Canada thistle (Cirsium arvense) is a perennial, developing from deep and extensive horizontal roots. Stems are 1 to 4 feet tall, ridged, branching above. Leaves are alternative, lacking petioles, oblong or lance-shaped, divided into spiny-tipped irregular lobes. Flowers are unisexual, on separate plants, flowers purple (occasionally white) in heads 1/2 to 3/4 inch in diameter. These male and female flower heads make the Canada thistle unique from other true thistles. The weed is a native of southeastern Eurasia. It was introduced to Canada as a contaminant of crop seed as early as the late 18th century. This aggressive weed is difficult to control.
Biological control agents for Canada thistle include a stem and shoot gall fly (Urophora cardui), a seed head weevil (Larinus planus), a stem mining weevil (Ceutorhynchus litura) and a foliage feeder (Cassida rubiginosa). Since 1993, biological control agents for Canada thistle have been released by the South Dakota Department of Agriculture, several county weed and pest boards along with several land managers. The establishment and efficacy of the biological control agents will continue to be monitored. Successful establishment has begun to allow for collection and redistribution of some of the above-mentioned bioagents.