Impacts of the introduced biocontrol agent, Rhinocyllus conicus (Coleoptera: Curculionidae), on the seed production and population dynamics of Cirsium ownbeyi (Asteraceae), a rare, native thistle

The release of non-native insects to control noxious weeds is commonly used to combat invasions without disturbing the environment through chemical or mechanical methods. However, introduced biological control agents can have unintended effects. This study was initiated to evaluate potential non-target effects of the flowerhead weevil, Rhinocyllus conicus Frölich, on Cirsium ownbeyi S.L. Welsh, a rare, native and short-lived perennial thistle in northwestern Colorado, northeastern Utah, and southwestern Wyoming. C. ownbeyi represents one of 22 known native hosts on which this introduced weevil has naturalized. The study population remained stable over the eight years of the study despite floral damage by the biocontrol beetle. The growth rate (λ) from a count-based population viability analysis of the population was 1.03; however, large inter-year variation indicates this rare species is still vulnerable to local extirpation. The biocontrol weevil consistently damaged the developing seeds over the course of the study independent of changes in overall population size and variation in the number of flowering individuals. The target species, Carduus nutans L. (musk thistle) is generally absent near the study plots, which may limit the population levels of R. conicus that can be sustained in this area. Although R. conicus utilizes C. ownbeyi as a host plant, the late flowering period of this native thistle and the small size of the flower heads may limit the demographic impact of R. conicus on C. ownbeyi.