The seed-feeding weevil Rhinocyllus conicus (Froel.) was screened and later introduced into Canada to control the invasive nodding thistle (Carduus nutans). The weevil is also known to attack thistles in the Cirsium genus. A study was conducted in British Columbia to confirm whether the weevil?s lack of establishment and population growth on marsh thistle (Cirsium palustre (L.) Scop.) was because the thistle was a poor host or its local habitat was unsuitable for the control agent. Weevils were released at 10 sites over a 4-year period. The existence of individual weevils at different life stages was investigated. No failing was found at any life stage, which indicated that marsh thistle was an adequate host plant for the weevil. The weevil?s spread from the point of release and its preferences for solar radiation as influenced by slope and aspect were documented. Additionally, although the numbers of weevils released annually had declined, the numbers marginally increased in some instances, potentially indicating acclimation to the site and population growth.
Susan C. Turner, Rick E. Tucker, Reg F. Newman, Sandy C. Cesselli. 2021. Survival of the Biological Control Agent Rhinocyllus conicus on Marsh Thistle (Cirsium palustre) in British Columbia. FLNRORD. Technical Report (FLNRORD). TR136