RNAi delivery for emerald ash borer: translocation and persistence of pest-specific dsRNA in the host plant

RNA interference (RNAi) is a sequence-specific gene silencing mechanism triggered by exogenous double-stranded RNA (dsRNA). Exploiting the RNAi mechanism to silence essential genes in insects has emerged as a promising pest control strategy, and RNAi-based products are being developed for plant protection. RNAi has proven effective against the highly invasive emerald ash borer (EAB) (Agrilus planipennis) (Coleoptera: Buprestidae); however, a practical delivery method remains a barrier to deployment. In this study I evaluate the movement and retention of topically applied dsRNA in green ash (Fraxinus pennsylvanica), to investigate the feasibility of dsRNA delivery through the host plant. Seedlings are exposed to EAB-specific dsRNA as a root soak and as a foliar spray, destructively sampled 3, 7, 14, 21, and 30 d after exposure, and sectioned into root, woody-stem, soft-stem, and leaf tissues. Reverse Transcription-PCR is used to assess the recovery of the topically applied dsRNA in the different tissues. Bioactivity and delivery to the target insect were confirmed by gene expression studies evaluating EAB larvae exposed to dsRNA-sprayed seedlings. Preliminary results detected the exogenous dsRNA 3 and 7 d after foliar spray application, and 7 and 14 d after root soak treatment; the bioassay demonstrates gene silencing in EAB larvae recovered from sprayed seedlings. My results provide foundational knowledge of in planta translocation and persistence of dsRNA, and delivery of RNAi to EAB through the host tree. This research demonstrates the feasibility of RNAi for single tree protection, thereby contributing to protection of our future forests.