Plant growth, stem numbers per plant, receptacle diameters, number of Rhinocyllus conicus Froelich (Coleoptera:Curculionidae) larval cells per seed head, and mortality of the weevil within cells were monitored in infestations of the thistles Carduus thoermeri Weinmann and C. macrocephalus Deft. Both species of thistle provided similar seed head area for weevil development, and no differential effects on the biology of R. conicus were detected.

Populations of large-flowered taxa of Carduus from many locations in the United States and Canada were grown in a common nursery from 1975 to 1983 at Lincoln, NE. Dates of first bloom and associated degree-day accumulation data exhibited some consistent relationships. Four species appeared to be represented in the collection. These included Carduus thoermeri Wein., C. macrocephalus Desf., and musk thistle (C. nutans L. ♯ CRUNU). A collection from British Columbia should be accorded species status but apparently has not been named in the literature. Interbreeding has taken place in the nursery and these combinations are intermediate between C. thoermeri and C. macrocephalus and between C. thoermeri and the British Columbia material. Some morphological characteristics supported these separations.