The widespread use of pesticides around the world raises the possibility that non-target organisms might also be affected.  Although short-term laboratory experiments are helpful in understanding the toxic effects of pesticides, they lack the incorporation of realistic factors found in nature.  For the world’s leading herbicide, glyphosate, we know little about how the natural stress of competition, along with application time, amount, and frequency, affect the toxicity to non-target organisms.  To explore these interactions, we used a series of two outdoor mesocosm experiments to, 1) assess the role of glyphosate application over ontogeny, and 2) examine the potential synergistic effects of competition and glyphosate on communities containing larval amphibians.   

Results/Conclusions

In the first experiment, we exposed two species of larval amphibians (Rana sylvatica and Bufo americanus) to different glyphosate (as Roundup Original MAX®) applications over ontogeny.  Consistent with past studies, we found that exposures of up to 3 mg acid equivalent (a.e.)/L caused substantial amphibian death. However, the amount of death was considerably higher when the herbicide was applied earlier in the experiment than later in the experiment. Also, single, large applications (at different times) had larger effects on tadpole mortality and growth than multiple, small applications (of the same total amount). 

In the second experiment, we exposed three species of larval amphibians (R. clamitans, R. catesbeiana, and Hyla versicolor) to a factorial combination of three glyphosate concentrations (0, 1, 2, or 3 mg a.e/L of the commercial formulation Roundup Original MAX®) and three tadpole densities (low, medium, or high). We found that increased tadpole density caused declines in tadpole growth and made the herbicide significantly more lethal to one species. Whereas the LC50 values were similar across all densities for green frogs (R. clamitans; 2.0 to 2.5 mg a.e./L) and gray treefrogs (H. versicolor; 1.7 to 2.3 mg a.e./L), the LC50 values for bullfrogs (R. catesbeiana) were 2.0 to 2.1 mg a.e./L at low and medium densities, but declined to 1.6 mg a.e./L at high densities.  In addition, we found a significant effect of temperature on glyphosate stratification in both experiments.  These experiments demonstrate the importance of examining application time and multiple stressors on natural communities.  They will also add to the limited research on glyphosate and environmental stressors.