Evaluating novel methods for control of Culicoides sonorensislarvae

Culicoides sonorensis (Diptera: Ceratopoginidae) is an economically important vector of livestock pathogens. RNA interference (RNAi) is an encouraging control method wherein double-stranded RNA (dsRNA) can lead to reduced target gene expression. The objectives of these studies were to characterize RNAi as a bio-rational insecticide for C. sonorensis, to investigate likely inhibitors of the mechanism, and to evaluate alternative approaches. We found that cultured cells respond efficiently to dsRNA, but the RNAi response is limited in larval stages. Seven nucleases capable of degrading dsRNA were characterized in the midge and were shown to degrade dsRNA in larval midge guts—a likely barrier to an efficient RNAi response. We developed strategies to enhance RNAi, but ultimately these did not significantly affect RNAi efficiency in larvae. As an alternative approach, we tested the efficacy of Bacillus thuringiensis subsp. israelensis (Bti) to cause lethal effects in C. sonorensis. Bti and Bti products were ineffective in C. sonorensis when compared to a susceptible mosquito species, and the acidic larval gut likely plays a role in preventing Bti toxin activity. These findings provide information on the basic mechanisms that prevent these techniques from being effective and provide a foundation for designing strategies to overcome the obstacles.