Toxic Effects Of Two Neonicotinoids Pesticides On Four Anural Species Tadpoles

The global amphibian populations and species has declined dramatically in recent decades.The previous researches have shown clearly this kind of decline is caused by a variety of factors,such as pesticides.In this paper,selected toxicities(including acute,subchronic,physical and nuclear),of two neonicotinoid pesticides(thiamethoxam and acetamiprid)to the tadpoles of four common amphibian species in southern China(Bufo gargarizans,Duttaphrynus melanostictus,Rhacophorus dennysi,Rana zhenhaiensis)are tested.Hopefully,it can provide more evidence for the protection of amphibians and the natural environments.It is also useful to application of amphibians to monitor environmental changing.The results were as follows:Acute toxicity tests found that the 96 h half lethal concentrations(96 h-LC₅₀)of thiamethoxam on tadpoles of four anuran species respectively were 377.742 mg/L,274.912 mg/L,632.644 mg/L and 215.745 mg/L;and the 96 h-LC₅₀of acetamiprid on these tadpoles respectively were 384.892 mg/L,332.903mg/L,594.211 mg/L and 227.545 mg/L.The results showed that the toxic degree of the two pesticides to the four tadpoles is as follows:R.zhenhaiensis>D.melanostictu>B.gargarizans>R.dennysi,and thiamethoxam and acetamiprid were both microtoxic to tadpoles of all four species.The growth rates of tadpoles of two toad species were observed by subchronic toxicity test to understand the influence of drugs on their growth and development.The results showed that thiamethoxam and acetaminidine had certain inhibitory effects on the growth of B.gargarizans tadpoles and D.melanostictu tadpoles.The higher the drug concentration,the longer the infection time,and the stronger the inhibition on the growth and development of tadpoles.After 28 days of exposure to thiamethoxam solution,the body length and weight of B.gargarizans tadpoles respectively decreased by 34.3%and49.4%at 50 mg/L.The body length and weight of D.melanostictu tadpoles respectively decreased by 29.2%and 63.1%at 60 mg/L.After 28 days of exposure to high concentrations of amidine,the body length and weight of B.gargarizans tadpoles respectively decreased by 31.5%and 55.1%at 50 mg/L;The body length and weight of D.melanostictu tadpoles respectively decreased by 28.9%and 66.2%at 60 mg/L.Physiological toxicity tests showed that thiamethoxam and acetamidine had different effects on the antioxidant defense system of tadpoles of three amphibians(B.gargarizans,D.melanostictus,R.dennysi).Generally,the activities of the antioxidant system of tadpoles were first activated within a short period of time.However,with the prolongation of the exposure time and the increase of drug concentration,they were then inhibited.The amount of malondialdehyde(MDA)in tadpoles increased with the increase of drug concentration.The result of micronucleus test showed that thiamethoxam and acetaminidine had a significant induction effect on the micronucleus and nuclear abnormalities of erythrocytes in tadpoles of R.dennysi.In a certain concentration range,the micronucleus rate of tadpole's erythrocytes increased with the extension of the time of poisoning and the increase of pesticide concentration,but too high concentration would inhibit the division of erythrocytes and the micronucleus rate would decrease.The rates of nuclear abnormalities increased with the drug concentration,demonstrating a significant dose-effect relationship.A single cell gelel etrophoresis(comet assay)showed that thiamethoxam and acetamiprid caused DNA damage to erythrocytes in R.dennysi tadpoles,with a significant linear correlation between the concentrations of the two drugs and the extent of DNA damage in tadpole's erythrocytes.Experiments show that thiamethoxam and acetamiprid have certain toxic effects on tadpoles.Therefore,rational and standardized use of neonicotinoids drugs is very important for protections of both environment and amphibians.