Butachlor (N-butoxy-methyl-2-chloro-20,60-diethyl-actanilide) is the most commonly used herbicide on rice paddy fields in Taiwan and throughout Southeast Asia. Paddy fields are man-made habitats that are commonly used for reproduction by many species of frogs. However, little is known regarding the ecological and physiological effects of butachlor on frogs.

A study published in Ecotoxicology examined acute and chronic effects of butachlor on tadpoles of the alpine cricket frog (Fejervarya limnocharis), an Asian species that breeds in rice paddies when they are first flooded. The timing coincides with typical butachlor application regimes, and cricket frogs can be exposed to higher concentrations of butachlor than other frogs.

Tadpoles were hatched from egg masses collected from paddy fields and then assigned to treatments of zero, 0.05, 0.1, 0.2, 0.4, 0.8, 1.6 and 3.2 mg/l of butachlor. All concentrations were below the recommended application rate of 4.8 mg/l of butachlor in paddy water.

Analysis of the experiment data showed that 0.87 mg/l of butachlor can kill 50% of the tadpoles after 96 hours of exposure. This demonstrated that cricket frog tadpoles were less sensitive than other amphibians that breed in rice paddy fields, such as the narrow-mouthed toad (Microhyla ornata; 0.53 mg/l) and Guenther’s frog (Rana guentheri; 0.74 mg/l). The surviving tadpoles exhibited a range of impacts, including delayed metamorphosis. Also, the number of DNA strand breaks in the red blood cells of cricket frog tadpoles increased with increasing butachlor concentrations.

Butachlor’s actual impact on wild frog populations may depend on its short half-life and the pattern and timing of its application. Staggered spraying in adjacent fields may create refugia with lower, decaying concentrations, which adult frogs may be able to detect.

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