Comparative Insecticidal\nActivity and Mechanism of\nIsocycloseram versus Other GABAergic Insecticides against the Fall\nArmyworm

The fall armyworm (FAW) is a serious agricultural pest\nand has\ndeveloped resistance to multiple insecticides. It is necessary to\nintroduce novel insecticide(s) for controlling FAW. Isocycloseram\nis a completely novel isoxazoline insecticide. However, its activity\nand mode of action against FAW have not been reported. In this study,\nisocycloseram exhibited a higher insecticidal activity (LC<sub>50</sub> = 0.26 mg/kg) than fipronil (LC<sub>50</sub> = 7.72 mg/kg) against\nFAW. The median inhibitory concentration (IC<sub>50</sub>) of isocycloseram\n(IC<sub>50</sub> = 8.52 nM) was almost equal to that of the desmethyl-broflanilide\n(IC<sub>50</sub> = 7.32 nM) to the <i>Sfr</i>RDL1 receptor.\nThe IC<sub>50</sub> of isocycloseram to the <i>Sfr</i>RDL2\nreceptor was 11.13 nM, which was obviously less than that of desmethyl-broflanilide,\ndieldrin, fipronil, fluxametamide. Compared with the <i>Sfr</i>RDL2 receptor, the <i>Sfr</i>RDL1 receptor exhibited higher\nsensitivity to GABAergic insecticides. The recombinant <i>Sfr</i>GluCl receptor was successfully stimulated by l-glutamate;\nhowever, the currents were low and weakly inhibited by isocycloseram\nat 10 μM. In conclusion, our results provided the theoretical\nbasis for usage of GABAergic insecticides for controlling FAW.