Supply can’t meet demand for thistle-munching weevils

Stem-mining weevils love Canada thistle but getting enough of the bugs has been a challenge for forage association

Canada thistle is a huge problem in ranchers’ fields — but unfortunately, demand for one of the solutions has outstripped supply.

West Central Forage Association has been selling stem-mining weevils since about 2014 but recently its supplier in Montana has had production problems and been unable to fill its orders.

“The last couple of years, including this year, we’ve had to reimburse people who purchased the weevils from us,” said Rachael Nay, the association’s general manager.

“We’re hoping to eventually get our own weevil collection going, but at the current moment, we don’t have the manpower to do that.”

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Although a tray of the thistle-loving weevils costs $210, the association received 200 orders last year and was only able to bring in 88 trays, and had to reimburse everyone else.

And while they’re not a guaranteed fix, the weevils have been shown to slow down the spread of Canada thistle, said Nay.

“We do have some of our applied research sites that we’ve set out,” she said. “Some of the sites have taken off and they have established colonies. Others not so much.

“We’ve heard the same from some of our producers who have got weevils. Some have said they didn’t establish, but we have also heard from quite a few who have established.”

Orders are taken in March, with the weevils arriving in September. The adult weevils are released in a thistle patch where they hibernate for the winter in the topsoil or leaf litter. In spring, they emerge, mate and lay eggs in the rosette portion of the thistle plant. Larvae feed on the thistles, damaging the stem and leaving them more susceptible to pathogens. They pupate in the soil and begin the cycle again.

“They find partners and lay eggs and keep reproducing if they have been established,” said Nay. “The larger the number you release at one site, the faster they will multiply and spread out.”

Producers from Northern Sunrise County all the way down to the southern border have been ordering the bugs. Yellowhead County has several Canada thistle sites that are controlled with weevils and interested residents can go and tour the sites. Nay said anyone who wants to do so can contact the West Central Forage Association during the summer.

And the weevils can’t fly off like the stem gall flies, which the association also distributes. (These insects cause galls to form on the Canada thistle plants, effectively stopping them from growing as quickly.)

“We haven’t had supply issues with those,” said Nay. “They are a little more touchy because they are flies. The day we deliver them, producers have to put them out in their field.”

The flies, which prefer shaded and wetter areas, haven’t been as popular as the weevils, but some producers have had success with them, including ones in the south of the province. Northern Sunrise County is also experimenting with the flies.

“One of our big success stories is in one place in the Wagner natural area near Edmonton (and Spruce Grove). Because it’s a wetland area, they really took off and spread throughout the thistle they had there. It was an ideal spot for them.”

Stem gall flies cost about $145 plus GST and can be ordered by calling 780-727-4447 or [email protected].

About the author


Alexis Kienlen

Alexis Kienlen lives in Edmonton and has been writing for Alberta Farmer since 2008. Originally from Saskatoon, Alexis is also the author of two collections of poetry, a biography, and a novel called "Mad Cow."



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