Combined toxicity of imidacloprid and three insecticides to the earthworm, Eisenia fetida (Annelida, Oligochaeta).

Although the earthworm Eisenia fetida has been used in many ecotoxicological studies in recent years, most of these studies have only focused on assessing the effects of individual insecticides. In the present study, we aimed to compare the individual and combined toxic effects of imidacloprid and three insecticides (phoxim, chlorpyrifos, and lambda-cyhalothrin) on E. fetida. We showed that imidacloprid had the highest intrinsic toxicity to the worms in filter paper contact test, followed by phoxim and lambda-cyhalothrin, while the least toxicity was found from chlorpyrifos. Moreover, 14-day soil toxicity test revealed that the highest toxicity was still detected for imidacloprid with an LC50 value of 2.82 (2.61∼3.17) mg a.i. kg-1 dry weight (DW), followed by chlorpyrifos with an LC50 value of 384.9 (353.5∼440.3) mg a.i. kg-1 DW. Meanwhile, a relatively less toxicity was found for lambda-cyhalothrin with an LC50 value of 560.3 (475.9∼718.5) mg a.i. kg-1 DW, while the lowest toxicity to E. fetida was observed for phoxim with an LC50 value of 901.5 (821.3∼1017) mg a.i. kg-1 DW. In addition, significant synergistic responses were found from the ternary mixture of imidacloprid-phoxim-lambda-cyhalothrin and quaternary mixture of imidacloprid-phoxim-chlorpyrifos-lambda-cyhalothrin in both bioassay systems. Therefore, our findings highlighted that the simultaneous presence of several insecticides in the soil environment might lead to increased toxicity, resulting in serious damage to the nontarget organisms compared with individual insecticides.