There’s a reason why stem-mining weevils are the priciest livestock in Alberta

The insects are mowing down Canada thistle across the province, and attracting lots of eager buyers

The stem-mining weevil larvae live up to their name, burrowing down the length of Canada thistles and feeding on their leaves after becoming adults.
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Tens of thousands of weevils have been pouring into Alberta from Montana — and producers are clamouring for more.

“It’s a biological control and it’s been going great,” said Rachael Nay, conservation agriculture extension co-ordinator with the West-Central Forage Association.

The association began the project to import stem-mining weevils four years ago as a way to combat Canada thistle, which can devastate the productivity of pastures if it gains the upper hand. The noxious perennial weed is hard to control, and difficult to attack. Each thistle produces thousands of seeds and individuals send up numerous shoots — as many as 170 shoots have been found in a single square metre, with just a fraction of that causing major productivity losses.

However, while cattle won’t eat thistles, stem-mining weevils will.

The insects lay their eggs in thistle rosettes and the larvae live up to their name by munching their way down the stem of the thistle as it grows. After exiting and pupating in the soil, the adults emerge and feed on thistle leaves.

“It’s a great alternative to herbicide,” said Nay.

The weevils are non-invasive (they only eat Canada thistle and some types of plumeless thistles) and were first approved as a biological control agent half a century ago.

Each tray, containing 105 weevils, costs $200 plus GST. How many you need varies.

“It really depends on how big of an area they have to cover,” she said.

What are we bid for this handsome devil? With tax, producers are paying $2 apiece for adult stem-mining weevils. But the insects and their larvae are voracious feeders on Canada thistle and over time can bring severe infestations under control photo: West-Central Forage Association

West-Central Forage suggests three methods. An aggressive approach is the most costly — buying as many as you can afford and topping them up with one or two releases in following years. But you can also release a tray or two in the worst area of Canada thistle infestation and release one or two more in following years. Or just release one tray and wait for them to slowly spread. (One Canadian study found the insects took six years to spread 90 metres, but a Montana study found that after a slow start, they had travelled nine kilometres from the release site a decade later.)

Producer interest in the weevils has spread much faster. The association gets daily calls from producers who are interested in purchasing the insects, said Nay.

“The program is getting bigger every year,” said Nay, adding her association has been working with other forage groups to get the word out and producers from all over the province have been placing orders.

Producers need to order them in spring with trays arriving at the end of August, which gives time for the adults to adjust and successfully overwinter. For more information, contact West-Central Forage at [email protected] or 780-727-4447.

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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