Oviposition site selection: pesticide avoidance by gray treefrogs

Abstract

Effects of glyphosate-formulated herbicides on nontarget organisms have received much recent attention. Although previous studies have explored the effects of pesticides on growth, development, and mortality of various amphibian species, no studies have tested the potential effects of herbicides on oviposition site selection by amphibians. Recent studies have found that a combination of pesticide and predatory cues lead to significantly increased mortality of tadpoles of several anuran species relative to that caused by pesticide alone. In the present study, I tested two hypotheses: First, adult gray treefrogs avoid oviposition sites based on the presence of glyphosate formulation (Roundup). Second, pesticide avoidance is manifested to a greater degree when combined with predatory cues. In the spring of 2006, I conducted an outdoor experiment using artificial ponds by setting up four treatments: Predatory fish cue, Roundup (2.4 mg glyphosate acid equivalent [a.e.]/L), a combination of predatory fish cue + Roundup, and a control. This experiment was designed to assess oviposition site choice among the four treatments by gray treefrogs based on the number of eggs laid in each treatment. Gray treefrogs avoided oviposition in pools contaminated with fish cue and/ or Roundup and placed the significant majority of their eggs in control pools, which suggests that breeding adults may be able to prevent lethal exposure of herbicide to their offspring through oviposition site selection. The present study provided the first evidence that the concentration of herbicide that is expected to be found in the field potentially alters oviposition site choice by amphibians. However, the concentration of 2.4 mg a.e./L is unlikely ubiquitous in nature. Thus, the further investigation of environmental relevancy of this finding is critical.