Indirect effects of an invasive biocontrol weevil on native insect floral herbivores in temperate prairies

Classical biological control of weeds, using imported exotic insect herbivores, has been advocated for natural areas, such as public lands and nature reserves, in both temperate and tropical ecosystems. However, evidence of significant direct effects on non-target native plants by several imported exotic insect species, such as the thistle flower head weevil Rhinocyllus conicus, has emerged. Further, the probability and consequences of indirect effects on native insects resulting from biocontrol agent use of non-target native plants remain uninvestigated. Using experimental tests in a retrospective study of R. conicus effects, we found that this biocontrol weevil competes with and displaces a key floral guild herbivore, Paracantha culta (Tephritidae), from the main flower heads of its new, non-target native host plant, Platte thistle (Cirsium canescens), a characteristic sparse species in temperate sand prairie. The results also clearly demonstrate that the floral herbivore guild of the native plant is not saturated, and that the mechanism underlying competitive process and outcome varies from flower head to flower head. The net effect of the weevil on the native fly within each head was determined by the interaction of four factors: flowering phenology, insect phenology, floral resources available, and fly behavior. The conservation implications from this temperate ecosystem are that deliberate introductions of exotic insects into new communities and environments in an effort to limit population density and growth of a weed can affect multiple interactions and processes, most of which remain unexplored and unknown.