Short-term detection of imidacloprid in streams after applications in forests

This paper explored the leaching of soil-applied imidacloprid in eastern US forests into streams. Imidacloprid is commonly applied in eastern forests to manage the hemlock woolly adelgid (Adelges tsugae), a pest of eastern hemlock trees (Tsuga canadensis). Streams flowing through hemlock forests tend to contain high aquatic invertebrate species richness. Other studies have found imidacloprid in streams adjacent to treated hemlock stands in the years after treatment, but this study sought to understand the initial leaching and concentrations of imidacloprid within a year after treatment. 

The authors measured the presence and concentration of imidacloprid in three streams adjacent to treated hemlock, along with two control streams. One control stream was removed from analysis because imidacloprid had been applied to the headwaters area 1-3 years before sampling, and imidacloprid was still being detected (0.028 - 0.068 µg/L). Samples from the treatment and control streams were collected monthly for a year 10-50m downstream from the imidacloprid-treated areas, as well as during one rain event. Imidacloprid was detected in all three streams at concentrations ranging from 0.053 to 0.833 µg/L during a single rain event 184-196 days after treatment. These samples all exceeded the US EPA chronic benchmark for freshwater invertebrates, and one stream exceeded the acute benchmark. Imidacloprid was not detected in any other samples. 

The stream that had been treated prior to the study showed relatively consistent detections despite changes in precipitation, while the streams treated during the study showed peaks in detections associated with precipitation. This suggests that peaks in concentrations may be more likely in the first year after treatment. While the benchmarks were exceeded in this study, other studies that examined macroinvertebrate communities in forested areas treated with imidacloprid did not identify negative effects despite identifying similar levels of contamination. This suggests that more research may be needed, especially focusing on water quality in various soil types, for longer durations after treatment, and different stream setback distances.

Authors: 
Wiggins, G., E. Benton, J. Grant, M. Kerr, and P. Lambdin
Journal: 
Journal of Environmental Quality
Year published: 
2018