Clothianidin alters leukocyte profiles and elevates measures of oxidative stress in tadpoles of the amphibian, Rana pipiens

Neonicotinoid pesticide use is widespread and highly debated, as evidenced by recent attention received from the public, academics and pesticide regulatory agencies. However, relatively little is known about the physiological effects of neonicotinoid insecticides on aquatic vertebrates. Amphibians (larval stages in particular) are excellent vertebrate bioindicators in aquatic systems due to their risk of exposure and sensitivity to environmental stressors. Previous work with wood frog (Rana sylvatica) tadpoles exposed to formulated products containing thiamethoxam or clothianidin in outdoor mesocosms found significant shifts in leukocyte profiles, suggesting the tadpoles were physiologically stressed. The main objective of the present study was to characterize this stress response further using complementary measures of stress after exposure to clothianidin on northern leopard frogs (Rana pipiens) during their aquatic larval stages. Laboratory static-renewal exposures were conducted over eight weeks with the technical product clothianidin at 0, 0.23, 1,10 and 100 mu g/L, and diquat dibromide at 532 mu g/L was used as a positive control. We assessed tadpole leukocyte profiles and measures of oxidative stress as these sublethal alterations could affect amphibian fitness. We found changes in several types of leukocytes at 1 and 10 mu g/L, suggesting that these tadpoles exhibited signs of mild physiological stress. Clothianidin also induced an oxidative stress response at 0.23, 1 and 100 mu g/L. However, we found no differences in survival, growth, development time or hepatosomatic index in frogs exposed to clothianidin. Our study indicates that tadpoles chronically exposed to clothianidin have increased stress responses, but in the absence of concentration-response relationships and effects on whole-organism endpoints, the implications on the overall health and fitness of these changes are unclear. Crown Copyright (C) 2021 Published by Elsevier Ltd. All rights reserved.

Automated summary

Tadpoles exposed to clothianidin at low concentrations exhibited mild physiological stress and oxidative stress response. However, this did not impact the survival, growth, development time, or hepatosomatic index of frogs. Therefore, the implications of these changes on overall health and fitness remain unclear.