Plant phenology and larval instar influence the efficacy of Bt maize against Spodoptera frugiperda (Lepidoptera: Noctuidae)

The fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), is one of the most destructive pests of maize worldwide, and its management relies heavily on hybrids expressing insecticidal proteins derived from Bacillus thuringiensis ( Bt ). However, the efficacy of Bt maize may vary according to plant developmental stage, canopy position, larval instar, and insect susceptibility. This study evaluated the mortality of first- and third-instar S. frugiperda larvae from laboratory and field populations fed with leaves collected from different canopy sections of VTPRO4® maize expressing Cry1A.105, Cry2Ab2, Vip3Aa20, and Cry3Bb1 proteins at the V3 and V8 phenological stages. Bt maize significantly increased larval mortality compared with the conventional hybrid, although mortality was incomplete under all evaluated conditions. First-instar larvae were consistently more susceptible than third-instar larvae, and the field population exhibited lower susceptibility than the laboratory population. Mortality was generally higher in leaves collected from the upper canopy at the V3 stage, whereas Bt efficacy declined markedly at the V8 stage across all canopy sections. These results demonstrate that Bt -mediated toxicity is dynamic and influenced by the interaction among plant ontogeny, canopy position, larval developmental stage, and population susceptibility. The reduced efficacy observed in older plants and against later larval instars may create survival windows for partially resistant individuals under field conditions, potentially accelerating resistance evolution and highlighting the importance of incorporating spatiotemporal variability into insect resistance management strategies for tropical maize production systems.