Musk thistle (Carduus nutans) seed production from early developing (terminal and first lateral) heads was reduced by 10% in 1973 and 75% in 1974 due to larval feeding of Rhinocyllus conicus. This was caused by heavy infestation of the early heads (45% in 1973, 70% in 1974) with a subsequent 5-fold increase in the number of weevil pupation chambers in these heads. Total seed production was reduced by 35-36% in both years despite increased plant vigor due to better growing conditions in 1974. Larval feeding reduced viability of mature musk thistle seeds. Plumeless thistle (C. acanthoides) seed production from early heads decreased only 5% in 1973 and 4% in 1974 due to low rates of infestation (9% in 1973, 5% in 1974). Total seed production was reduced 0.2% in 1973 and 0.1% in 1974. Weevil larvae developing in plumeless thistle heads were more heavily parasitized than those in musk thistles. Parasitoids suffered high mortality due to entrapment in weevil pupation chambers (Aliolus curculionis, 837; Bracon mellitor, 10%), and did not inhibit weevil population buildup. These combined factors resulted in successful control of musk thistle at 1 Virginia site. R. conicus showed ovipositional preference for C. nutans over C. acanthoides heads both in the laboratory and under field conditions. Oviposition of an established weevil colony was better synchronized with musk than with plumeless thistles at a site where both thistles were present. Sequentially released, overwintered R. conicus were better synchronized with Carduus thistle development because of an extended ovipositional period, but the weevils suffered reduced fecundity. Ovipositional preference was for the earlier stages of musk thistle heads. Intermediate stages of plumeless thistle heads were preferred, and larval survival was reduced because of insufficient time for weevil development and subsequent depletion of food supply. Musk thistles subjected to mechanical and R. conicus-induced damage to terminal heads were shorter than control plants and suffered reduced seed production from terminal heads. Total head and seed production per thistle were not significantly reduced. Emerging adult weevils decreased in weight relative to crowding; no difference was detected between sexes (female, 13.4mg; male, 13.8mg). / Ph. D.