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.

Any plant not sown from seed is often labeled a weed in improved pastures of New Zealand. Most improved pastures are a mix of perennial ryegrass and white clover but generally are infested with broadleaf weeds. Changes in forage production due to individual weeds were determined using measurements of perennial ryegrass and white clover before and after dairy cattle, beef cattle, or sheep grazing under, near, and far from individual plants of six rosette-forming weed species throughout a growing season. The larger weeds, bull thistle and musk thistle, reduced the amount of forage utilized 42 and 72%, respectively, in beef cattle– and sheep-grazed hill-country pastures. Forage production under and near Canada thistle, hedge mustard, broadleaf plantain, and hairy buttercup in a dairy pasture was greater (136, 140, 178, and 450%, respectively) than in the control areas. Although the dairy pasture was grazed following recommended grazing procedures, our results indicate that this grazing system was not maximizing forage yield potentials of perennial ryegrass and white clover and that these weeds served as an indicator that the pasture was being overgrazed.