Abstract
Musk (Carduus nutans group) and Canada (Cirsium arvense) thistles differ from each other in that they are in separate genera and have different growth habits. Musk thistle can act as an annual, a winter annual, or a biennial, and reproduces totally by seed, whereas Canada thistle is a perennial and reproduces mainly by vegetative regrowth and continues to emerge year after year from the same plant root material. Rhinocyllus conicus is a seed head attacking weevil which was introduced into North America for the biocontrol of musk thistle. Larvae of a second weevil, Trichosirocalus horridus, feed on the apical meristem of the musk thistle rosette and developing stems. Larvae of Cheilosa corydon mine directly into the tender, young musk thistle shoots, then mine up and down the stem, causing the stems to become easily broken and/or prematurely dried. The weevil Ceutorhynchus litura was released in Canada in 1965 for the biological control of Canada thistle. Larvae of Urophora cardui burrow into the stem where they induce the plant to create large galls, which act as a nutrient sink and stress the plant.
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