The effects of insecticides on the early development of wild frogs inhabiting paddy fields

Abstract

Insecticides are chemicals used in agriculture that may have negative impacts on human health and the survival of non-target organisms. Many amphibian species in Japan depend on paddy fields during their early development, which overlaps with rice cultivation and insecticide application to paddy fields. These early developmental stages include organ formation, when insecticide exposure could cause embryological disturbances and have serious effects on larval growth and survival. In this study, we focused on two amphibian species, the Japanese tree frog (Hyla japonica) and the Montane brown frog (Rana ornativentris) as indicator species of paddy ecosystems. We examined the toxicities of three insecticides: clothianidin (a neonictinoid), cartap (a nereistoxin), and chlorantraniliprole (a diamide). We measured larval sensitivity to these insecticides and differences in toxicity between the active ingredients and formulations. Our results indicate that exposure to cartap increased the malformation and the mortality rates in both frog species, while no teratogenicity was observed in clothianidin and chlorantraniliprole. The sensitivity to cartap differed between the two frog species; in Montane brown frogs, deformity and mortality rates occurred at higher concentrations (0.2 mg/L) than in Japanese tree frogs (0.02 mg/L). Cartap exposure caused deformities including spinal bend and spinal blisters in both species, and albinism in the Japanese tree frog. The spinal bends occurred after exposure to concentrations of 0.2 mg/L cartap in the Montane brown frogs and 0.02 mg/L cartap in Japanese tree frogs. Blisters developed after exposure to a concentration of 2 mg/L cartap in both species. Experimental treatments resulted in a greater incidence of spinal bend and a smaller incidence of spinal blisters, which occurred after exposure to the cartap formulation rather than exposure to a single active ingredient. Our results indicate that cartap has a serious impact on the early development of paddy-dwelling frogs, especially Japanese tree frogs. Spinal bends and albinism caused by cartap exposure can seriously effect larval survival and may occur at the concentrations used in standard applications of cartap in paddy fields.