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

Abstract

Chemical insecticides are an important tool to control damaging pest infestations. However, lack of species specificity, the rise of resistance to these pesticides 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 that use 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, are recalcitrant to RNAi when dsRNA is orally administered, due to degradation by gut nucleases and slow cellular uptake pathways. Thus, delivery vehicles that protect dsRNA and deliver it to insect gut cells are highly desirable. In this work, we demonstrate the complexation of D. suzukii-specific dsRNA for degradation of vha26 mRNA with bespoke, hydrophilic, diblock copolymers. To demonstrate the efficiency of this delivery system, we study the ex vivo protection of dsRNA against enzymatic degradation by gut enzymes. Flow cytometry is then used to investigate the cellular uptake of Cy3-labelled dsRNA, with confocal microscopy confirming cytoplasmic delivery after complexation. The polymer/dsRNA polyplexes induced significantly decreased 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. 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.