Weevils

The Canada thistle, despite its name, is native to Europe and is an invasive and harmful plant in Canada. Anyone who has walked Camrose's trails has run into the large spiked weeds. Besides being irritating and painful for us and our pets, they are also a huge problem for agriculture, as they deter cattle from grazing and compete with other crops, drastically reducing yields. With thousands of seeds and 6 meter roots, they spread rapidly and are difficult to eradicate. Mechanical and chemical control methods are often expensive and not always effective, so what else can we do?

The City of Camrose is beginning a trial of introducing the “Canada thistle stem mining weevil.” These weevils have been approved and used in Canada since 1965 to control Canada thistle, only eating this species and a few close relatives. They will be released in two large thistle patches on the north and south ends of the river valley and will use the thistles as food and habitat until they begin hibernation in the soil. In early spring, as the first thistles begin to grow, the weevils will emerge and continue eating. After a few weeks, eggs are laid in holes in the plant. When they hatch, the larvae tunnel downwards towards the roots while eating and, once fully fed, enter the soil to undergo metamorphosis. Now fully grown, the weevils begin eating again and continue this cycle.

The weevils do not eat the entire plant, but they cause enough damage to allow other microorganisms to enter the thistle and kill it in addition to lowering the plant's chances of surviving the winter. The insects will spread, with one study seeing them move 9km ten years after release. Unfortunately, this is not a quick solution, as most of us would love to see the thistles gone as soon as possible, but this is a more permanent and ultimately cheaper solution than continued picking or spraying only to have more thistles pop up next year.

It is hoped that the weevils will spread throughout the City, decrease the current infestations of thistles, and stay in the environment to combat any new weeds that are blown in. For the next few years, you will see stands of thistles that are left untouched instead of being destroyed as they usually are and this is to provide good habitat for the weevils as they become established. Once they do, the patches should clear out in no time!