New Frontiers in Pest Control: Chitosan Nanoparticles-Shielded dsRNA as an Effective Topical RNAi Spray for Gram Podborer Biocontrol

Chickpea pod borer, Helicoverpa armigera, displays resistance to chemical insecticides and transgenics. The potential nontransformative RNAi approach of specific gene silencing by mRNA breakdown through exogenous double-stranded (dsRNA) delivery to Helicoverpa faces problems of degradation by nucleases and insect gut pH. We demonstrate that chitosan nanoparticles (CNPs) effectively mediate specific dsRNA delivery against Helicoverpa armigera juvenile hormone methyltransferase (JHAMT) and acetylcholine esterase (ACHE) target genes. Ionotropically synthesized cationic CNPs (100 nm size, +32 mV charge) loaded dsRNA efficiently and protected it effectively from degradation by nucleases and insect gut pH. Tagging CNPs with Calcofluor fluorescence illustrated its efficient uptake in columnar insect gut cells. The potential of CNPs-mediated dsRNA delivery was elucidated with effective silencing of green fluorescent protein transformed Sf9 cells. Furthermore, CNPs-dsRNA complexes were stable for 5 d on leaf surfaces, and their ingestion with leaf effectively silenced H. armigera JHAMT and ACHE genes to suppress related enzyme activities and caused 100% insect mortality. Further, in planta bioassay with CNPs-dsRNA spray confirmed the RNAi induced insect mortality. Moreover, CNPs-dsRNA fed nontarget insects Spodoptera litura and Drosophila melanogaster were unaffected, and no toxicity was observed for CNPs in cell line studies. Remarkably, only two low dose (0.028 g/ha) topical CNPs-ache-dsRNA sprays on chickpea displayed reduced pod damage with high yields on par with chemical control in the field, which was followed by CNPs-jhamt-dsRNA nanoformulation. These studies can pave the way for the development of topical application of CNPs-dsRNA spray as a safe, specific, innovative insecticide for sustainable crop protection.