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 four aquatic invertebrates: Chironomus dilutus, Hyalella azteca, Lumbriculus variegatus, and Neocloeon triangulifer. Acute (96-h) LC50s were derived for individual compounds and used to design a binary mixture study to determine cumulative effects. LC50s showed imidacloprid was more potent than tebuconazole by 1-3 orders of magnitude for four species. Lethality data from mixture experiments were analyzed using MIXTOX to determine deviations from independent action (IA), followed by the model deviation ratio (MDR) technique to determine biological significance and reproducibility of observed mixture effects. MIXTOX showed cumulative toxicities of IMI-TBZ differed between the species; for C. dilutus there was no deviation from IA; however, for H. azteca the mixture was antagonistic (specifically dose-ratio dependent), and for N. triangulifer was synergistic. The MDR method showed that only observations with H. azteca significantly deviated from IA. Due to the lack of evidence of clear deviation from IA and the much greater potency of imidacloprid, the weight of evidence indicates the presence of tebuconazole is unlikely to appreciably increase the hazard from imidacloprid exposure to aquatic invertebrates. This article is protected by copyright. All rights reserved.