Investigating new bio-rational larvicides against the biting midge Culicoides sonorensis

The biting midge Culicoides sonorensis (Diptera: Ceratopoginidae) is an economically important vector of numerous livestock pathogens. Traditional chemical insecticides are non-specific and harmful to the environment, but RNA interference (RNAi) is an encouraging alternative control method. Here, double-stranded RNA (dsRNA) can lead to reduced target gene expression. However, the RNAi mechanism is not equal in all insects. 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 nanoparticle carriers, dsRNA-expressing E. coli, and stronger RNA structures to enhance RNAi, but ultimately these enhancers did not significantly affect RNAi efficiency in larvae. As an alternative approach, we used Bacillus thuringiensis subsp. israelensis (Bti)—a bacterium used in mosquito control for its insecticidal proteins—to examine lethal effects in C. sonorensis. While it was found that Bti and Bti products worked poorly in C. sonorensis, we characterized an inhibitory mechanism and found supplementary conditions that enhanced Bti. There is much to investigate towards translating these findings to the field, but this work creates a strong foundation for future studies. These findings could lead to safer insecticid to control this important pest.