Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae) is one of the most destructive agricultural pests. We investigated the cumulative lethal effects of two nanostructure materials by feeding nanostructural form of copper oxide (CuO) or copper oxide doped with silver (CuO:Ag), nanosheets-treated diet to third and/or fifth instars of S. littoralis until pupation. Nanosheet concentrations were individually incorporated into 150 mL of artificial diet and offered as treated food. Both nanosheets had no immediate effect but caused cumulative larval mortality. At high concentrations and longer exposure times, nanosheet had a significant insecticidal effect against S. littoralis larvae. Nanosheets at 300 mg had the highest insect mortality effect, reaching 97.6% and 100% among CuO-treated third and fifth instars compared with 57.6% and 48.2% among the same instar treated with CuO:Ag. Treated larvae exhibited higher pupal mortality, and pupal and moth deformities compared with untreated larvae. There were more deformities among those fed CuO:Ag. Incorporation of nanosheets into the diet had a significant effect on the timing of larval development. Results showed that CuO was more influential than CuO:Ag at all concentrations and with similar exposure durations. Our results suggest that nanosheets may have important implications on the population dynamics of S. littoralis.