RNA interference (RNAi) is a transforming technology with high potential for practical applications in biology, including specific and safe insect pest control. For developing RNAi-based pest-control products no general recommendations exist and the best strategy needs to be determined for each insect pest separately on a case-by-case basis. In this research, the potential of silencing the genes encoding the subunits of the ecdysone receptor complex, EcR and Ultraspiracle (USP) by RNAi was evaluated in the corn borer, Sesamia nonagrioides, using different delivery approaches and targeting different developmental stages. In conjunction with our previous research it is demonstrated that prepupae are sensitive to RNAi triggered by dsRNA injection and that feeding of dsRNA-expressing bacteria throughout S. nonagrioides’ larval life can lead to limited developmental malformations with no potent insecticidal results. Our results, consistent with previous studies, indicated a great fluctuation of exogenous RNAi effectiveness in the Lepidopteran species, suggesting that further factors should be taken into consideration in order to expand this very promising field into the ‘’RNAi-resistant’’ insect species.