Large‐Scale Monitoring, Biochemical Mechanism, and Fitness Cost of Resistance to Chlorfenapyr in a Near‐Isogenic Strain of Spodoptera litura ( Lepidoptera : Noctuidae ) From China

Spodoptera litura is a globally important polyphagous herbivore that has developed resistance to many widely used insecticides. Chlorfenapyr is a pyrrole insecticide with demonstrated efficacy against lepidopteran pests, but the susceptibility baseline and field-evolved resistance in S. litura remain poorly characterized. In this study, we built a near-isogenic strain (JZ-NIL) based on one field-collected chlorfenapyr-resistant population of S. litura and characterized patterns of cross-resistance, fitness costs, and mechanism of metabolic resistance. In comparison with the susceptible Lab-S strain, the field-sampled JZ population displayed approximately 225.0-fold resistance to chlorfenapyr. The JZ-NIL strain was built and showed 124.2-fold chlorfenapyr resistance, with no apparent cross-resistance to broflanilide, chlorpyrifos, chlorantraniliprole, emamectin benzoate, indoxacarb, and spinetoram. Assays of enzyme activity and tests of synergism showed that diethyl maleate (DEM) made a synergistic ratio of 1.9 against chlorfenapyr, and glutathione S-transferase (GST) activity in JZ-NIL was 1.8-fold significantly higher than in the strain of Lab-S, indicating that GST-mediated metabolism contributes to chlorfenapyr resistance. In addition, pupation rate, fecundity, and hatchability were significantly lower in JZ-NIL than in Lab-S. The relative fitness of JZ-NIL was 0.54 compared with Lab-S. Overall, these results indicate that GST-mediated detoxification contributes to chlorfenapyr resistance in S. litura and that this resistance is associated with significant fitness costs. Further studies are needed to clarify the metabolic and target-site mechanisms underlying chlorfenapyr resistance.