Modelling and mapping geographic ranges to evaluate weed biocontrol agents: a case study using Altica carduorum (Coleoptera: Chrysomelidae) and Cirsium arvense (Asteraceae).
Leaf-feeding beetles, Altica carduorum, from a population in NW China have been identified as candidate agents for biocontrol of Canada thistle, Cirsium arvense. The potential of beetles from this population to establish on the Canadian prairies was assessed. A phenological model was applied to determine where sufficient heat accumulates to allow completion of at least one generation per year. The model was applied both with and without a submodel of adult thermoregulation. The model was driven using meteorological data from a grid covering the agricultural region of the Canadian prairies, i.e., ca. 670 000 km2, at a resolution of ≤ 50 × 50 km. In each grid square, the percentage of the years 1960-89 for which the model indicated that at least one generation could have been completed was calculated. These proportions were then categorized (0-80%, 80-99% and 100%) and mapped. Maps of C. arvense density over the simulation region were also produced, and compared to the maps of predicted beetle distribution. The model suggests that A. carduorum could establish over much of the prairies. Without thermoregulation, the predicted range (i.e. the area in which one generation was completed in every year) is restricted to the warmer parts of the prairies, where C. arvense is generally sparse. Inclusion of thermoregulation almost doubled the predicted range to cover most of the range of C. arvense on the Canadian prairies, missing only the cooler peripheral areas. These results suggest that this strain of A. carduorum has potential to overlap the range of C. arvense on the Canadian prairies, and so has potential to control this weed.