Acute toxicity tests on Japanese amphibian larvae using thiobencarb, a component of rice paddy herbicides

Acute toxicity tests were carried out on five species of Japanese amphibian larvae, at different developmental stages, to assess the risk posed by thi obencarb, a component of rice paddy herbicides. Test substances were four t ypes of commercially formulated herbicide containing mainly thiobencarb, an d the 24 h, 48 h, 72 h and 96 h LC50 (median lethal concentration) values o f these herbicides were calculated by probit analysis. These values ranged from 0.9 to 6.5 mg/l of thiobencarb. Newly hatched larvae seemed to be slig htly more resistant to the herbicides than well-developed larvae in all tes t species. There were no clear interspecific differences in responses. The actual thiobencarb concentration in paddy water was measured with indoor mo dels for two weeks, and it ranged from <0.005 to 3.1 mg/l. Some of the meas ured concentrations exceeded the LC50 values. Thiobencarb residue in paddy water can therefore be lethal to amphibians throughout larval development. Tests with Xenopus laevis produced approximately the same LC50 values as th ose 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.