Identification of insecticide resistance mechanism of Spodoptera frugiperda against deltamethrin and development of its molecular diagnostic tools

Fall armyworm (FAW), Spodoptera frugiperda (Lepidoptera: Noctuidae) is one of the major invasive pests of various cereal crops in many Asian countries including Korea. In Korea, various types of insecticides are used to control FAW, and a high degree of resistance has been identified, especially to pyrethroid insecticides such as deltamethrin. Selecting the field population with deltamethrin consistently for more than 10 generations, resistant strain resulted in resistance levels 794 times higher than susceptible strain reared in the lab without insecticide exposure judged with LC₅₀ vales. Bioassay results after treatment with or without inhibitors of specific detoxification enzymes, i.e. synergists, confirmed the involvement of cytochrome P450 and esterases in deltamethrin resistance, with SR (synergistic ratio) values of 0.41 and 0.76 when treated with PBO and TPP. Comparison of gene expression of insecticide susceptible and resistant strains by RNAseq showed the same results as the bioassay, and CYP9f2 and FE4 type CCEs were highly overexpressed only in resistant strain. Other detoxification gene families showed little resistance-specific expression patterns. In many previous studies, the primary mechanism of pyrethroid insecticide resistance has been reported to be sodium channel gene mutations, but this study confirmed that deltamethrin resistance development of FAW was based on detoxification enzyme expression particularly P450s and CCEs without any previously reported sodium channel mutations, even though a significantly higher level of resistance of almost 800-fold was identified. These results can be utilized for the development of molecular diagnostics using specific resistance-related genes and for insecticide resistance management of FAW.