Seasonal Mortality of Cassida rubiginosa (Coleoptera: Chrysomelidae) Due to Incomplete Development and Parasitism in Southwestern Virginia

Abstract

Releases of Cassida rubiginosa Müller in Montgomery Co. in Virginia in 1973 have been successful and the beetle is consistently the most conspicuous 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 objective was to determine the effect of parasitism on C. rubiginosa more than a decade after its release in southwestern Virginia. A two-year study involving three sites revealed that parasitism and incomplete development were major mortality factors. Four species of parasites, Eucelatoriopsis dimmocki (Aldrich) (Diptera: Tachinidae), Itoplectis conquisitor (Say) (Hymenoptera: Ichneumonidae), Conura torvina (Walsh) (Hymenoptera: Chalcididae), and Aprostocetus sp. (Hymenoptera: Eulophidae), were collected from larvae or pupae of C. rubiginosa. No egg or adult parasites were detected. The major parasite, Aprostocetus sp., was collected from up to 80% of C. rubiginosa pupae during the latter part of the season. Incomplete development accounted for up to 91% of the mortality at later sampling dates. Combined seasonal mortality of larvae and pupae ranged from 12 to 47% in 1989 and 8.6 to 80% in 1991.