Phytophagous Insect Faunas of the Two Most Common Native Cirsium Thistles, C. californicum and C. proteanum, in Southern California

Phytophagous insects representing six orders, 26 families, 49 genera, and 59 species are reported as composing the insect fauna of Cirsium californicum Gray (Asteraceae) in southern California. Similarly, phytophagous insects representing six orders, 20 families, 29 genera, and 31 species are reported from C. proteanum J. T. Howell. Most insects feeding on these thistles are polyphagous, ectophagous, sap- and foliage-feeding species. Ten (17%) of 58 species assessed for host specificity on C. californicum are stenophagous (i.e., their host plants restricted at least to Cynareae); 7 (23%) of 31 species on C. proteanum are stenophagous. The six oligophagous species (i.e., only attacking Carduinae) on C. californicum and four oligophagous species on C. proteanum include the flower head-infesting weevil Rhinocyllus conicus (Froelich), introduced and established for the biological control of alien, asteraceous thistles of European origin. Species packing within flower heads of both thistles averaged approximately two (range, 0–4) native insect species. Species packing in or on shoots of C. californicum averaged 10.5 ± 0.9 (⁠|x| ± SEM) species; for C. proteanum, 8.4 ± 0.8 species.