Rhinocyllus conicus Establishment for Biocontrol of Thistles in Virginia

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Abstract
Rhinocyllus conicusFroel. (Col.: Curculionidae) larvae feeding within the capitula ofCarduusthistles may reduce production of viable seeds. EachR. conicuslarva destroyed 9.7 developing musk thistle (Carduus nutansL.) seeds. An average musk thistle capitulum, 29.2 mm diameter, produced 15.3 viable achenes per millimeter of diameter for an average of 447 viable seeds. Surveys of 23 release sites in Virginia revealed that thistles at 12 had sustained a population of weevils. Weevil eggs at six sites have increased in abundance, and establishment appears assured. Colonization ofR. conicuswas enhanced by spring or early summer releases; larger stocks of insects were required for late summer releases. The weevils exhibited better synchronization with musk thistle than with plumeless thistle (Carduus acanthoidesL.); and best success has been achieved where extensive, persistent stands of musk thistle were available.

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