The Impacts of Glyphosate and Common Co-Formulations with Surfactant and Dye on American Toad Fitness

Anthropogenic activities can increase contaminants in aquatic environments. These pollutants can have negative consequences for aquatic taxa, particularly amphibians, because of their permeable skin and biphasic life cycle. Water pollution is contributing to the global decline of amphibians, so understanding the impact of contaminants on these taxa is imperative. Glyphosate herbicides are typically formulated with a surfactant to increase effectiveness and with a soluble dye to reveal coverage. Typical formulations result in increased mortality of several amphibian species, but we do not have a good understanding for which formulation component (or components) reduce fitness. We conducted an outdoor mesocosm experiment to assess the individual and interactive effects of glyphosate, surfactant, and dye on survival, days to metamorphosis, and mass at metamorphosis of American Toads (Anaxyrus americanus). Our results showed that none of our fitness variables responded to glyphosate itself in two concentrations commonly found in contaminated freshwater habitats nor to the soluble marker dye. In contrast, survival in surfactant treatments was, on average, 6.6% compared to 81% for the control treatment, but surfactant led to larger mass at metamorphosis of larvae that did survive. These results suggest that surfactant is the main problem in glyphosate application and that formulations without the surfactant may be the best choice for riparian management.