Efficacy of endosulfan and fipronil and joint toxic action of endosulfan mixtures against Leptinotarsa decemlineata (Say)

The Colorado potato beetle (Leptinotarsa decemlineata [Say]) in the north Xinjiang Uygur autonomous region in China has evolved resistance to pyrethroids and carbamates. Mutations resulting in target site insensitivity, namely L1014F in LdVssc1 voltage-sensitive sodium channel and S291G in acetylcholine esterase, confer, at least partially, the resistance. Insecticides with different modes of action may serve as likely replacements. Endosulfan and fipronil are GABA-gated chloride channel-blocking insecticides. In this article, we found that the contact toxicities of the two compounds were among the highest ever estimated to the 4th-instar larvae and the adults and affirmed that they were useful for L. decemlineata control in north Xinjiang. Regarding stomach toxicities of the two compounds, adult beetles were less sensitive than 2nd-, 3rd-, and 4th-instar larvae, suggesting that the appropriate timing for spraying is the early larval stage. Mixtures of endosulfan and α-cypermethrin at 1:160, 1:80, and 1:40 ratios, of endosulfan and phoxim at 1:24 ratio, and of endosulfan and isocarbophos at 1:72 and 1:288 ratios, significantly increased toxicity in a field population. The combination indices were significantly below 1 at both LD50 and LD90 levels, revealing synergistic effects. Our results demonstrated that endosulfan and fipronil could be applied alone and endosulfan may also be used in binary mixtures to restore pyrethroid susceptibility. These findings may have considerable practical implications for L. decemlineata resistance management.