Effective delivery and selective insecticidal activity of double-stranded RNA via complexation with diblock copolymer varies with polymer block composition

BACKGROUND Chemical insecticides are an important tool to control damaging pest infestations. However, lack of species specificity, the rise of resistance and the demand for biological alternatives with improved ecotoxicity profiles means that chemicals with new mode-of-actions are required. RNA interference (RNAi)-based strategies using double-stranded RNA (dsRNA) as a species-specific bio-insecticide offer an exquisite solution that addresses these issues. Many species, such as the fruit pest Drosophila suzukii, do not exhibit RNAi when dsRNA is orally administered, due to degradation by gut nucleases and slow cellular uptake pathways. Thus, delivery vehicles that protect and deliver dsRNA are highly desirable.

RESULTS In this work, we demonstrate the complexation of D. suzukii-specific dsRNA for degradation of vha26 mRNA with bespoke diblock copolymers. We study the ex vivo protection of dsRNA against enzymatic degradation by gut enzymes, which demonstrates the efficiency of this system. Flow cytometry then investigates the cellular uptake of Cy3-labelled dsRNA, showing a 10 fold increase in the mean fluorescence intensity of cells treated with polyplexes. The polymer/dsRNA polyplexes induced a significant 87% decrease in the odds of survival of D. suzukii larvae following oral feeding, only when formed with a diblock copolymer containing a long neutral block length (1:2 cationic block/neutral block). However, there was no toxicity when fed to the closely related D. melanogaster.

CONCLUSION Thus, we provide evidence that dsRNA complexation with diblock copolymers is a promising strategy for RNAi-based species-specific pest control, however, optimisation of polymer composition is essential for RNAi success.