Comparative Survey of the Phytophagous Insect Faunas of Italian Thistle, Carduus pycnocephalus , in Southern California and Southern Europe Relative to Biological Weed Control

Abstract

A diverse assemblage of phytophagous insects representing 7 orders, 23 families, and more than 40 species fed or reproduced on Italian thistle, Carduus pycnocephalus L. (Compositae) in southern California. Most of the insects attacking this introduced annual weed were euryphagous, ectophagous, sap- and foliage-feeding species. Half of the identified insect associates were economic species. In southern Europe, Italian thistle hosts a far greater diversity of phytophagous insects; those detected to date represent 8 orders, 31 families, and more than 80 species. All major plant parts are damaged by one or more species of insects, unlike the situation in southern California, where this thistle was relatively free of deleterious insect injury. About a third of the European insect associates of Italian thistle were endophagous as immatures. A third of the associates identified to species were stenophagous, restricted to host-plants belonging to the tribe Cynareae. Only 3 species (ca. 4%) of the insect enemies of Italian thistle in Europe immediately appeared promising for use as biological control agents in southern California: Psylliodes chalcomera Illiger (Coleoptera: Chrysomelidae), Ceutorhynchus trimaculatus F. (Coleoptera: Curculionidae), and Rhinocyllus conicus Froelich (Coleoptera: Curculionidae). The last-named, seed-destroying species was initially colonized on Italian thistle in southern California in 1973.