Assessing the effects of imidacloprid and binary mixtures of the neonicotinoids imidacloprid in fathead minnow larvae (Pimephales promelas)
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Neonicotinoid insecticides were designed to be selective for insect nicotinic acetylcholine receptors (nAChRs), however recent studies indicate they may cause subtle toxicity in fish. Imidacloprid (IM) and thiamethoxam (TM) are neonicotinoids found together in Wisconsin surface waters, yet no studies have evaluated their combined toxicities to fish. The aim of this study was to better understand the potential for IM and IM:TM mixtures to activate the nAChR and to affect the development (hatching, length, survival) or behavior (embryonic motor activity, predator escape response) of fathead minnows (Pimephales promelas). Embryos and larvae were exposed to IM or to mixtures of IM:TM. Only survival was significantly affected. Chronic exposure to 0.2 μg IM/L and 1:1 0.02 and 200 μg IM/L:μg TM/L resulted in reduced survival. Survival improved in those exposed to mixtures (1:1 2, 20 μg IM/L:μg TM/L, 1:4 0.05 μg IM/L: 0.20 μg TM/L, and 1:5 0.05 μg IM/L: 0.25 μg TM/L); however, high mortality was observed in controls. The effect on survival, but not behavior implies that IM and TM may have low affinities for the vertebrate nAChR and interact with a different system in the fish, which was supported by molecular modeling.