RNA interference (RNAi) technology is a promising approach used in pest control. The efficiency of RNAi varies considerably among different insect species, and growing evidence suggests that degradation of double-stranded RNA (dsRNA) prior to uptake is an important factor that limits RNAi efficiency in insects. Our recent work on fall webworm ( ), an important invasive pest in China, showed a relatively low silencing efficiency of RNAi through dsRNA injection, which is considered the most feasible dsRNA delivery method for inducing RNAi, and the factors involved in the mechanism remain unknown. Herein, we first detected the dsRNA-degrading activity in the hemolymph and gut content of in ex vivo assays and observed rapid degradation of dsRNA, especially in the hemolymph, which was complete within only 10 min. To determine whether dsRNA degradation could contribute to the low effectiveness of RNAi in , four dsRNA nuclease ( ) genes, , and were identified by homology searching against the transcriptome database, and their transcript levels were subsequently investigated in different tissues, developmental stages, and after dsRNA injection. Our results show that are highly expressed mainly in gut tissues and hemolymph, and the expression of and were significantly upregulated by dsGFP induction. RNAi-of-RNAi studies, using as a reporter gene, demonstrated that silencing and significantly increases RNAi efficacy via dsHcCht5 injection, and co-silencing these two genes results in a more significant improvement in efficacy. These results confirm that the RNAi efficacy in through dsRNA injection is certainly impaired by dsRNase activity, and that blocking could potentially improve RNAi, providing a reference for related studies on insects where RNAi has low efficiency.