Evaluation of ecological risk to populations of a threatened plant from an invasive biocontrol insect

Abstract. Controversy exists over estimation of ecological risk in biological control. At present, the risk to the rare, federally listed Pitcher’s thistle (Cirsium pitcheri) in North America from Rhinocyllus conicus, a biological control weevil now feeding on many native thistles, is unknown. We hypothesized that quantification of host specificity and potential phenological overlap between insect and plant would improve assessment of the magnitude of risk. In laboratory host specificity tests, we found no significant difference in R. conicus feeding or oviposition preference between the rare C. pitcheri and the targeted exotic weed (Carduus nutans) or between C. pitcheri and Platte thistle (C. canescens), a closely related native North American species known to be affected by R. conicus. In a garden environment, R. conicus spontaneously found, oviposited, and developed completely on C. pitcheri. Taller plants with more flower heads were significantly more vulnerable, suggesting that the greatest impact is likely to be on individuals that generally contribute the most to recruitment and population persistence. For eight sites in two national parks over three years, the calculated period of expected R. conicus activity overlapped 99 % and 78 % of the flower heads initiated by C. pitcheri in the southern and the northern park, respectively. A de