The Herpetological Journal

pp.73-81

Authors: Masahiro Saka

Abstract: Acute toxicity tests were carried out on five species of Japanese amphibian larvae, at different developmental stages, to assess the risk posed by thiobencarb, a component of rice paddy herbicides. Test substances were four types of commercially formulated herbicide containing mainly thiobencarb, and the 24 h, 48 h, 72 h and 96 h LC₅₀ (median lethal concentration) values of these herbicides were calculated by probit analysis. These values ranged from 0.9 to 6.5 mg/I of thiobencarb. Newly hatched larvae seemed to be slightly more resistant to the herbicides than well-developed larvae in all test species. There were no clear interspecific differences in responses. The actual thiobencarb concentration in paddy water was measured with indoor models for two weeks, and it ranged from Xenopus laevis produced approximately the same LC₅₀ values as those of Japanese amphibians. This indicates that experimental frogs such as Xenopus laevis can act as a model for these native and wild amphibians when toxicity tests are conducted.

Keywords: Japanese amphibians, herbicide, thiobencarb, acute toxicity, risk assessment