Weevils to wage war on Canada thistle


Forage association testing biological control in Mountain View County

Forage specialists plan to release hundreds of predator insects in Mountain View County this fall to test the effectiveness of a biological control on a very common noxious weed.

Much research has been done on the effectiveness of stem mining weevil to control Canada thistle in the U.S., but little is known about the insect’s effectiveness and survivability in the Prairie climate despite its use here for several years.

“We can take that research with a grain of salt or we can try and answer some of the many questions we have,” said Devin Knopp, a forage and livestock agronomist with Grey Wooded Forage Association (GWFA).

Standing in knee-high thistles at times, Knopp, association summer student Enna Graham, and Lorelee Grattidge, a Mountain View County sustainable agricultural specialist, staked out trial plots on riparian land surrounding the county’s municipal office off Highway 2A last Thursday.

Weevils will be released into the plots this fall kicking off what’s expected to be a five-year project.

“We’ve got some main questions we’re trying to answer,” said Knopp. “How likely are they to survive the winter? How likely are they to spread outside the monitoring area? How likely are they able to control Canada thistle?”

Canada thistle is widespread in Alberta and is classified as a noxious weed. It is of concern to many landowners as it spreads easily and plants can crowd out forage grasses in pastures and rangelands, reducing yields and productivity.

It can be tricky to eradicate, Knopp said, as roots have been found to extend up to 25 feet into the ground.

“They reach nutrients and water our grasses can only dream about,” said Knopp.

Herbicide has proven effective but its use is restricted by municipal and provincial governments in some areas like in riparian areas near waterbodies or courses.

For these areas specifically, as well as for any land, it is hoped the stem mining weevil can prove effective over time.

The insect has evolved to feed mainly on Canada thistle and offers a one-two punch to aid in eradicating a plant.

Although feeding on the Canada thistle doesn’t necessarily kill it, Knopp said it does cause stress on the plant. And when larvae exit the stem, they create opportunities for other insects to enter it.

GWFA is embarking on two different loosely defined “experiments” said Knopp.

A monitoring project is occurring in Lacombe and Wetaskiwin counties as well as with the Medicine River Watershed Society. One cup of weevils, amounting to 150 insects, will be released into the centre of each location’s test plots once thistle counts have been taken in each.

Thistle dissections will be taken at random next year to determine if weevils are reproducing and in three years plant stand counts would be taken as well as weevil population counts.

In Mountain View County, GWFA is launching an establishment project.

“In this one we also want to find out if we can overload the population,” said Knopp.

Four different trial plots measuring four metres by four metres have been established at the Mountain View County office site and another four trial plots have been established at a location south of Sundre.

Random plant counts have been taken at each.

At each of the locations in the fall, one cup of weevils will be released into one plot, two cups into another, and three into another, leaving one plot at each location free of weevils.

“We’re going to see if we can speed up control by overloading,” said Knopp.

The establishment project will also be ongoing for five years with similar plant dissections, plant and insect counts being done throughout the years as the monitoring project.

Mountain View County is a member of the forage association and its sustainable agricultural specialist said the project holds promise for area landowners.

“We’re hoping this is going to be an effective option to other control methods like herbicides,” said Grattidge.

Herbicide use in riparian areas, what she described generally as “an area too wet to farm, too dry to fish,” is restricted.

There’s also people who don’t like using herbicides for a variety of reasons.

“So this is just a natural way, per se. We’re not adding something that isn’t necessarily found in nature.”


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Lea Smaldon

Lea Smaldon joined Mountain View Publishing as managing editor in 2006.