Acute Effects of Binary Mixtures of Imidacloprid and Tebuconazole on 4 Freshwater Invertebrates

Receiving waters from agricultural areas can contain multiple pesticides such as the neonicotinoid imidacloprid and the fungicide tebuconazole, leading to the potential for aquatic life to be exposed to such mixtures. In the present study, the effects of tebuconazole were tested alone and in binary mixtures with imidacloprid on 4 aquatic invertebrates: Chironomus dilutus, Hyalella azteca, Lumbriculus variegatus, and Neocloeon triangulifer. Acute (96-h) median lethal concentrations (LC50s) were derived for individual compounds and used to design a binary mixture study to determine cumulative effects. The LC50s showed that imidacloprid was more potent than tebuconazole by 1 to 3 orders of magnitude for the 4 species. Lethality data from mixture experiments were analyzed using MIXTOX to determine deviations from independent action, followed by the model deviation ratio (MDR) technique to determine the biological significance and reproducibility of observed mixture effects. MIXTOX showed that the cumulative toxicities of imidacloprid-tebuconazole differed between the species: for C. dilutus there was no deviation from independent action; however, for H. azteca the mixture was antagonistic (specifically dose ratio-dependent), and for N. triangulifer it was synergistic. The MDR method showed that only observations with H. azteca significantly deviated from independent action. Because of the lack of evidence of a clear deviation from independent action and the much greater potency of imidacloprid, the weight of evidence indicates that the presence of tebuconazole is unlikely to appreciably increase the hazard from imidacloprid exposure to aquatic invertebrates. Environ Toxicol Chem 2019;00:1-17. © 2019 SETAC.