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Rhinocyllus conicusFroel. (Col.: Curculionidae) larvae feeding within the capitula ofCarduusthistles may reduce production of viable seeds. EachR. conicuslarva destroyed 9.7 developing musk thistle (Carduus nutansL.) seeds. An average musk thistle capitulum, 29.2 mm diameter, produced 15.3 viable achenes per millimeter of diameter for an average of 447 viable seeds. Surveys of 23 release sites in Virginia revealed that thistles at 12 had sustained a population of weevils. Weevil eggs at six sites have increased in abundance, and establishment appears assured. Colonization ofR. conicuswas enhanced by spring or early summer releases; larger stocks of insects were required for late summer releases. The weevils exhibited better synchronization with musk thistle than with plumeless thistle (Carduus acanthoidesL.); and best success has been achieved where extensive, persistent stands of musk thistle were available.

The impact of a weevil(Rhinocyllus conicusFroel.) (Coleoptera: Curculionidae) onCarduusthistles in Virginia was evaluated in terms of seed reduction and viability during 1973 and 1974. Musk thistle(Carduus nutansL.) seed production from terminal and first lateral heads was reduced by 10% in 1973 and 75% in 1974 due to larval feeding ofR. conicus.The greater seed reduction in 1974 was caused by heavy infestation (45% in 1973, 70% in 1974); there was a five-fold increase in the number of weevil pupation chambers per head over the previous year. Total seed production per plant decreased by 35 to 36% in both years despite increased plant vigor due to better growing conditions in 1974. Larval feeding reduced viability of mature musk thistle seeds. Although individual heads of plumeless thistle(Carduus acanthoidesL.) were frequently destroyed by larval feeding, seed production from early heads of this thistle decreased by only 5% in 1973 and 4% in 1974 due to low rates of infestation (9% in 1973, 5% in 1974). Total seed reduction per plant for each of the two years was less than 0.2%. Infested heads of plumeless thistle did not produce sufficient seeds for evaluation of the weevil's impact on seed viability.

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