Abstract
Testing of the crown weevil , Trichosirocalus horridus, against a range of Australian Aster aceae species confirmed its specificity to nodding thistle , Carduus nutans, and permission was granted for its release in Australia . It has established at the release site , and has been redistributed to other areas in eastern Australia . A field experiment designed to study the impact of this weevil on rosette growth and reproductive potential of the thistle is described . Results indicate that the weevil can kill some plants , and reduce rosette growth of survivors by 50% . The reproductive potential of the attacked plants , measured as seeds per plant , was reduced by 67% . The interaction of this biological control agent with two others already established in Australia , the receptacle weevil , Rhinocyllus conicus, and the seed fly , Urophora solstitialis, is discussed .

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