Inheritance and Resistance Mechanisms of Field-Evolved Resistance to Pyrethroids in a Fall Armyworm (Spodoptera frugiperda J.E. Smith) (Lepidoptera: Noctuidae) Strain from Puerto Rico

This study examines resistance inheritance to the pyrethroid insecticides esfenvalerate and deltamethrin in a Puerto Rican strain of fall armyworm (FAW), Spodoptera frugiperda, a major global pest of corn. The resistant strain (PPR) showed significantly higher resistance compared to a susceptible strain (SUS), with a 62-fold X-linked and 15-fold autosomal-linked resistance ratio (RR50) for esfenvalerate and deltamethrin, respectively. Resistance was incompletely dominant for both insecticides. Synergist bioassays revealed that detoxification enzymes play a key role in resistance, with PPR exhibiting increased toxicity across all tested synergists, especially with a 12-fold increase when all were combined. Deltamethrin assays confirmed the importance of these enzymes, with a 17-fold increase in PPR toxicity when combined with esterase inhibitors. These findings highlight the complexity of pyrethroid resistance, involving multiple non-target site mechanisms, and suggest that heterozygous individuals could survive in treated crops due to incomplete dominance. The results emphasize the need for diversified pest management strategies, including insecticide rotation, to effectively control FAW populations.