Nontarget host utilization of thistle species by introduced biological control agents and spatial prediction of non-target feeding habitats

Rhinocyllus conicus Fröelich and Trichosirocalus horridus (Panzer) were introduced from Europe into North America as biological control agents of the exotic weed species Carduus nutans L. Concern exists over the feeding of these weevils on at least 25 species of native Cirsium thistles. Research was conducted to 1) estimate phenological synchrony of the eight thistle species in Tennessee with R. conicus and T. horridus, 2) investigate naturally-occurring populations of the five native Cirsium thistle species for non-target activity by R. conicus and T. horridus, 3) quantify the impacts to plants of each thistle species to feeding of R. conicus and T. horridus, and 4) identify potential areas of non-target feeding by the weevils using spatial analysis. Phenologies of two native species, C. carolinianum and C. horridulum, are synchronous with R. conicus reproduction, and all eight thistle species are phenologically synchronous with the reproduction of T. horridus. No non-target activity by R. conicus was observed in naturally-occurring populations of Cirsium thistles, but adults of T. horridus were documented for the first time on the native species C. carolinianum, C. horridulum, and C. muticum. In caged plant studies, larvae of R. conicus completed development in heads of C. carolinianum and C. horridulum and reduced seed numbers of both native species. Basal meristems of all eight thistle species exposed to T. horridus were damaged at varying levels in caged plant studies, but no other impacts to plants were observed. Spatial analyses showed associations between Mahalanobis distance values and plant counts of Carduus nutans and Cirsium carolinianum in predicted habitats, and the occurrence of Carduus nutans was associated with the occurrence of both weevil species in these habitats. About 12% of the total study area consisted of habitats where C. nutans and Cirsium carolinianum overlap. The potential exists for these weevils to utilize native Cirsium species found in Tennessee as plant hosts. The spatial model developed during this study not only allows potential monitoring of populations of C. carolinianum to be more targeted, but also may be modified to apply to other systems involving interactions among introduced and native species.