RNAi-based technologies are ideal for pest control as they can provide species specificity and spare nontarget organisms. However, in some pests biological barriers prevent use of RNAi, and therefore broad application. In this study we tested the ability of a synthetic cationic polymer, poly[N-(3-guanidinopropyl)methacrylamide] (pGPMA), that mimics arginine-rich cell penetrating peptides to trigger RNAi in an insensitive animal Spodoptera frugiperda. Polymer dsRNA interpolyelectrolyte complexes (IPECs) were found to be efficiently taken up by cells, and to drive highly efficient gene knockdown. These IPECs could also trigger target gene knockdown and moderate larval mortality when fed to S. frugiperda larvae. This effect was sequence specific, which is consistent with the low toxicity we found to be associated with this polymer. A method for oral delivery of dsRNA is critical to development of RNAi-based insecticides. Thus, this technology has the potential to make RNAi-based pest control useful for targeting numerous species and facilitate use of RNAi in pest management practices.