Resumen
Diffuse and spotted knapweed (Centaurea diffusa Lam and C. stoebe L.) are two closely related invasives found in many parts of British Columbia?s Southern Interior, causing substantial economic losses in rangelands. Beginning in 1970, the provincial government initiated a long-term biological control effort against the knapweeds, introducing 10 different insect agents from 1970 to 1987. In an effort to evaluate the efficacy of the program, archival (1983?2008) data was amassed from 19 vegetation monitoring sites that contained knapweed. In 2010, these sites were relocated and re-monitored and cover values were analyzed. Diffuse knapweed showed significant declines at 14 of 15 sites; spotted knapweed declined at three of four sites. Possible alternative explanations for the decline are discussed. Evidence strongly points to a suite of biocontrol agents (seed feeders and root feeders) as the primary drivers of knapweed decline in British Columbia?s Southern Interior.