Azadirachtin toxicity against larval instars of Spodoptera frugiperda (J.E. Smith): ontogenetic thresholds and lethal time dynamics under high-mountain laboratory conditions

BACKGROUND Azadirachtin, derived from Azadirachta indica A. Juss., is increasingly valued as a botanical insecticide for Integrated Pest Management (IPM) programs. However, most toxicological data on azadirachtin against Spodoptera frugiperda (J.E. Smith) have been generated under lowland tropical conditions (20–30°C), leaving a critical knowledge gap regarding its efficacy in Andean high-mountain agroecosystems characterized by cooler temperatures. RESULTS Dose–response and lethal time bioassays against all six larval instars were conducted at 2,586 m a.s.l. (17 ± 1°C, 65 ± 10% RH). Early instars (L1–L3) showed significantly higher susceptibility, with LC₅₀ values below 703 µL/L at 96 h and mean LT₅₀ of 43.3 h. A critical physiological threshold between L3 and L4 was identified by convergent evidence from Probit analysis (R² = 0.986, p < 0.001) and lethal time curves, with LT₅₀ increasing abruptly by 2.7-fold from L3 to L4 (37.9 vs. 145.9 h). Three-way ANOVA confirmed that larval instar (partial η² = 0.399, p < 0.001) and exposure time (partial η² = 0.355, p < 0.001) were the dominant factors determining mortality. CONCLUSION Azadirachtin retains biologically significant activity against S. frugiperda under high-mountain conditions, but its efficacy is strongly dependent on larval ontogeny. The L3–L4 threshold represents an actionable decision point for IPM applications: treatments should be timed to target L1–L3 instars to maximize cost-effectiveness and minimize the risk of inconsistent field outcomes. These findings provide the first comprehensive toxicological characterization of azadirachtin against S. frugiperda in Andean high-altitude environments.