Response of the mayfly (Cloeon dipterum) to chronic exposure to thiamethoxam in outdoor mesocosms

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 for mayflies (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. This article is protected by copyright. All rights reserved