Common insecticide is more harmful to aquatic ecosystems than previously thought

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The insecticide fipronil and some of the compounds it degrades to are more toxic to aquatic insects, such as mayflies and stoneflies, than previously estimated, reports a new study from the USGS Regional Stream Quality Assessment.

The insecticide fipronil was detected in 22% of 444 small streams sampled across five major US regions; where fipronil was detected it was commonly at concentrations likely to be toxic to the most sensitive species.

USGS researchers investigated fipronil toxicity using “mesocosms” in which natural communities of bottom-dwelling organisms were relocated from a pristine stream to a laboratory setting and exposed for 30 days to water containing fipronil, one of its four degradates, or no insecticide at all. The goal was to determine at what concentrations each fipronil compound is toxic to the myriad organisms that live in streams, and to determine how fipronil compounds affect the way such aquatic communities function.

Fipronil is widely used to control ants, termites, fleas, and many other insects and for seed treatment of corn. Until now, information on aquatic life responses to the insecticide fipronil has been limited to a few species and to short exposures of a few days.  Concentrations of fipronil compounds determined to be toxic by the new mesocosm study are 3 to 2,600 times lower than those reported in the literature, depending on the fipronil compound, in part because of the scarcity of published data. The study also was able to determine a “benign” level of fipronil compounds, that is, the combined concentration unlikely to be toxic to 95% of species. Finally, the researchers found that higher concentrations of fipronil compounds decreased the diversity of species present, altered when juvenile aquatic insects changed into terrestrial flying adults, and disrupted the food web.

18% of the streams sampled nationally had fipronil compound concentrations, averaged over a 4-week sampling period, that exceeded the benign level determined by the mesocosm experiment. In the Southeast, where fipronil was detected more frequently than in the other four regions, 52% of streams sampled had fipronil compound concentrations that exceeded the benign level. Additionally, stream biodiversity in four of the five regions decreased with an increase in the concentration of fipronil compounds, indicating that fipronil compounds may be contributing to a decrease in stream biodiversity.

This study is the second in a series of three mesocosm studies that investigate the effects of commonly used insecticides—bifenthrin, fipronil, and imidacloprid—on aquatic communities. These three insecticides were chosen for the studies because they were those that occurred most frequently at potentially toxic levels and that showed associations with impaired aquatic communities studied in the Regional Stream Quality Assessment.

Citation:  J. L. Miller, T. S. Schmidt, P. C. Van Metre, B. J. Mahler, M. W. Sandstrom, L. H. Nowell, D. M. Carlisle, P. W. Moran, Common insecticide disrupts aquatic communities: A mesocosm to field ecological risk assessment of fipronil and its degradates in U.S. streams. Sci. Adv. 6, eabc1299 (2020). DOI: 10.1126/sciadv.abc1299

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