Studies over a two-year period were undertaken to evaluate the potential and effect of Rhinocyllus conicus as a biological control agent for Carduus nutans and C. acanthoides in Virginia. Over an 81 day range, 25 females oviposited an average of 100.5 eggs. Life expectancy did not differ between sexes. Examination of 1999 C. nutans plants in Pulaski Co. showed: 26.7% of all blooms were infested (46.7% terminals and 18.2% laterals); no weevil orientation relative to plant density but a preference for terminal blooms and no bloom size preference for oviposition. Evaluation of 261 capitula revealed that an infested bloom produced 37 mature seeds/mm of diameter. Each larva destroyed 9.7 achenes.

Weevil mortality on C. nutans capitula occurred primarily during the egg stage (58.3%). Mechanical egg dislodgement was not a major mortality factor. Ege and/or adult parasitism was not observed. Aliolus curculionis, Bracon mellitor, and Campoplex polychrosidis parasitized the weevil larvae and pupae. Twelve other possible parasites were collected. Crowding was not a significant mortality factor. An egg maturation of 31.9% was noted, and 3.1 weevils emerged per infested bloom. Mortality among sites ranged from 56.1% (adjacent to trees) to 95.8% (unprotected).

Weevil adults exhibited thigmotaxis by overwintering in the cracks of the framework of a large outdoor rearing cage. They apparently preferred this site rather than soil litter and debris.

Best release establishment and dispersal occurred on C. nutans which was preferred to Carduus acanthoides.