Post-release assessment of the efficacy of Urophora cardui and Hadroplontus litura, biological control agents of Canada thistle

Biological control of Canada thistle (Cirsium arvense (L.) Scop.), is one of the oldest programs in the U.S. but there are only few studies assessing the impact of biological control on this invasive Eurasian perennial thistle. Two biological control agents are still considered environmentally safe and are permitted for release on C. arvense, the stem-galling fly Urophora cardui (Diptera, Tephritidae) and the stem-mining weevil Hadroplontus litura (Coleoptera, Curculionidae), first released in the U.S. in 1978 and 1972, respectively. To assess biological control efficacy, we set up study sites using a standardized impact monitoring protocol (SIMP), consisting of ten 0.125m² plots along permanent 20m transects at C. arvense infestations across Idaho (n=7) and South Dakota (n=9). At each infestation, four transects were set up at least 1 km distant from each other and releases of either biocontrol agent alone or combined were randomly assigned to the four transects, resulting in a total of 64 permanent study transects. We measured vegetation cover and C. arvense stem density in plots and assessed insect abundance in 2008, 2009 and 2011. Data varied greatly between study sites, years and biocontrol agent treatments.  Generally, U. cardui and H. litura are widespread but occur at low abundances regardless of whether insects were released at a transect or not.  Data analyses are preliminary, in part because abiotic covariates have not been included yet. Based on the available data we conclude that biological control has no impact on C. arvense in Idaho or South Dakota.