Seven neonicotinoid insecticides are used in the United States. These neonicotinoids are frequently detected as mixtures in streams and can be toxic to aquatic invertebrates. However, little is known about how runoff that contains more than one insecticide affects life in streams. To better understand the impacts of insecticide mixtures on freshwater ecosystems, this study used a 30-day laboratory test in which invertebrates, algae, and other microbes that live on the bottom of a river were relocated to an artificial stream in a laboratory and exposed to a range of concentrations of 2 neonicotinoids (imidacloprid and clothianidin). The goal was to generate species-specific effects for each neonicotinoid and a mixture of the two on a broad range of organisms representative of actual stream communities. Results from the experiments were then compared to a field dataset to test if effects observed in the laboratory were consistent with those observed in Coastal California streams.  

Aquatic insects were sensitive to the neonicotinoids imidacloprid and clothianidin in the artificial stream laboratory experiments. The results indicate that the stream communities tested were either marginally protected or under-protected by EPA regulatory levels meant to protect freshwater biodiversity. Importantly, mixtures of imidacloprid and clothianidin caused effects greater than was predicted by summing the effects of the individual compounds.  

As part of the U.S. Geological Survey Regional Stream Quality Assessments, 1 of 7 neonicotinoids were measured in 72% of the 85 Coastal California streams sampled and 56% of streams sampled were observed to have 2 or more neonicotinoids. Average neonicotinoid concentrations in Coastal California streams were more than 7 times greater than EPA guidelines set to protect ecosystem health and were correlated with poorer condition of the observed ecological communities.  

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