Gollot, L.; Tebby, C.; Frattaroli, L.; Beaudouin, R.; Royaute, R.; Fabure, J.

Soils are vital reservoirs of biodiversity and providers of ecosystem services, yet they are increasingly threatened by agricultural intensification and pesticide use. Residues often persist as complex mixtures, while environmental risk assessment still largely focuses on single substances, potentially underestimating mixture effects. Earthworms play a key role in soil functioning and are particularly vulnerable to pesticide contamination. We investigated the effects of a binary mixture of epoxiconazole and imidacloprid, two persistent and frequently detected pesticides, on life-history traits of Aporrectodea caliginosa. We estimated each compound relative potency using dose-response experiments on juvenile growth and cocoon production. Next, we assessed the potential for synergy or antagonism in a fixed-ratio ray design including five concentration ratios and seven additive isoboles (36 conditions). Both compounds showed significant toxicity. Imidacloprid showed high potency (juvenile growth NOEC = 0.28 mg/kg; reproduction EC50 = 0.55 mg/kg), whereas epoxiconazole had moderate effects (juvenile growth NOEC = 9.3 mg/kg; reproduction EC50 = 126.8 mg/kg). Reproductive endpoints were more sensitive than adult growth, with juvenile growth being the most sensitive overall. Mixture analysis using Jonker's models revealed significant deviation from Independent Action only under the simple interaction model, indicating synergism, consistent with cytochrome P450 interference reported in other taxa. Field-reported imidacloprid concentrations often approach effect thresholds, suggesting potential risks for earthworm populations. Overall, the combined effects of epoxiconazole and imidacloprid may exceed predictions not taking interactions into account. These results highlight the need to incorporate pesticide mixture effects into environmental risk assessment.