This study investigates the antifeedant activity of silver (Ag) and zinc oxide (ZnO) nanobiopesticides (NBPs) synthesized from C. collinus against third instar larvae of Helicoverpa armigera. Using a no-choice bioassay method, cotton leaf discs treated with different concentrations of Ag-NBPs and ZnO-NBPs were exposed to the larvae, and their feeding behavior was monitored. The results demonstrated a significant, concentration-dependent reduction in leaf consumption, indicating strong antifeedant effects of both NBPs. The antifeedant efficacy was higher at increased concentrations, suggesting that these NBPs effectively deter the pest from feeding, which could substantially reduce crop damage. These findings highlight the potential of Ag-NBPs and ZnO-NBPs as eco-friendly alternatives for pest management strategies targeting H. armigera.