Biology: Canada thistle is a perennial, deep rooted colony forming, aggressive weed. Aerial shoots are produced from creeping lateral roots. Can spread by seed, however, vegetative spread is the more common method. Tillage acts to increase the size of infestation as fragments of the roots will easily grow into a new stem. Canada thistle plants are dioecious, meaning they can have either all male or all female flowers. Typically a colony of Canada thistles consists of genetically identical all male or all female plants, and this condition is unique to North American thistles.

Range: Canada thistle is found throughout North America, Europe, North Africa, Asia from Siberia through Japan, China to Afganistan.

Control: Achieving control is difficult, as tillage worsens the problem by dragging and spreading rhizomes, while herbicides are prohibitively expensive in low value pastures or forage. The extensive and deep root system also makes complete herbicide translocation difficult to achieve total control.

Past work on biological control in Nova Scotia had concentrated on Urophora cardui, a gall insect, and Hadroplontus litura, a stem-mining weevil (Sampson and Ingraham, 1990).

In Nova Scotia, U. cardui has become established throughout the province, but does not provide good control with the exception being years when adults emerge early and achieve gall formation on main stems causing stunted plants with reduced flowering vigour.

Hadroplontus litura was released into Nova Scotia from 1984-1990. It was recovered in 1989 (Sampson and McSween, 1992) but was not observed in later surveys until 2004 when feeding damage was discovered in Eastville, Nova Scotia (Colchester County) on the few scattered remaining plants left over from a much larger infestation in 1990. This insect although elusive, appears to give excellent control once populations reach high enough levels.

Rhinocyllus conicus, a seed-feeding weevil, was released on Canada thistle in Nova Scotia in 1989. This insect was recovered in 1990 but the current status is unknown.

Cassida rubiginosa, when present in large quantities can extensively defoliate localized patches of Canada thistle. This insect is established throughout eastern Canada.

Vanessa cardui, a butterfly whoses larvae occasionally defoliates thistles over large areas (Harris, 2003). Orellia ruficauda is a seed feeding insect and larvae of this fly feed extensively the seeds of Canada thistle. These insects are found in all provinces but are not host specific, thus unsuitable for biological control.