Biology of immature Culicoides variipennis ssp. australis (Coq.) (Diptera:Ceratopogonidae) at Saltville, VA

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Virginia Polytechnic Institute and State University


The larval and pupal biology of a unique population of gulicoides variipennis inhabiting the brine ponds of Saltville, VA was studied. Developmental threshold temperatures (OC) and thermal constants (Odays) for larvae and pupae were 9.6OC and 387Odays (larval stage) and 9.6OC and 3OOdays (pupal stage) respectively. Accumulated heat units recorded in the field ranged from 366—376Odays between successive generations in the summer. Heat accumulations required for completion of immature development of Q. variipennis were found to be much greater (83lOdays) for the overwintering generation. During the summer, larval/pupal distribution within the littoral zone of a brine pond was confined to the surface cm of mud at or near the shoreline. Insects overwintered farther offshore, mostly as 3rd instars. In ear1y' March, most larvae had xnolted to 4th instars and migrated above shoreline to pupate. Adult emergence occurred in April. Three summer generations were documented for 1983-1984 at Saltville._ Life tables and survivorship curves were calculated for the overwintering generation and the first summer generations for 1983 and 1984. For the overwintering generation, there was a relatively constant mortality rate between successive ageclasses (Type II survivorship curve). During the summer, there was relatively little mortality between successive larval age—classes but a dramatic increase in mortality was evident at the pupal stage (Type I survivorship curve). Late instar larvae were found to migrate from the shoreline onto the exposed mudflats to pupate, thus becoming vulnerable to predation by ants and carabid beetles. Excellent survival rates of the larvae during the summer was attributed to habitat stability, the paucity of predators and parasites and abundant microfloral content (i.e. food} of the pond water. Intra-specific competition for food resources appeared to be alleviated somewhat by partitioning of those resources on a diurnal cycle.