A weed only a goat could love
BY THE BLAINE BUG CREW
Canada thistle is a gypsy at heart. Its seeds travel by rivers, streams and wind; they will find fertile soil and overtake the natural plants. Canada thistle thrives in any moist or/and shady space. The reason it spreads so fast is because its seeds launch themselves into the water supply, or birds eat the seeds and then later… but that is a discussion for another day.
Canada thistle is an aggressive, colony-forming perennial weed that reproduces by seed and deep, extensive horizontal roots. Flowering occurs from June through August. The flowers are urn-shaped, purple, and male and female flowers occur on separate plants with heads ranging from 1/2 to 3/4 inches in diameter. Fruits are about 1/8-inch long and brownish with a tuft of hairs at the top. Stems are typically 1-4 feet tall with alternate, oblong or lance-shaped leaves divided into spiny-tipped irregular lobes.
Canada thistle is a native of southeastern Eurasia and was introduced in gardens and flowerbeds – yes, someone used to actually plant it.
Canada thistle can tolerate a wide range of environmental conditions but requires good light intensity for optimal growth.
Now you’re probably wondering how we take care of this problem. The Blaine Bug Crew has the answer! URCA is a biological control agent that attacks Canada thistle stems and forms a gall, which acts as a metabolic sink. The metabolic sink decreases the nutrient supply available to the plant, thereby reducing plant vigor and growth. Adult flies emerge from galls in late spring to early summer, mate, and females deposit one to 30 eggs in Canada thistles’ vegetable shoots. Fly larvae initiate gall development by tunneling into the plant stem. Larvae grow slowly while the gall is developing and attain 98 percent of their body weight as a gall matures. Multiple larvae (three to 10) can be found within large galls. Pupation occurs within the gall in early spring. Galls resemble a small green crabapple, generally in the middle of the stem, and can vary in size, but they are generally marble to walnut-sized. The fly does best in semi-shaded, moist, disturbed areas with scattered Canada thistle plants.
If you have a large Canada thistle infestation that has no mowing, grazing, spray, or cultivation and think it might be a good spot for biological control, please contact the Blaine Big Crew at (208) 316-0354.