Environmental Stress Increases Synergistic Effects of Pesticide Mixtures on Daphnia magna

Some widely used pesticide mixtures produce more than additive effects according to conventional combined effect models. However, synergistic effects have been so far generally observed at unrealistically high pesticide concentrations. Here, we used Daphnia magna as a test organism and investigated how food limitation-a common ecological stressor-affects the mixture toxicity of a pyrethroid insecticide and azole fungicide. We also compared three models regarding the prediction of mixture effects, including concentration addition (CA), effect addition (EA), and stress addition model (SAM). We revealed that especially under low food, the strength of synergism between esfenvalerate and prochloraz increased with an increasing concentration of prochloraz independent of the null model. Under high food conditions and at concentrations of prochloraz >= 32 mu g/L, we observed a marginal synergistic effect with a model deviation ratio (MDR) = 2.1 at 32 mu g/L prochloraz and 2.2 at 100 mu g/L prochloraz when using CA as the null model. In contrast, the combination of both pesticides and food stress caused synergistic effects shown by an MDR = 10.9 even at 1 mu g/L of prochloraz that is frequently detected in the environment. The combined effects of pesticides and food stress could be predicted best with the SAM that showed the lowest mean deviation between effect observation and prediction (mean deviation SAM = 16 [SD = 28], EA = 1072 [2105], CA = 1345 [2644]). We conclude that common environmental stressors can strongly increase the synergistic effects of toxicants. This knowledge is especially relevant considering current efforts to include the additional risk of pesticide mixtures and environmental stressors into the environmental risk assessment of pesticides.