Double trouble: The synergistic threat of environmental stressors and pesticide mixtures

Halting biodiversity loss demands a clear understanding of how multiple stressors interact across levels of biological organization. Here, we investigate these interactions at the individual level using Daphnia magna as a model species. We exposed D. magna to a single pesticide (esfenvalerate) and a field-relevant mixture of 13 pesticides - both characterized by equivalent toxic units (TU_Esf = TU_Sum-mix) - under two common climate-related stressors: food limitation and elevated temperature. The pesticide mixture behaved additively, conforming to Concentration Addition (CA); in contrast, environmental stressors produced synergistic effects accurately predicted by the Stress Addition Model (SAM). Food limitation amplified toxicity approximately four-fold for esfenvalerate and ten-fold for the mixture (model-deviation ratios, MDR = 4 and 10, respectively). Adding elevated temperature (+5 °C above optimum) further intensified these interactions, driving MDRs to 19 for esfenvalerate and 42 for the mixture, with latent interactions strengthening throughout the 21-day test. The 19-fold synergism was driven entirely by the two non-chemical stressors, whereas adding pesticide mixtures only doubled it. Consequently, LC₅₀ under environmental stress decreased from TU 3.48 (0.45 μg/L) to TU 0.18 (0.024 μg/L) for esfenvalerate, and from TU 2.83 to TU 0.068 for the pesticide mixture, while LC₁₀ dropped by more than two orders of magnitude. Food limitation and elevated temperature were the primary drivers of synergism. Although pesticide mixture slightly deviated from additivity, environmental stress greatly intensified toxicity. Therefore, ecological risk assessments should account for both chemical mixtures and co-occurring environmental stressors.