The Toxicity And Mechanisms Of Neonicotinoid Insecticide On Earthworm(Eisenia Fetida)

Neonicotinoids are most of widely used insecticides applied to crops. They were utilized throughout the world and accounted for one-fourth of the entire insecticide market because of their excellent insecticidal activity. The neonicotinoids are particularly suited for controlling many insects with biting and sucking mouthparts. They were also used in seed dressing for protection from soil insects. Neonicotinoids have lower toxicity to birds, fish and mammalian but higher toxicity to bees and earthworm. Earthworms （Eisenia fetida）, an animal in soil ecosystem, play an important role in maintaining the ecological function of soil. Earthworms are important index to evaluation the ecological safety.In the present study, acute and chronic toxicity of six neonicotinoids were evaluated. The results showed that the14d half-maximal lethal concentration （LC50） of imidacloprid, acetamiprid, nitenpyram, clothianidin, thiacloprid and guadipyr was3.05,2.69,4.34,0.93,2.68and>100mg/kg, respectively. Reprduction of earthworms were significantly reduced with a56d half-maximal effective hatchability concentration （EC50） was0.954mg/kg、0.318mg/kg、0.261mg/kg、0.368mg/kg、0.202mg/kg and>100mg/kg, respectively. Mean cocoons per earthworm, mean hatchlings per cocoon, and mean cocoon weight were also significantly decreased compared with controls.The DNA damage effects of imidacloprid, acetamiprid, nitenpyram, clothianidin, thiacloprid and guadipyr on earthworms were conducted with comet assay, results showed guadipyr didn’t induce DNA damage in earthworm with100mg/kg. While imidacloprid and thiacloprid induced DNA damage with1.0mg/kg.The effects of the six neonicotinoids on E. fetida were further sought by studying CAT、 SOD and AchE activity. After exposure to1.0mg/kg neonicotinoids for3,7,14and21days, the activity of SOD and CAT in earthworm increased and then decreased to control level. AChE activity decreased at3days and7days at higher concentrations and then increased to control level, significant increase of CAT activity in earthworm was observed at day7,14and21after exposed to50.0（excluded day21） and100mg/kg. SOD activity had a different change from both AChE and CAT. The first significant increases of SOD activity was found as early as day3under10.0mg/kg guadipyr exposure, while day7for CAT and SOD increase peaks were founded at day7under50.0and100mg/kg and at day14under100mg/kg. At day21, SOD activity recovered to original level of control.The expression levels of SOD, CAT and AhE gene of earthworms were studied as toxicity endpoints. E. fetida was exposed to1.0mg/kg neonicotinoids for3,7,14and21days. The results showed that neurological dysfunctions gene EW1_F1P04_C04（F1P04）, EW1_F1P10_E08 （F1P10） and EW1_F2P14_D06（F2P14） and Lr_PAHCF₆4C08（64C08） of calcium binding were found to be down-regulated after3days and7days exposure; while signal transduction gene EW1_F1P07_H02（F1P07） was significantly up-regulated at a7day and21day exposure; SOD gene of earthworms started to be up-regulated after3and14days expose, and than down-regulated to control levels. In contrast, CAT gene was founded to be significantly up-regulated after14days and21days exposure.The epidermal and midgut of earthworm were examined after14days exposure of imidacloprid, acetamiprid, nitenpyram, clothianidin, thiacloprid and guadipyr. The results showed that neonicotinoid insecticide could damage the epidermal and midgut cells of E. fetida, meanwhile, the midgut cells were more sensitive to neonicotinoid insecticide, the effect of acetamiprid on the epidermal and midgut was more dramatic, which could affect epidermal and midgut cells at0.025mg/kg, while the other neonicotinoids damaged epidermal and midgut cells with higher concentrations. To guadipyr, it could damage the midgut at100mg/kg, but not to epidermal.