Plumeless thistle is a European Asteraceae naturalized in Argentina, where it is a troublesome pasture and range weed. A European weevil (Rhinocyllus conicus) was imported for biocontrol of this thistle. Oviposition patterns, egg to adult survival, and the weevil's effect on the production of achenes by thistles were monitored over 2 yr. The oviposition period did not begin simultaneously in all thistle populations, but finished in late November. Egg to larvae survival was 63% and egg to adult survival was approximately 50%. No important egg predators were detected. The weevils destroyed 80% of the propagules per capitulum and showed a slight tendency to adapt to the plumeless thistle cycle.
