Establishment of star polycation-based RNA interference system in all developmental stages of fall armyworm Spodoptera frugiperda

RNA interference (RNAi) has become an essential technique in functional analysis of insect genes and pest management. However, insensitivity to RNAi is the main obstacle that limits the identification of functional genes in lepidopteran insects. Hereby we explored a nanocarrier (star polycation, SPc)-mediated dsRNA delivery system to achieve efficient RNAi in lepidopteran insect Spodoptera frugiperda. Double-stranded RNA (dsRNA) was synthesized targeting a consistently expressed gene V-type proton ATPase subunit d (ATP-d) as well as enhanced green fluorescent protein (eGFP). At the egg and early larval stages, SPc-loaded dsRNA could penetrate the physical obstacles of eggshell and larval body wall. Thus, soaking and topical applications of SPc-loaded dsRNA were suitable for efficient RNAi at egg and 1st-2nd instar larval stages, respectively. The SPc could protect dsRNA from degradation by midgut fluid of mid-late instar larvae. Thereby oral feeding of SPc-loaded dsRNA silenced the target gene at mid-late larval stages more efficiently compared to dsRNA alone. Finally, for pupal and adult stages, injection of SPc-loaded dsRNA was enough to induce high RNAi effect (about 67% for pupa and adult). A key developmental gene, Chitin deacetylase 1 (CDA1), was taken as an example to test the pest control efficiency of SPc-based RNAi system. The constructed system could remarkably induce the knockdown of CDA1 and lead to growth retardation and mortality. Overall, the present study constructed an efficient nanocarrier-based RNAi system, which would provide a powerful tool for identifying functional genes in Spodoptera frugiperda.