Canada Thistle Stem-Mining Weevils
On September 21, 2017, I hopped in a van with a partner from West Central Forage Association that was heading south to collect Lac Ste. Anne’s newest weed-fighting residents, Canada Thistle Stem Mining Weevils. These weevils were provided by a company called Weedbusters Biocontrol; you can find more information about their services at their WEBSITE. (http://www.weedbustersbiocontrol.com/aboutcontact.html)
After over 30 hours of driving in the rain, snow, more rain, then finally sunshine, and speaking with over 150 individuals to arrange weevil order pick-ups, our LSAC weevils had to spend one last night sleeping in a refrigerator before they could be released the following morning. After giving them 30-45 minutes minutes to wake up, they all looked lively and ready to nibble on thistle when they were released.
Why bring them?
The aim for bringing these insects into Lac Ste. Anne County is that they will establish a high enough population to reduce the amount of Canada Thistle that is present here, in a long term and sustainable fashion.
With any biological control agent, the results are slow to appear. Often, it can be over five years before any significant reduction is seen. Hopefully, at that time, the population will have grown large enough to allow us to collect weevils from our sites and provide them to residents for their own personal thistle control. We are incredibly excited to have these insects, and hope that you are too!
What are “Stem-mining Thistle Weevils”?
By using the common name of stem-mining weevils, you are referring to either Ceutorhynchus litura or Hadroplontus litura. For our purposes, these species are not differentiated as they produce the same effects, and are all-around very similar insects. They have been used and studied in places like the United States, New Zealand, and elsewhere in Canada. They are host specific to Canada Thistle (Cirsium Arvense) and some Carduus species (plumeless thistles).
There is more information available on this FACT SHEET (http://www.westcentralforage.com/media/24017/Weevil%20info%20sheet.pdf)
Why do we care so much about Canada thistle?
Unlike the name would portray, Canada thistle actually originated in Europe, with this weevil as its natural enemy. Canada thistle entered the USA from Canada, so it was assumed that Canada was its point of origin. Before the mistake could be rectified, the common name of Canada thistle stuck. The early settlers who brought Canada thistle with them as an addition to their salads did not realize the implication of bringing this plant into a new place without its natural enemy. Being able to out-compete native plants with its combination of high seed production, vegetative reproduction, and deep reaching roots, Canada thistle was one of the first weeds to be named on the Weed Control Act in 1907, and is the only weed that has remained on the Act for over 130 years.
Canada thistle has the same growing capabilities in Europe as it does in North America, and – for the most part – easier growing conditions with a gentler climate, and lots of moisture…. So, why is Canada thistle a noxious weed in Canada, and not in Europe? Because in Europe, Canada thistle has a natural enemy to keep its population in check. Since Canada thistle did not originate in Canada, no natural enemies to the plant live here.
Aside from their flowers, North American animals and insects do not enjoy consuming Canada thistle, as their sharp thorns are painful to eat. Canada thistle causes a reduction in diversity and the health of sensitive ecosystems (like riparian areas and wetlands); this means those ecosystems are less able to maintain a healthy, natural population of flora and fauna. In agroecosystems, Canada as a whole loses over $200,000,000 every year due to yield loss, specifically from thistle competition with crops.
Additionally, a high thistle population reduces the carrying capacity of pastures (how many animals a pasture can feed).
How do they work?
The weevils arrived in Lac Ste. Anne County as adults. As it is fall, and getting cooler, they will soon bury into the soil where they will overwinter as adults. In the spring, they will emerge, and lay their eggs on the stem of the thistle. The adults will die soon after egg laying/fertilization. The newly hatched larvae will mine into the stem of the thistle. Near the end of the August, the larvae will burrow into the soil, pupate, and emerge as adults. The adults will nibble on the thistle plants briefly, before burrowing back into the soil to hibernate again for the winter.
The weevils do not cause the direct death of Canada thistle plants. By boring holes in the plant, they weaken it since it cannot move water and nutrients as effectively, as well as leaving the plant susceptible to weather conditions, winterkill, other insects, and plant diseases.
How soon will they work?
Although weevils can fly, they don’t fly long distances unless they are forced to. It is unlikely that the weevils will move more than a couple metres per season.
For these weevils to work at all, it is required that they establish after release. Many things can impede their establishment, including: dying in transport, getting sick, extreme hot or cold, disturbance (like cultivation or mowing while they are in the ground or in the plant), etc.
In most cases, it’s a minimum of three years before you see effects, but it can take as long as five to eight years, dependent on the weevils and the weather conditions.
What if they become invasive, and take over the ecosystem?
They will not. There are incredibly strict regulations that biological control agents must pass before they are permitted to enter Canada. One such test: they are placed in a closed system with economically or nationally valuable plants. In a situation like that, only permitted response a biological control agent can have is to starve if its specific host is not present. In our weevils’ case, they would rather starve to death than eat anything other than Canada thistle.
Are they dangerous?
Only to Canada thistle. They are host specific, are not vectors for disease, and carry no zoonotic diseases (diseases that can be passed to other species, animals, plants, or humans). Again, there are incredibly strict tests and regulations that these weevils must pass before they are permitted into Canada. This would be the first question answered, and if there is even a possibility of a biological control agent causing any harm to humans, it would immediately be removed as an option.
Will they work?
Time will tell. As mentioned, it takes a long time for biological control agents to take effect. Once they do establish, they will never produce 100% control on their given target, as that target is their food source. Through using biological control, we can reduce the plant stand, and maintain Canada thistle at a manageable level.
However, weed control is best done through Integrated Pest Management programs. This means using a combination of biological, mechanical, cultural, and chemical tools to control weeds in a long term and sustainable fashion. The weevils (biological) are most successful where a healthy, desirable plant stand is promoted (cultural), and some herbicide (chemical) or mowing (mechanical) is done to control the thistle spread until the weevils establish in a high enough population.
Can I get my own weevils?
You definitely can! Contact West Central Forage Association to order a tray. However, as these are living creatures, weevil collection is weather and weevil dependent, so you may have to be patient. Some people are on wait-lists for two or three years before they can get themselves a tray.
I have more questions that weren’t answered here.
Please, contact Agriculture Services at 780-785-3411 or via email for more information.