Neonicotinoid insecticide detections in Wisconsin's groundwater resources

Neonicotinoids are a popular and widely-used class of insecticides whose water-soluble nature and 20-year usage history has led to questions about their accumulation in groundwater resources.  Wisconsin  vegetable  growers  continue  to  rely  heavily  on  neonicotinoid  insecticides  for  the control of damaging populations of key insect pests through a combination of in-ground and foliar applications.  Thiamethoxam, one of the three most widely-used neonicotinoids, has been detected in Wisconsin monitoring wells since monitoring began in 2008 at concentrations nearing 9.0 µg/L and also in private potable wells where levels have not exceeded 1.61 µg/L.  In 2011, neonicotinoids were also detected in irrigation water delivered by high-capacity center-pivot irrigation systems.  The reported risks to human health at these levels is considered negligible, however there is concern among the scientific community that exposure to low doses of neonicotinoid insecticides causes harm to native and managed pollinators.  In cooperation with six commercial crop producers, we sampled over 70 active center-pivot irrigation wells in seven discrete clusters (3.5 km min. separation) over three years in the Wisconsin River drainage basin, and thiamethoxam concentrations were quantified using competitive ELISA kits.  Well detections ranged from < 0.05 µg/L (quantification limit) to 1.766 µg/L.  Significant within- and between-cluster variation was observed at large scale (50km clusters) and small scale (10km clusters).  Relationships between specific well detects and associated agricultural, spatial, and hydrological factors are explored.  In addition, detects at individual wells varied over time, with evidence of broad seasonal trends related to insecticide use throughout the growing season.