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Response of the Mayfly (Cloeon Dipterum) to chronic exposure to Thiamethoxam in outdoor Mesocosms

: Pickford, D.B.; Finnegan, M.C.; Baxter, L.R.; Böhmer, W.; Hanson, M.L.; Stegger, P.; Hommen, U.; Hoekstra, P.F.; Hamer, M.

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Environmental toxicology and chemistry (2017), Online First, 40 pp.
ISSN: 0730-7268
Journal Article, Electronic Publication
Fraunhofer IME ()
mesocosm; insecticide; Neonicotinoids; risk assessment; Mayfly

Thiamethoxam (TMX) is a widely used neonicotinoid insecticide that has been detected in surface water monitoring programs in North America and Europe. This has led to questions about its toxicity to non-target insects, specifically those with an aquatic life stage. In order to address the uncertainty associated with possible impacts related to environmental exposures, a chronic (35-d) outdoor mesocosm study with a formulated product containing TMX was conducted. The specific focus of the study was the response of mayflies (Ephemeroptera), which have been reported to be particularly sensitive in laboratory studies. A range of concentrations (nominally 0.1, 0.3, 1.0, 3.0, 10.0 µg/L TMX), plus an untreated control were tested and the abundance and emergence of mayflies (Cloeon dipterum) was assessed on a weekly basis for 35 days. Mean measured time-weighted average exposures were within 6% of nominal over the duration of the study with the mean half-life of TMX in each treatment ranging from 7 to 13 days. Statistically significant reductions in both larval abundance and adult emergence were observed at 10.0, 3.0, and 1.0 ug/L following 1, 2, and 3 weeks of exposure, respectively. Exposure to 0.1 and 0.3 µg/L TMX had no statistically significant effect on larval mayfly abundance or adult emergence at any point in the study. These findings support a 35-d NOEC of 0 .3 µg TMX/L f or m ayflies (C. dipterum) under chronic conditions. Further, since the 95th centile of environmental concentrations has been reported at 0.054 µg/L, these results indicate that populations of C. dipterum and similarly sensitive aquatic insects are unlikely to be significantly impacted by TMX exposure in natural systems.