Widespread agricultural use of neonicotinoid insecticides has resulted in frequent detection of mixtures of these compounds in global surface waters. Recent evidence suggests that neonicotinoid mixtures can elicit synergistic toxicity in aquatic insects under acute exposure conditions, however this has not been validated for longer exposures more commonly encountered in the environment. Therefore, we aimed to characterize the chronic (28-day) toxicity of imidacloprid, clothianidin, and thiamethoxam mixtures under different doses and mixture ratios to determine if the assumption of synergistic toxicity would hold under more environmentally realistic exposure settings. The sensitive aquatic insect Chironomus dilutus was used as a representative test species, and successful emergence was used as a chronic endpoint. Applying the MIXTOX modeling approach, predictive parametric models were fitted using single-compound toxicity data and statistically compared to observed toxicity in subsequent mixture tests. Imidacloprid-clothianidin, clothianidin-thiamethoxam and imidacloprid-clothianidin-thiamethoxam mixtures did not significantly deviate from concentration-additive toxicity. However, the cumulative toxicity of the imidacloprid-thiamethoxam mixture deviated from the concentration-additive reference model, displaying dose-ratio dependent synergism and resulting in up to a 10% greater reduction in emergence from that predicted by concentration addition. Furthermore, exposure to select neonicotinoid mixtures above 1.0 toxic unit tended to shift sex-ratios toward more male-dominated populations. Results indicate that, similar to acute exposures, the general assumption of joint additivity cannot adequately describe chronic cumulative toxicity of all neonicotinoid mixtures. Indeed, our observations of weak synergism and sex-ratio shifts elicited by some mixture combinations should be considered in water quality guideline development and environmental risk assessment practices for neonicotinoid insecticides, and explored in further investigations of the effects of neonicotinoid mixtures on aquatic communities.

农业上对新烟碱类杀虫剂的广泛使用已导致在全球地表水中频繁检测到这些化合物的混合物。最近的证据表明，新烟碱类混合物可在急性暴露条件下在水生昆虫中引起协同毒性，但是尚未被环境中更长时间的暴露所验证。因此，我们旨在表征吡虫啉，噻虫胺和噻虫嗪混合物在不同剂量和混合比例下的慢性（28天）毒性，以确定在更现实的环境暴露条件下协同毒性的假设是否成立。敏感的水生昆虫Chironomus dilutus曾被用作代表性的测试物种，而成功的出苗被用作了慢性终点。应用MIXTOX建模方法，使用单化合物毒性数据拟合预测性参数模型，并在随后的混合物测试中与观察到的毒性进行统计比较。吡虫啉-噻虫胺，噻虫胺-噻虫嗪和吡虫啉-噻虫胺-噻虫嗪的混合物没有明显偏离浓度加和毒性。但是，吡虫啉-噻虫嗪混合物的累积毒性偏离了浓度加成参考模型，显示出剂量比依赖性的协同作用，导致浓度增加所预测的出芽率降低多达10％。此外，暴露于选择高于1的新烟碱类混合物。0毒性单位倾向于使性别比例向更多男性主导的人群转移。结果表明，与急性暴露相似，联合添加的一般假设不能充分描述所有新烟碱混合物的慢性累积毒性。确实，我们在一些新烟碱类杀虫剂的水质准则制定和环境风险评估实践中应考虑我们对某些混合物组合引起的弱协同作用和性别比变化的观察，并应在进一步研究新烟碱类混合物对水生社区的影响时加以探索。