Toxicity of azadirachtin against larval instars of Spodoptera frugiperda (J.E. Smith, 1797): ontogenetic thresholds and lethal time dynamics under high-mountain laboratory conditions

Abstract

Azadirachtin, derived from Azadirachta indica A. Juss., is a botanical insecticide of growing relevance for Integrated Pest
Management (IPM). Most toxicological data on azadirachtin against Spodoptera frugiperda (J.E. Smith, 1797) have been
generated under lowland tropical conditions (20-30°C), leaving a critical knowledge gap regarding its efficacy in Andean high-mountain agroecosystems. 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 through Probit analysis (R² = 0.986, p < 0.001) and lethal time curves, with LT50 increasing 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. 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: treatments should target L1-L3 instars to maximize cost effectiveness and minimize the risk of inconsistent field outcomes.