Effect of release herbicide on mortality, avoidance response, and growth of amphibian larvae in two forest wetlands

Effects of Release herbicide (triclopyr butoxyethyl ester, [TBEE]) on mortality, avoidance response, and growth of larval amphibians (Rana clamitans, Rana pipiens) were investigated using in situ enclosures deployed in two forest wetlands in northern Ontario, Canada. Release was applied at nominal concentrations ranging from 0.26 to 7.68 mg TBEE acid equivalents (AE)/L. No significant deleterious effects of this herbicide on larval growth were detected. However, concentration-dependent mortality and abnormal avoidance response were observed. Most mortality occurred within 96 h following treatment. Median lethal concentration (LC50) values for each species and experimental site ranged from 2.79 to 3.29 mg AE/L, while median effective concentration (EC50) values (abnormal avoidance response) ranged from 1.67 to 3.84 mg AE/L. The LC10 and EC10 endpoints approximated aqueous concentrations (0.59 mg AE/L) expected under direct aerial overspray scenarios, indicating a potential risk of impacts for a small proportion of native amphibian larvae. However, given the low frequency and limited use of this herbicide formulation in Canadian forestry, these risks are considered negligible. Changes in usage patterns would require concurrent chemical and biological monitoring of operational spray programs to accurately quantify the probability and magnitude of real-world exposures and to relate these exposure levels to concentration-response relationships including those described in this study.