Effects of two neonicotinoid insecticides on blood cell profiles and corticosterone concentrations of wood frogs (Lithobates sylvaticus).

Neonicotinoids are widely used insecticides that are detectable in agricultural waterways. These insecticides are of concern due to their potential impacts on non-target organisms. Pesticides can affect development of amphibians and suppress the immune system, which could impact disease susceptibility and tolerance. No previous studies on amphibians have examined the effects of these insecticides on differential blood cell proportions or corticosterone concentrations (a general stress hormone). We investigated the effects of chronic exposure to two neonicotinoids, thiamethoxam and clothianidin, on immunometrics of wood frogs (Lithobates sylvaticus). Frogs were exposed to single, chronic treatments of 2.5 µg/L or 250 µg/L of clothianidin or thiamethoxam for seven weeks from Gosner stage 25 to 46. The juvenile frogs were then maintained for three weeks post-metamorphosis without exposure to neonicotinoids. We measured water-borne corticosterone twice: six days and eight weeks after exposure in larval and juvenile frogs respectively. We assessed differential blood cell profiles from juvenile frogs. Corticosterone was significantly lower in tadpoles exposed to 250 µg/L of thiamethoxam compared to other tadpole treatments, but no significant differences in corticosterone concentrations were found in treatments using juvenile frogs. Anemia was detected in all treatments compared to controls with the exception of tadpoles exposed to 2.5 µg/L of clothianidin. Neutrophils to leukocyte and neutrophil to lymphocyte ratios were elevated in frogs exposed to 250 µg/L of thiamethoxam. Collectively, these results indicate that chronic exposure to neonicotinoids has varied impacts on blood cell profiles and corticosterone concentrations of developing wood frogs, indicative of stress. Future studies should investigate whether exposure to neonicotinoids increases susceptibility to infection by parasites in both larval and adult wood frogs. This article is protected by copyright. All rights reserved.