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

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

Joel Price, Mark Schwarzländer and Bradley L. Harmon

Although biological control of Canada thistle (Cirsium arvense (L.) Scop.), is one of the oldest programs in the U.S., there are only few studies assessing the efficacy the two biological control agents approved for the control of Canada thistle. These are the stem-galling fly Urophora cardui (L.) (Diptera, Tephritidae) and the stem-mining weevil Hadroplontus litura (F.) (Coleoptera, Curculionidae), first released in the U.S. in 1978 and 1972, respectively. We set up permanent study sites using a standardized impact monitoring protocol (SIMP), consisting of ten 0.125m² plots along 20m transects at Canada thistle infestations in the State of Idaho (n=7) and South Dakota (n=9). At each study site, four transects were set up at least 1km distant from each other and releases of either biocontrol agent alone or combined were randomly assigned among the four transects, resulting in a total of 64 permanent study transects. We measured vegetation cover and Canada thistle stem density in plots and assessed insect abundance between 2008 and 2012. Data varied greatly between study sites, years and biocontrol agent treatments.  Generally, U. cardui and H. litura are widespread but occur only 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 Canada thistle populations in Idaho or South Dakota.