Comparative Insecticidal Activity and Mechanism of Isocycloseram versus Other GABAergic Insecticides against the Fall Armyworm

The fall armyworm (FAW) is a serious agricultural pest and has developed resistance to multiple insecticides. It is necessary to introduce novel insecticide(s) for controlling FAW. Isocycloseram is a completely novel isoxazoline insecticide. However, its activity and mode of action against FAW have not been reported. In this study, isocycloseram exhibited a higher insecticidal activity (LC₅₀ = 0.26 mg/kg) than fipronil (LC₅₀ = 7.72 mg/kg) against FAW. The median inhibitory concentration (IC₅₀) of isocycloseram (IC₅₀ = 8.52 nM) was almost equal to that of the desmethyl-broflanilide (IC₅₀ = 7.32 nM) to the <i>Sfr</i>RDL1 receptor. The IC₅₀ of isocycloseram to the <i>Sfr</i>RDL2 receptor was 11.13 nM, which was obviously less than that of desmethyl-broflanilide, dieldrin, fipronil, fluxametamide. Compared with the <i>Sfr</i>RDL2 receptor, the <i>Sfr</i>RDL1 receptor exhibited higher sensitivity to GABAergic insecticides. The recombinant <i>Sfr</i>GluCl receptor was successfully stimulated by l-glutamate; however, the currents were low and weakly inhibited by isocycloseram at 10 μM. In conclusion, our results provided the theoretical basis for usage of GABAergic insecticides for controlling FAW.