Quantitative genetic analysis and risk assessment of field-evolved emamectin benzoate resistance in Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae)

A population of Spodoptera litura originating from table grapes in northwest Western Australia in 2024 had 55.1% survival on a discriminating dose of emamectin benzoate in the F1 generation. Survival was stable in this strain (BM6xTG) for the next 7 generations under laboratory selection at the discriminating dose, with 28.9%, 19.6%, and 21.8% mortality in the G4, G6, and G7, respectively. A series of iso-female lines were established from G8 adults and used to generate proxy F2 recombinants (where parental G7 moths were considered as the F0 generation), which were then screened with the discriminating dose of emamectin benzoate. Three lines exhibited 100% survival and were presumed homozygous for resistance. Subsequent quantitative genetic analysis showed that resistance was autosomal, incompletely dominant (DLC = 0.85), and conferred by 1 or a small number of closely linked loci. While there was no major cross-resistance to chlorantraniliprole, indoxacarb, methoxyfenozide, or Vip3A in the BM6xTG strain, it was significantly less sensitive to spinetoram than the laboratory strain. There was a lack of resistance suppression by metabolic inhibitors triphenyl phosphate, diethyl maleate, and piperonyl butoxide, suggesting that metabolic detoxification may not play a role in resistance. A fourth iso-female line was completely insensitive to both emamectin benzoate and chlorantraniliprole, and a complementation test for allelism confirmed multi-resistance to Group 28 and Group 6 insecticides in this strain. The risk that these resistance variants pose to management of S. litura is discussed.