Synergistic enhancement of emamectin benzoate and Metarhizium anisopliae against late-instar Spodoptera frugiperda by promoting fungal pathogenicity

Spodoptera frugiperda, a globally invasive pest, has developed severe resistance to chemical insecticides, necessitating sustainable control strategies. This study investigated the synergistic interaction between emamectin benzoate (EB) and the entomopathogenic fungus Metarhizium anisopliae CQMa421 against fourth-instar larvae. Bioassays revealed a synergistic ratio (SR) of 4.19 when EB was combined with CQMa421 spores (2 × 10⁷ spores/mL), reducing EB usage by 76.1 %. CQMa421 did not cause significant effects on S. frugiperda, whereas emamectin benzoate (EB) significantly disrupted feeding, development, and molting, particularly delaying fourth-instar molting by 4.65 days. Compared to EB alone, the combined treatment delayed larval molting by 1.2 days, suppressed feeding (18.3 % reduction in frass weight and 11.1 % weight loss), reduced adult emergence by 12.3 %, shortened oviposition duration by 1.5 days, and decreased total fecundity by 42.5 %. Additionally, the combined treatment increased pupal deformity by 4.4 %, collectively inhibiting population recovery. Mechanistic studies demonstrated that EB accelerated fungal spore germination (GT₅₀ reduced by 3.4 h), appressorium formation (AT₂₅ advanced by 4.5 h), and penetration of the cuticle of Spodoptera frugiperda. Furthermore, the combined treatment significantly downregulated immune-related genes (Attacin: 86.6 %; Cecropin: 61.2 %; MyD88: 50.5 %). This indicates that EB enhances the pathogenicity of the M. anisopliae synergistically by disrupting the host's immune barrier and delaying larval molting. This research provides a strategy and theoretical basis for controlling the resistance of Spodoptera frugiperda through reduced pesticide use and increased effectiveness.