Amphibian populations are declining globally at an alarming rate. Pesticides are among a number of proposed causes for these declines. Though a sizable data-base examining effects of pesticides on amphibians exists, the vast majority of these studies focus on toxicological effects (lethality, external malformations, etc.) at relatively high doses (ppm). Very few studies focus on effects such as endocrine disruption at low concentrations. Further, the majority of studies examine exposures to single chemicals only. The current study examined nine pesticides (four herbicides, two fungicides, and three insecticides) used on cornfields in the mid-western US. Effects of each pesticide alone (0.1 ppb) or in combination were examined. In addition, we examined atrazine and S-metolachlor (0.1 or 10 ppb each) or the commercial formulation, Bicep II Magnum, which contains both of these herbicides. These two pesticides were examined in combination because they are persistent throughout the year in the wild. We examined larval growth and development, sex differentiation, and immune function in leopard frogs (Rana pipiens). In a follow-up study, we also examined the effects of the nine-compound mixture on plasma corticosterone levels in male African clawed frogs (Xenopus laevis).
Though some of the pesticides inhibited larval growth and development, the pesticide mixtures had much greater effects. Larval growth and development were retarded, but most significantly, pesticide mixtures negated or reversed the typically positive correlation between time to metamorphosis and size at metamorphosis observed in controls: Exposed larvae that took longer to metamorphose were smaller than their counterparts that metamorphosed earlier. The nine-pesticide mixture also induced damage to the thymus, resulting in immunosuppression and contraction of flavo-bacterial meningitis. The study in X. laevis revealed that these adverse effects may be due to an increase in plasma levels of the stress hormone, corticosterone. Though it cannot be determined whether all of the pesticides in the mixture contribute to these adverse effects or whether some pesticides are “effectors”, some are “enhancers”, and some are “neutral”, the current study revealed that estimating ecological risk and the impact of pesticides on amphibians using studies that examine single pesticides at high concentrations, only, may lead to gross underestimations of the role of pesticides in amphibian declines.