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Canada Thistle

By The Blaine Bug Crew

Today, we are going to talk about the noxious weed called Canada thistle and how to keep it contained naturally.

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 (sometimes white), and male and female flowers occur on separate plants. Fruits are brownish, with a tuft of hairs at the top. Stems are typically 1 to 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 to Canada as a contaminant of crop seed in the 18th century.  It can commonly be found in gardens, flowerbeds, pastures, cultivated fields, rangelands, forests, and along river banks, ditches and roadsides. Canada thistle can tolerate a wide range of environmental conditions, but requires good light intensity for optimal growth. It is highly competitive with crops and, in heavy concentrations, effectively prevents grazing. Only two insects are approved for release to contain Canada thistle—Urophora cardui and Hadroplontus litura.

The Hadroplontus litura is a stem-mining weevil. It attacks the stems and rosettes of Canada thistle. Adults feed on rosette leaf foliage in the spring, and larvae consume tissues while mining within the shoots. At low densities, larvae and adult feeding does not significantly impact populations of Canada thistle directly. Feeding does cause secondary damage, however, as pathogens and other organisms enter the stem of Canada thistle via holes made by exiting larvae. At high densities, feeding by Hadroplontus litura will reduce the vigor of both rosettes and flowering stems to the point of deterioration.

Overwintering weevils emerge from soil litter and feed on leaf and stem tissue in early spring. Eggs are laid in spring within Canada thistle’s boiling shoots, with hatching larvae mining in the stems and root crowns throughout spring and summer. Multiple larvae (up to 20) can be found in individual stems. Mature larvae tunnel out of the stems, drop to the soil surface, and pupate in the soil. Adults of the new generations emerge in August and overwinter in soil litter. There is one generation per year. The weevil does best in open but moist areas with scattered Canada thistle plants.

If you are aware of an invasion of Canadian thistle that needs attention, please let the Blaine County Weed Department know at (208) 788-5574.

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