A sprayable long hairpin dsRNA formulated with layered double hydroxide against the western flower thrips, Frankliniella occidentalis: Control efficacy in a greenhouse and influence on beneficial insects

RNA interference (RNAi) is a cellular mechanism regulating gene expression at a post-transcriptional level in eukaryotes. Suppression of vacuolar-ATPase B subunit (vATPase-B) expression resulted in lethality to the western flower thrips, Frankliniella occidentalis, following oral administration of the gene-specific double-stranded RNA (dsRNA) for RNAi. This study aimed to enhance the insecticidal activity of sprayable dsRNA against the thrips. Initially, the study screened for differences in insecticidal activity across various frames of the target gene's open- reading frames using similarly sized (approximately 300 bp) dsRNAs, observing minimal variation. Subsequently, the optimal length of dsRNA was determined by preparing samples ranging from 100 to 700 bp, with lengths over 300 bp demonstrating high insecticidal activities. The study also compared linear and hairpin forms of dsRNA, with hairpin dsRNA exhibiting higher insecticidal activity. Additionally, two formulations of chitosan and layered double hydroxide (LDH) nanoparticles were assessed with dsRNAs against the same target region; the LDH formulation outperformed the chitosan in insecticidal activity. The effects of dsRNA on non-target organisms (NTOs) were evaluated against two honey bees, Apis mellifera and A. cerana, and a natural enemy, Orius laevigatus, where some dsRNAs with high sequence homology to the NTOs caused significant mortalities. The optimal size of hairpin dsRNA, formulated with LDH and harmless to NTOs, was then sprayed on F. occidentalis infesting greenhouse-cultivated hot peppers. The LDH-hairpin dsRNA spray achieved a significant reduction in thrips population, comparable to the control efficacy of the chemical insecticide, spinosad.