Monitoring insecticide resistance and target-site mutations in field populations of <i>Spodoptera frugiperda</i> (Lepidoptera: Noctuidae) in China

The fall armyworm Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), a notorious invasive pest, has been widely monitored for insecticide resistance. Since its invasion of China in late 2018, early to mid-stage monitoring of resistance is particularly crucial to inform effective control strategies. Resistance ratios (RRs) derived from bioassays offer valuable insights into the overall resistance levels of field populations, while the detection of resistance allele frequencies helps uncover the potential causes of resistance variation. In this study, we established a baseline of susceptibility in third-instar larvae to 7 insecticides using a laboratory strain Xinzheng2019 and assessed the resistance levels of 9 populations collected from central and southern China between 2022 and 2023. Compared to the susceptible Xinzheng2019 strain, 2 field populations showed low-level resistance (RR = 5-10) to indoxacarb, and one exhibited low-level resistance to chlorantraniliprole. Minor resistance (RR = 3-5) was observed in 2 populations to lambda-cyhalothrin, 5 to indoxacarb, and 5 to chlorantraniliprole. All populations remained susceptible (RR < 3) to spinetoram, emamectin benzoate, chlorfenapyr, and lufenuron. Molecular analysis of 11 mutation sites across 3 key resistance-related genes (Ace-1, GluCl, and voltage-gated sodium channel [VGSC]) revealed that 52.1% of all tested samples carried either homozygous or heterozygous resistance alleles at the F290V locus of the Ace-1 gene, while no resistance mutations were detected at other sites. Our findings offer valuable insights into the insecticide resistance status of S. frugiperda field populations in China and provide guidance for effective chemical insecticide use.