Susceptibility to cyantraniliprole in Australian Spodoptera frugiperda (Lepidoptera Noctuidae) and establishment of diagnostic tests for resistance management

Diamides are an important class of insecticides for control of insect pests in Australian farming systems. Their favourable toxicological and environmental profiles have led to broad registration across multiple pest species and crop types. However, widespread use is known to compromise insecticidal efficacy due to resistance, and concerns have been raised about the sustainability of these insecticides in Australian agriculture, particularly since the incursion of Spodoptera frugiperda in 2020. As a first step in pre-emptive management of S. frugiperda, baseline susceptibility to cyantraniliprole was determined in larval bioassays performed on 16 field populations collected from maize and sweet corn between 2020 and 2021 during the initial period of establishment in Australia. There was a narrow (2.1-fold) range of intra-specific variation in cyantraniliprole susceptibility amongst field populations. The average median lethal concentration was 0.088 mg L⁻¹, which was significantly higher than in Helicoverpa armigera (LC₅₀ = 0.037 mg L⁻¹). An analysis of relative diamide toxicity showed that while the median concentration of chlorantraniliprole was significantly higher than cyantraniliprole in S. frugiperda, there was no significant difference between the toxicity of chlorantraniliprole and cyantraniliprole at the LC_99.9 level. Using the dose–response data generated from these bioassays and taking into account empirical survival, a concentration of 2 mg L⁻¹ of cyantraniliprole was determined to be a suitable dose for discriminating between cyantraniliprole-susceptible and cyantraniliprole-insensitive phenotypes of S. frugiperda.