Thesis (M.S., Plant, Soil and Entomological Sciences)--University of Idaho, June 2014 | Classical biological control of weeds has been used almost 50 years to control Cirsium arvense (Canada thistle) in the United States. However, few field studies have assessed the efficacy of approved biological control agents, a stem-galling fly Urophora cardui and stem-mining weevil Hadroplontus litura. We reviewed literature on C. arvense control, and between 2008 and 2012, monitored release transects of U. cardui, H. litura, both species, or no insects released at C. arvense infestations across the western United States. At each study transect (n = 87), we measured U. cardui parasitism pressure, biological control agent establishment and abundance, vegetation cover, C. arvense density, and abiotic variables; each as potential factors in inter-annual C. arvense density fluctuation. We found that U. cardui galls are attacked by five parasitoid species at low rates and that C. arvense infestations are negatively affected by perennial grass competition and not by biological control agent herbivory.