One of the most widely used herbicides for commercial and home use is glyphosate, the active ingredient in Roundup® Regular. We examined toxicity of the herbicide Roundup® on 6 amphibian species: Ambystoma gracile, Ambystoma macrodactylum, Anaxyrus [Bufo] boreas, Pseudacris regilla, Rana cascadae, and Rana luteiventris. Larvae were exposed to 6 different Roundup® Regular treatments (0 (control), 0.1, 0.5, 1.0, 2.0, and 5.0 mg AI/L dilutions of glyphosate) and monitored for 16 d. Estimated acute lethal concentrations at 24 h (LC50) varied significantly among species (ANOVA, F(3, 56) = 3.54, p < 0.0202), with concentrations ranging from 0.43 mg AI/L of Roundup® for P. regilla to 2.66 mg AI/L for A. boreas. Bufonid and ambystomatid larvae were less sensitive than Ranid and Pseudacrid species tested, with no salamander larval mortality occurring at 24 h. Mean time-to-death varied from 1 d for P. regilla to 8.3 d for A. gracile, respectively (ANOVA, F(5, 971) = 108, p < 0.0001). For exposure times longer than 24 h, the A. boreas was not significantly different than the salamanders for time-to-death, based on Tukey-Kramer comparisons. Results suggest Roundup® Regular is highly toxic to the amphibians at levels below EPA standards for drinking water and at concentrations they may be exposed to during overspray. We recommend the use of less toxic glyphosate-based herbicides in aquatic systems, if applications are necessary, or made during times of year when amphibian larvae are not present.
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