Host Utilization of Native Cirsium Thistles (Asteraceae) by the Introduced Weevil Rhinocyllus conicus (Coleoptera: Curculionidae) in California

The flowerhead weevil Rhinocyllus conicus (Froelich) was introduced from Europe into California in the early 1970's for biological control of weedy thistles Carduus nutans L., Carduus pycnocephalus L., and Silybum marianum (L.) Gaertn. (Cardueae). In this study, adult R. conicus was reared from flowerheads collected from natural populations of 12 species of native, nontarget Cirsium thistles in California: Cirsium andersonii (Gray) Petrak, C. brevistylum Cronq., C. californicum Gray, C. campylon H. Sharsm., C. callilepis (Greene) Jeps., C. ciliolatum (L. Henders.) J. T. Howell, C. cymosum (Greene) J. T. Howell, C. douglasii DC., C. hydrophilum (Greene) Jeps. var. vaseyi (Gray) J. T. Howell, C. pastoris J. T. Howell, C. proteanum J. T. Howell, and C. tioganum Congd. These are new host records for this weevil. This extension of its host range is not surprising because European Cirsium was known to be within the host range of R. conicus. The impact, if any, of these new host associations is not yet known. The weevil was also reared from the naturalized weedy thistles C. nutans, C. pycnocephalus, C. tenuiflorus W. Curt., Cirsium arvense (L.) Scop., Cirsium vulgare (Savi) Tenore, and S. marianum. Quantitative rearing data are reported for each thistle host.