The primary biocontrol agent currently in use against Canada thistle is the stem-gall forming fly, Urophora cardui. Urophora lays eggs in the stems of Canada thistle and large galls develop around the stem-dwelling larvae. Larvae pass through the winter within the protective galls and emerge in the spring as adults. This agent is not effective in killing the plant but may weaken the plant, making it more susceptible to other stresses.
A stem mining weevil, Ceutorhynchus littura, has been released in Colorado but reliably collectable populations have yet to be established. Since there is some evidence that these beetles may be effective in combination with the gall flies the Department of Agriculture is working to establish collection sites for C. littura for use in combination with U. cardui.