Effect of Urophora cardui (L.) (Diptera : Tephritidae) and Ceutorhynchus litura (F.) (Coleoptera : Curculionidae) on the weed Canada thistle, Cirsium arvense (L.) Scop.

Stress caused by two biocontrol agents on their weed host Canada thistle [ Cirsium arvense (L.) Scop., Asteraceae] was investigated. The combined attack of the gall-fly Urophora cardui (L.) (Diptera: Tephritidae) and the stem-mining weevil Ceutorhynchus litura (F.) (Coleoptera: Curculionidae) caused a significant reduction in the vigour of the attacked plants as measured by the number of live buds in the 1st year and by the dry weight of the roots in the 3rd year. The attacks did not significantly reduce the number of aborted buds, mature seed heads, and new shoots, nor the combined dry weight of the leaves, stems, and galls as compared with Canada thistle plants that had not been galled or mined, in any of the 3 years. Non-structural carbohydrate content of the roots of attacked shoots was found significantly reduced only at the end of the mining period.