Relationships between biotic indices and neonicotinoid insecticide detections in Wisconsin’s groundwater-fed streams

The use of neonicotinoid insecticides has attracted recent controversy due to their sub-lethal effects on bees and their implication in global pollinator decline.  Neonicotinoid groundwater contamination and declining freshwater fish populations have also attracted the attention of investigators.  Thiamethoxam, one of the most widely-used neonicotinoids, has been detected in Wisconsin groundwater monitoring wells since 2008 at concentrations up to 9.0 µg/L, in private potable wells at levels approaching 1.61 µg/L, and in center-pivot irrigation systems at levels of 1.76 µg/L.  Sub-lethal effects of neonicotinoids on beneficial insects have been noted at exposures between 1 and 10 µg/L.  Wisconsin’s Central Sands Eco-region, an intensively-farmed area of the state where a large fraction of neonicotinoid inputs occur, is bisected by a number of groundwater-fed Class I and Class II trout streams where increasing pollutant levels and subsequent declines in invertebrate populations may threaten ecosystem health and recreational value.  The Wisconsin Department of Natural Resources (DNR) has been monitoring the health of these streams for many years by computing Hilsenhoff Biotic Indexes (a measure of the diversity and resilience of aquatic invertebrates).  In order to compare neonicotinoid insecticide contamination levels in these streams with the DNR’s stream health metrics, we identified 36 sample sites along four trout streams in the Central Sands (two flowing west through intensive agriculture to the Wisconsin River, and two flowing east to the Fox River through less intensive agriculture).  Water samples were collected in July, September, and October (at 45 day intervals) and thiamethoxam concentrations determined using ELISA.