Order Your Thistle Stem Mining Weevils

  • 04 Mar 2019
  • Written by  Big Lakes County

Thistle Stem Mining Weevils will be available for pick-up at the Big Lakes County Administration Office in late August/Early September. Final pick-up instructions will be provided to all participants closer to the date. All orders must be paid for when placed. Participants are encouraged to follow the Weevil Release Pointers document provided by West Central Forage Association for best success.  The Weevils are $200/ tray for 100 Weevils. Below you will find the Weevil order form. Please fill it out and bring it in to the Admin office. 


What is Canada Thistle?
Canada thistle (Cirsium arvense) is a vigorous, highly adaptable, highly competitive weed that occurs in a wide range of habitats. Canada thistle is the species most frequently declared noxious under state or provincial weed control legislation in the United States and Canada because it causes extensive crop yield losses. It is listed as “Noxious” under the Alberta Weed Control Act, meaning that the plants must be controlled. A density of 20 Canada thistle shoots per square metre can cause estimated yield losses of 34% in barley, 26% in canola, 36% in winter wheat, and 48% in alfalfa seed. Field infestations can reach 173 shoots per square metre. The prickly mature foliage is also thought to reduce productivity of pastures by deterring livestock from grazing.

How to prevent it?
The Canada thistle stem-mining weevil (Hadroplontus litura, formerly Ceutorhynchus litura) occurs naturally in France, Switzerland, Austria, Germany, Britain, and southern Scandinavia. It was first introduced into Canada as a biological pest control agent in 1965 and into the US in the early 1970s.
Eggs are laid in the mid-vein of the rosette leaves in early spring, and hatch after five to nine days. Larvae internally mine the inside of the stem of the thistle plant as the shoot elongates during the summer. Fully developed larvae will exit the plant at the root and enter the soil to pupate. They will emerge again in their adult form later in the summer, and feed on thistle leaves before winter. Adults will over winter in the soil, ready to attack the emerging thistle the following spring. The adults are cold hardy and can tolerate wet spring snow storms, and some light flooding, without difficulty. When the larvae mine the stem, they consume plant tissue, and leave exit holes when they emerge, which may allow other micro-organisms to enter the thistle stem, with adverse consequences for the thistle.

Will the bugs become invasive?
NO. They would not be approved as bio-control agents, and therefore would not be allowed in the country, if they were a risk to agricultural crops. The host range of the weevil is restricted to C. arvense and some Carduus species (plumeless thistles, but not nodding thistle).

How quickly will they work?
Even in the most successful of cases, years are required before an insect can catch up with an exotic weed species. It is not a quick fix, but a permanent, self-perpetuating weed control tactic. Canadian field studies indicate a spread on average of 90 m in 6 years, but results seem to vary regionally. Infestation at several initial release sites located in Bozeman, Montana (where we get the bugs) was slow to expand in the first few seasons, but after ten years weevils were found 9 km from these release sites.

How many insects are enough for my infestation?
Unfortunately, it isn’t as simple as saying that you need two trays for your two acres of thistle. However, there are three approaches to determining how many you might need/want:
One approach is to inundate your infestation with as many releases as is affordable during the first couple years of introduction. After that, one should make additional releases in isolated areas in future years. This method is the most dramatic. It reduces the time, in years, for an insect to build up and disperse to the limits of your infestation. Another approach is to release insects into your most critical areas in the first year. Then, reinforce these releases with additional insects in future years. This method strives to achieve a balance between what is affordable and the degree of infestation in your area. The least costly approach is to introduce one or two releases into your infestation and do nothing more. This method will get a colony started. It may also take many more years for the insect to distribute itself throughout your entire infestation.

Should I spread each release over a large area?
No, concentrate each release. Your goal is to get the beneficial insect colonies established in the areas you have set aside for biological control. When starting any population in a new area, you need enough numbers to start with so you can withstand potential mortalities, but still have enough individuals left over to continue to build the population. With herbicide, you take a finite amount of material and dilute and spread the material over a large area. When introducing a biological control agent, one takes a finite amount of material (insects) and concentrates it in specific areas. In the initial releases, one wants to concentrate the insects so the insects can continue to find each other to mate and reproduce. Also, this allows one to more easily check for the insect in future years and for later redistribution to new areas.

How quickly do they work?
Even in the most successful of cases, years are required before an insect can catch up with an exotic weed species. It is not a quick fix, but a permanent, self-perpetuating weed control tactic. Keep in mind that it is the larvae that does the damage, not the adults, so control of the thistle won’t really start until the spring after release. You will see no difference in the establishment year. The next year you may start to see a slight thinning the thistle, but the third year is when you should really start to notice a difference. Literature research has shown that, upon release at new locations the weevils will spread slowly. In field studies in Canada, they spread on average 90 m in 6 years. Infestation at several initial release sites located in Bozeman, Montana (where WCFA import the weevils from) was slow to expand in the first few seasons, and after ten years weevils were found 9 km from these releases.

How can I tell if they are working?
There are several ways to tell if the weevils are being effective. First, place a stake into the ground at the release site, so you have a point from which to measure the thistle populations. Take pictures every year to have a visual point of reference, over time you should notice a change in the plants species at the release site; thistle numbers should decline and other plant populations should increase. Another option is to conduct plant counts before you release the weevils and yearly after that. If you want to check to see if the bugs are present and breeding, you can pull a few individual plants (look for plants that show outward signs of being unhealthy, like blackened stems) and slice the stems open lengthwise. You may find larvae present, or just the damaged plant.

Are herbicides compatible with bio-control?
Yes and no. Each weed and beneficial insect has specific, case by case, situations where one can integrate herbicides and bio-control. For most cases, while herbicides may not kill the bio-control agents, the damage done to the host plants prevents the insects from completing their development cycle. In other words, if all of the weeds die, then the insects won't have anything to live on. To integrate insects with herbicides, we recommend first releasing insects on an area away from livestock, and that is not mowed or sprayed. Then, at least two years should be allowed for the insect populations to build up. If herbicides are to be used at all, it should be after these natural enemy populations have increased. Even then, the timing of spraying is critical to minimizing the damage to the insect populations.

What happens to the insects after the weeds are gone?
Sorry, but your weeds may never be eradicated. In the most successful examples of biological control, there are always a small number of plants that do not fully succumb to the attack of the beneficial insect. This is good. It allows the insect population to sustain itself during years of low weed density. Once the weevils have exhausted a thistle patch, they will migrate to look for more food.

How important is competing for vegetation in successful weed control?
Biological control insects alone are not the answer. Without healthy stands of desirable vegetation to take the place of undesirable weeds, bio-control cannot be successful. As the insects reduce the weed population, useful plants take their places and gain a competitive advantage. Together, bio-control agents and competing vegetation will reduce weed infestations. Encouraging desirable plants, by re-seeding or reducing grazing pressure, will greatly help the insects do their job.