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 Michelle E. DePrenger-Levin a, * , Thomas A. Grant III b , Carol Dawson c a Denver Botanic Gardens, Research and Conservation Department, USA b Colorado State University, Graduate Degree Program in Ecology (GDPE) and the Department of Forestry, Rangeland and Watershed Science, USA c US Bureau of Land Management (BLM) – Colorado State Office, USA article info Article history: Received 17 November 2009 Accepted 19 July 2010 Available online 23 July 2010 Keywords: Carduus nutans Cirsium ownbeyi Colorado Native plants Non-target effects Rhinocyllus conicus abstract The release of non-native insects to control noxious weeds is commonly used to combat invasions with- out disturbing the environment through chemical or mechanical methods. However, introduced biolog- ical 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 Wyo- ming. C. ownbeyi represents one of 22 known native hosts on which this introduced weevil has natural- ized. The study population remained stable over the eight years of the study despite floral damage by the biocontrol beetle. The growth rate (k) 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 extir- pation. 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. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction The Eurasian flowerhead weevil, Rhinocyllus conicus Frölich, was introduced in 1963 as a classical biological control agent against a suite of invasive thistles, especially Carduus spp. (Kok and Surles, 1975; Turner et al., 1987; Kok, 2001). Successful or substantial bio- logical control of musk thistle (Carduus nutans L.) has been noted in much of the United States (Goeden and Ricker, 1977; Puttler et al., 1978; Batra, 1980; Littlefield, 1991; Lambdin and Grant, 1992; Boldt and Jackman, 1993; Buntin et al., 1993). Substantial non-tar- get impacts to native North American thistles have also occurred (Pemberton, 2000). While R. conicus was known to feed on Carduus, Cirsium, Silybum, and Onopordum species in its native European range, it was not expected to feed significantly on the native North American thistles. Initial R. conicus releases on Carduus nutans showed populations of the weevil increased greatly in range but not in density (Hodgson and Rees, 1976; Reed et al., 2006). Evidence of R. conicus on native Cirsium spp. was reported in grow- ing numbers of states including southwestern Montana in 1974 (Rees, 1977) and California in 1982 (Turner et al., 1987; Louda et al., 1997). Biocontrol releases continued through the 1970s de- spite knowledge that the weevil’s host plants included native North American thistles (Rees, 1977; Harris, 1988; Louda et al., 1997; Pemberton, 2000 and references therein). As confirmation of R. conicus feeding on native thistles spread across North America, so did concerns regarding the possibility of some native species becoming extinct (Louda et al., 1997, 1998, 2005; Rose et al., 2005). Pemberton (2000) reports that 22 of 90 North American Cirsium spp. are known hosts of R. conicus includ- ing Cirsium canescens Nutt., C. centaureae (Rydb.) K. Schum., C. hillii (Canby) Fernald, C. pulchellum (Greene) Woot. & Standl., C. tweedyi (Rydb.) Petrak, C. undulatum (Nutt.) Spreng., and C. undulatum (Nutt.) Spreng. var. tracyi (Rydb.) S.L. Welsh (Louda et al., 1997; Louda and O’Brien, 2002; Rose et al., 2005; Russell and Louda, 2005; Sauer and Bradley, 2008). Many states have ceased further releases of the biocontrol agent after observing damage on non- target native species (Sauer and Bradley, 2008). However, R. conicus populations continue to spread naturally and more information is 1049-9644/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.biocontrol.2010.07.004 * Corresponding author. Address: 909 York Street, Denver, CO 80206. Fax: +1 720 865 3683. E-mail address: deprengm@botanicgardens.org (M.E. DePrenger-Levin). Biological Control 55 (2010) 79–84 Contents lists available at ScienceDirect Biological Control journal homepage: www.elsevier.com/locate/ybcon