Impact of Trichosirocalus horridus (Coleoptera: Curculionidae) on Carduus thistles in pastures

Abstract

Trichosirocalus horridus (Panzer), a rosette-feeding weevil introduced from Europe, was released on musk thistle ( Carduus thoermeri Weinmann), plumeless thistle ( Carduus acanthoides L.) and mixtures of both thistle species from 1974 to 1976 in pastures in Virginia. The weevils became established at nine of the 10 release sites: three on musk, four on plumeless thistle and two on mixtures. Density of thistles was reduced in all established sites by 1981. Thistle reduction ranged from 1·3% for the most recently established site to 96·5% for the earliest established and most successful site, versus an 11·6% increase at the plumeless thistle site where T. horridus was not established, and a 9·7% increase at a mixed thistle site prior to establishment in 1980. A control musk thistle without any weevil release showed a 5·8% increase between 1976 and 1979, when Rhinocyllus conicus, a thistlehead weevil, which is well established in Virginia, dispersed into the area.