Early insecticide tolerance in an invasive fall armyworm (Lepidoptera: Noctuidae) population in relation to bioassay phenotypes and genetic haplotypes

Fall armyworm, Spodoptera frugiperda (J. E. Smith, 1797) (Lepidoptera: Noctuidae), is a major lepidopteran pest that has rapidly expanded across the Eastern Hemisphere, raising concerns about declining susceptibility to common insecticides. This study primarily evaluated larval susceptibility and tolerance patterns to emamectin benzoate, chlorantraniliprole, and lambda-cyhalothrin using time- and dose-response bioassays on second- and fourth-instar larvae from the Thai population. Emamectin benzoate induced delayed mortality, chlorantraniliprole caused rapid mortality even at low doses, and lambda-cyhalothrin produced acute knockdown effects, particularly in second instars. Although overall mortality was high, delayed death and partial survival at field-equivalent doses, especially among fourth instars, revealed instar-dependent tolerance heterogeneity. To provide population genetic context, mitochondrial cytochrome oxidase subunit I (COI) and nuclear triosephosphate isomerase (Tpi) haplotypes were analyzed in bioassay-tested and field-collected individuals. Extremely low maternal diversity was observed, with all specimens carrying the corn-strain mitochondrial haplotype COICSh4. In contrast, the nuclear Tpi locus showed higher diversity, comprising five haplotypes (TpiCa1a, TpiCa1b, TpiCa2a, TpiCa2b, and TpiRa1a) shared with other Asian and African populations. Notably, individuals surviving lambda-cyhalothrin exposure were disproportionately associated with the TpiCa2a haplotype. Although these genes are not resistance genes, these nonrandom associations suggested that the population genetics background may contribute to heterogeneity in insecticide tolerance. Together, these results highlighted the value of bioassays as a primary tool for detecting early shifts in susceptibility, with population genetic markers providing complementary insight into demographic structure and tolerance-associated patterns. Integrating phenotypic assays with genetic surveillance can strengthen early-stage resistance monitoring and inform insecticide-rotation and integrated pest management strategies for S. frugiperda and other invasive agricultural pests.