Synergistic Efficacy of Bacillus Thuringiensis ssp. Aizawai and Neem Oil Against Spodoptera Frugiperda: Investigating the “Dual-Hit” Mechanism from Laboratory to Field

Abstract

The invasive fall armyworm, Spodoptera frugiperda (J.E. Smith), poses a severe threat to global maize (Zea mays L.) production, necessitating     sustainable management alternatives. This study evaluated the laboratory and field efficacy of Bacillus thuringiensis ssp. aizawai (Bt) and neem oil, applied independently and in combination. Laboratory bioassays quantified mortality and sublethal impacts over 72 hours, while field trials measured damage severity and infestation levels, and non-target arthropod safety using Emamectin Benzoate as a synthetic benchmark. Results indicated that while standalone applications primarily induced sublethal developmental delays and weight reductions, the combined Neem–Bt treatment elicited a significant synergistic response. This “dual-hit” mechanism achieved larval mortality of 8.67 ± 1.53 larvae out of 10 (86.7%), comparable to the synthetic control. Notably, a physiological saturation point was identified at maximum concentrations, where neem’s antifeedant properties suppressed foliage consumption and limited Bt toxin uptake. Field evaluations corroborated these findings, demonstrating significant reductions in infestation rates and crop damage while maintaining minimal disruption to beneficial arthropod populations. The synergy likely involves Bt-induced midgut lesions that facilitate the systemic penetration of azadirachtin, leading to the potentiation of hormonal and nutritional disruptions. These results establish the Neem–Bt combination as a potent, environmentally sustainable component for Integrated Pest Management (IPM) programs designed to safeguard maize yield stability.