been for a number of years, host plant resistance,” Weaver said.

MSU developed one of the first varieties of solid-stem wheat called Rescue, so named because of its intended purpose of saving the industry from sawflies.

Weaver and Allen each said it and the solid-stem varieties that came after have certainly helped control the species, but it’s still an ongoing fight.

“It was supposed to rescue the spring wheat from the sawfly,” Allen said, “Well, we’re still fighting the sawfly.”

Weaver said the species is remarkably adaptable, like most insects are, having moved from spring wheat to winter wheat over the years, and they pose researchers like him and wheat producers’ unique challenges.

“They’re very tenacious,” he said, “They’re a native species so they’ve been here for longer than wheat has.”

Weaver said when sawfly larvae feed on the stems and can cause major damage, but they hardly ever feed on enough for pesticides to take effect.

“The biggest problem is they’re just not so vulnerable to insecticides, which is of course one of the things growers can most commonly rely on,” he said.

The primary issue with solid-stem wheat is that growers tend to see lower yields from those varieties as opposed to hollow-stem wheat.

Weaver said there is not an easily identifiable reason as to why this is, but it may simply be a function of time, with the development of solid-stem varieties always being a bit behind their counterparts.

“Most solid-stem varieties probably don’t perform agronomically quite as well. It’s not really clear what it is, I mean looking for genetic markers for yield varieties doesn’t show anything,” he said, “… There’s always a lag between the best hollow-stem varieties and the best solid-stem varieties.”

Weaver said that, due to the adaptable nature of these insects, the disadvantage of solid-stem wheat in the yield department may be something of a blessing in disguise because it prevents growers from overexposing the insects to something they could potentially adapt too given enough time.

“If they were used steadily for 30, 40, 50 years, in the same location, the same population, that’s probably a recipe for the insect to develop resistances,” he said.

Weaver said there may be pockets of sawflies that are better at cutting solid-stem wheat, but there is not indication that any full-blown resistant population is developing.

He said MSU and other organizations are continuing to study sawflies and develop more effective variations of solid-stem wheat as part of an ongoing fight against the insects.

“It’s the true story of sawfly,” he said, “Everything seems like it should be a straightforward solution and it’s not … It’s an agricultural challenge, for sure.”

Weaver said he understands that the sawfly issue can be an exhausting subject to talk about, but he thinks it’s important for the public to stay informed about the issue.

“I think it’s important to keep people aware of it,” he said, “I know it’s a chronic problem and that some people get tired of it.”

 

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