Toxicity of the neonicotinoid insecticides thiamethoxam and imidacloprid on tadpoles of four species of South American amphibians and effects of thiamethoxam on the metamorphosis of Rhinella arenarum

Abstract The present study examined the acute and chronic toxicity of the neonicotinoid insecticides imidacloprid and thiamethoxam in neotropical amphibian species from the Pampa region of Argentina. The median lethal concentration after 96h of exposure (96h-LC50) ranged between 11.28 and > 71.2 mg/L amongst all species and development stages tested; indicating that these pesticides are unlikely to cause acute toxicity in the wild. The subchronic toxicity was also low, with 21d-LC50 values ranging between 27.15 and > 71.2 mg/L. However, exposure of developing tadpoles of Rhinella arenarum to thiamethoxam from stage 27 until the completion of metamorphosis showed that treated animals present a significant reduction in metamorphic success together with a smaller size at metamorphosis and an asymmetry in the length of the left and right front arms. These effects, which are observed starting from the lowest concentration tested (9 mg/L), suggest an effect of thiamethoxam on the hypothalamic-pituitary-thyroid (HPT) axis. Given that the observed effects can clearly impair amphibian performance and survival in the wild, further studies employing the same experimental approach but looking at lower environmentally-relevant concentrations are needed.