Canada thistle: Meet your worst nightmare

Every rancher hates this hard-to-control weed, but stem mining weevils just eat them up (from the inside)

Weevil larvae offer up major damage to Canada thistle.
Reading Time: 3 minutes

Every year, Canada thistle takes a big bite out of the productivity of Alberta forage and grasslands. But an increasing number of producers is biting back — in a wholly natural way.

Larvae of an adult weevil.
Larvae of an adult weevil. photo: West-Central Forage Association

In late August, the West-Central Forage Association will once again import hundreds of cartons of little black bugs from an insectary in Montana. These weevils will make their way into the enthusiastically waiting hands of dozens of Alberta forage and livestock producers frustrated by costly and difficult-to-manage Canada thistle.

Once released into infestations, the weevils will deliver an all-natural, permanent, self-perpetuating biocontrol solution to the troublesome weed.

“I can’t believe how many phone calls we are getting from farmers about Canada thistle stem mining weevils,” Melissa Freeman, who is co-ordinating the importation of the stem mining weevils for the forage association, said earlier this year.

“The demand is definitely there, and growing. In 2014, we had 89 people order weevils through us. Last year, we had 120 people confirmed on our list, but a late snowfall down in Montana meant the adult insects couldn’t be collected so we weren’t able to bring any up. And this year, we’re getting tons of calls.”

Canada thistle is an ultra-competitive noxious perennial weed. Every pound of Canada thistle biomass reduces grazing land’s desirable biomass by two pounds.

The weed can seem almost impossible to attack. In addition to producing thousands of seeds, its deep and extensive root system can self-propagate, sending up shoots every few inches for 20 feet or more. Control options are limited as livestock refuse to eat the spiny leaves except when the plant is tiny; mowing allows the hardy root system to send up countless more shoots; tillage simply spreads the self-propagating roots; and herbicides are typically undesirable or impossible to apply on grazing land.

A heavy infestation of Canada thistle in 2010 (top) prior to the release of the weevils and the same patch of ground in 2012 (bottom).
A heavy infestation of Canada thistle in 2010 (top) prior to the release of the weevils and the same patch of ground in 2012 (bottom). photo: D. Engstrom

Canada thistle stem mining weevils, approved as a biological control agent in Canada 40 years ago, are attracting much greater interest because they are environmentally friendly — and they really work.

Freeman bought six trays of weevils for her farm two years ago.

“It takes awhile for the population to build up — as much as five years — so we didn’t expect much at first,” she said. “Last year, we saw a bit of a difference. This year when we went to check our pastures, there was almost no thistle. We’ve seen a very big difference.”

There’s no official recommendation regarding how many weevils are required — it depends on the size of your thistle infestation, budget, and how quickly you want to get rid of the thistle.

An entire tray should be placed in a single infestation in order to allow the bugs to build up a self-perpetuating population. Once the weevils gain decent control of the patch of thistle, they will travel short distances in search of more thistle. Their movement is relatively slow — Canadian field studies found they move about 90 metres from over six years.

Canada thistle stem mining weevils are available to purchase from Integrated Weed Control in Bozeman, Montana. This year, a tray of 105 weevils will cost US$125 (plus shipping). To bring the bugs across the Canadian border, an importer must hold a Canadian import certificate.

Because the weevils are susceptible to heat and cold, and shipped items are often delayed at the Canadian border, Canadian customers should plan to pick up their weevils at the U.S. insectary and then personally transport them across the border rather than shipping them.

While anyone may privately arrange an import certificate from the Canadian government, most Alberta producers instead opt to have the West-Central Forage Association handle the paperwork and customs requirements. In late August, forage association staff plan to drive down to Bozeman to pick up the insects, drive them across the border, and hand deliver them to producers along a set route up through Alberta.

For more information, contact the WCFA at [email protected] or 780-727-4447.

About the author



Stories from our other publications