Successful biological control of plumeless thistle, Carduus acanthoides L. [Campanulatae: Asteraceae (= compositae)], by Trichosirocalus horridus (Panzer) (Coleoptera: Curculionidae) in Virginia

Abstract

The plumeless thistle, Carduus acanthoides L., is a major weed of pastures, highway median strips, and road sides in much of western Virginia. It occupies a range similar to that of musk thistle, Carduus thoermeri Weinmann, and is often found together with musk thistle. A 15-year study at five sites with releases of Trichosirocalus horridus (Panzer), a ceutorhynchine weevil introduced into North America for the biological control of both musk and plumeless thistles, showed excellent control of plumeless thistle with mean ± SD reductions of 95 ± 4.9%, range 87–100%. Average initial thistle densities during weevil releases in 1975–1977 compared with those in 1990 were 8.7 and 0.3/m 2 , respectively. Mean time for the plumeless thistle population to collapse was 10.8 ± 2.3 (range 7–13) years compared with about half that for musk thistle.