A weevil wonder

A weevil wonder

Every year, Canada thistle takes a big bite from the productivity of countless acres of Albertan forage land. A master at pushing out desirable forage, the noxious weed hits producers where it hurts most: in their forage yields, which translates directly to their cash in pocket. The research is sobering: for every pound of Canada thistle biomass that grows in a field, almost two pounds of desirable forage biomass is lost. Because tillage spreads self-propagating pieces of root, mowing cuts the above ground portion of the thistle but leaves it hardy root system, and grazing is ineffective at all but the tiniest (rosette) plant stage, many producers assume they have exactly two choices: the (often-undesirable) option of applying herbicide or the (always undesirable) option of conceding defeat. Luckily, they are wrong. There is one more option: Hadroplontus litura, a little known bug more commonly known as the Canada thistle stem mining weevil.


The Canada thistle stem mining weevil first gained approval as a biological control agent in Canada in 1965, after proving effective and entirely target-species specific during trials in Lethbridge through the early 1960s. Since then, the tiny black bugs have been imported from colonies in the US in relatively small numbers. Despite their lack of uptake among agricultural producers until now, they are impressively effective, all-natural, and fully self-sustaining.


“The science behind these weevils is proven and complete: we know they won’t go rogue and they won’t become an agricultural pest,” says Kristina Pultz, who coordinates weevil imports into Alberta in her role as conservation agriculture and extension program manager with the West Central Forage Association.


“The issue we face is that many people don’t know they exist and don’t know they are allowed in Canada. And those who do know about them, well, it can be tough to convince people to give them a try. To be honest, they’re not cheap, they take time to be effective, and they are something very different than your usual chemical option, so the concept can be a tough sell.


But, she adds, “Our uptake is going up dramatically now. Farmers want something self-sustaining – they don’t want to be out there mowing and weeding - and a lot of people are really trying to get away from chemicals.”


Adult weevils live on and eat the thistle leaves and stems through late summer and early fall before burrowing into the soil to hibernate at the first heavy frost. In spring, the adults emerge from hibernation and mate. Each female chews a hole into the leaf of a Canada thistle rosette and deposits her eggs in the midvein. Approximately a week later, the eggs hatch and larvae begin to mine their way through the leaf, down the stem and into the root collar. By early summer, the larvae reach their maximum size and prepare for metamorphosis by chewing their way out of the thistle shoot near ground level and burrowing into the soil. Two to three weeks later, the newly adult-stage weevils emerge and begin feeding on the leaves of the thistle, preparing for winter hibernation.   


While adult stem mining weevils do eat a limited quantity of Canada thistle, it is actually the larvae that cause death in Canada thistles.


“The mining action of the weevil larvae and the exit hole they leave behind when they move from the plant into the soil leaves the plant vulnerable,” explains Pultz. “They don’t kill the plant directly but instead act as an agent to open the door to environmental stress such as cold/frost (which could be why they seem to do so well here) and disease.”


Though effective, the weevil’s action is not immediate. In fact, farmers should expect this form of bio-control to require multiple years.


“Let’s say you want to release them in 2016. You’d release them in the fall – September or October – because they are not reliant on soil at that stage of their lifecycle. They will chew on the leaves on the plant but not enough to be noticeable. In 2017, they’ll lay eggs and the first larvae will start chewing into the Canada thistle rosettes. But you will see little difference in the thistle population. By the second larval year, so that would be 2018, you might see a little thinning but you shouldn’t expect to see a really significant difference. By the third larval year – 2019 – you should see a real difference,” says Pultz. “Our rule is that you can expect to see a significant difference in three to four years. It can be hard to be patient but it is worth the wait.”


Adult weevils should be released all together into a single patch of healthy thistle to provide a dense enough population that they have a chance to get established. The patch of thistle should then be left undisturbed for the next multiple years so the weevils have a suitable environment to colonize and proliferate in.


Once the weevils gain the upper hand on the patch of thistle you’ve released them into, they will travel short distances in search of more thistle. Their movement is relatively slow: Canadian field studies show that weevils spread an average of 90 meters from their initial release location over six years.


Last year’s cost was $190 per tray of 105 live weevils, says Pultz. While any producer can privately arrange an import certificate from the Canadian Government, most Albertan producers instead opt to have the West Central Forage Association handle the paperwork and customs requirements, particularly since Pultz is willing to meet with and drop trays to producers anywhere along her travel route from the Canadian border up to the Peace Country.


In 2014, Pultz imported 250 trays of weevils. Last year, demand jumped to 600 trays, environmental conditions in Montana meant she was unable to complete the orders.


“Last year, Montana had a cold, wet summer. The bugs were like teenagers that don’t want to get out of bed – we didn’t have the right conditions for them to emerge when we needed them to. Last year’s conditions shouldn’t effect this year’s population, so we’re hopeful that we won’t have any problem importing them this year,” she says.


What she cannot yet do, unfortunately, is advise farmers as to how many weevils they should order.


“The science does not yet exist to say that if you have this much land and this much thistle, you need this many bugs,” says Pultz. “How many bugs you need depends on the size of your thistle infestation, how much money you want to spend, and how quickly you want to get rid of them.”

A big reason the weevils have not yet taken off among Canadian producers is the fact that they do not suit crop land, as soil disturbance from tillage and seeding disrupts their habitat.


“There’s real potential for these weevils to suit beef producers. Already, beef producers are one of our big buyers,” says Pultz. “When people take a look at photos of our demonstration sites, can see how the Canada thistle population has dwindled, and hear that the weevils will last indefinitely as long as there is a food supply for them, they get pretty excited.”


by Madeleine Baerg