Relationships among Edaphic, Climatic, and Vegetation Conditions at Release Sites and Aphthona nigriscutis Population Density

The role of site conditions on the success of leafy spurge control by Aphthona nigriscutis is poorly understood. Our objective was to determine the relationships among the climatic and edaphic conditions of A. nigriscutis release sites, A. nigriscutis population density, leafy spurge cover and density, and grass cover. We sampled 13 field sites in eastern Montana where A. nigriscutis had been established for 6 to 8 years. Sampling was done in June and July of 1998 along a randomly determined transect from the center of each release point to densely infested leafy spurge. Data were analyzed by step-down regression procedures using A. nigriscutis density, leafy spurge cover and density, cover of grass, forbs, litter, and bare ground as the dependent variables. Independent variables included vegetative and insect data sampled as well as site conditions. Average annual precipitation was the only edaphic or climatic site characteristic that influenced A. nigriscutis density; the beetle population density increased as precipitation increased. There was a negative association between A. nigriscutis and number of leafy spurge flowering stems and leafy spurge cover. Grass, litter, and bare ground cover increased as A. nigriscutis numbers increased. Within the range of site conditions sampled in this study, successful establishment and population expansion of A. nigriscutis appears to be related to the cover of leafy spurge, grass, forbs other than leafy spurge, litter, and bare ground. This information may aid in site selection for release of this important biological control agent or in creating conditions for optimum establishment.