SEASONAL MORTALITY OF CASSIDA-RUBIGINOSA (COLEOPTERA, CHRYSOMELIDAE) DUE TO INCOMPLETE DEVELOPMENT AND PARASITISM IN SOUTHWESTERN VIRGINIA

Releases of Cassida rubiginosa Muller in Montgomery Co. in Virginia in 1973 have been successful and the beetle is consistently the most con spicuous defoliator of Canada thistle in southwestern Virginia. As the original beetle populations collected in northern Virginia were only moderately parasitized, it appeared that the establishment and spread of C. rubiginosa in southwestern Virginia were not inhibited. Our obje ctive was to determine the effect of parasitism on C. rubiginosa more than a decade after its release in southwestern Virginia. A two-year s tudy involving three sites revealed that parasitism and incomplete dev elopment were major mortality factors. Four species of parasites, Euce latoriopsis dimmocki (Aldrich) (Diptera: Tachinidae), Itoplectis conqu isitor (Say) (Hymenoptera: Ichneumonidae), Conura torvina (Walsh) (Hym enoptera: Chalcididae), and Aprostocetus sp. (Hymenoptera: Eulophidae) , were collected from larvae or pupae of C. rubiginosa. No egg or adul t parasites were detected. The major parasite, Aprostocetus sp., was c ollected from up to 80% of C. rubiginosa pupae during the latter part of the season. Incomplete development accounted for up to 91% of the m ortality at later sampling dates. Combined seasonal mortality of larva e and pupae ranged from 12 to 47% in 1989 and 8.6 to 80% in 1991.