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

Abstract

Thiamethoxam 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 nontarget insects, specifically those with an aquatic life stage. To address the uncertainty associated with possible impacts from environmental exposures, a chronic (35-d) outdoor mesocosm study with a formulated product containing thiamethoxam 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, and 10.0 mg/L thiamethoxam), plus untreated controls were tested, and the abundance and emergence of mayflies (Cloeon dipterum) were assessed weekly for 35 d. Mean measured time-weighted average exposures were within 6% of nominal over the duration of the study, with the mean half-life of thiamethoxam in each treatment ranging from 7 to 13 d. Statistically significant reductions in both larval abundance and adult emergence were observed at 10.0, 3.0, and 1.0 mg/L following 1, 2, and 3 wk of exposure, respectively. Exposure to 0.1 and 0.3 mg/L thiamethoxam had no statistically significant effect on larval mayfly abundance or adult emergence at any point in the study. These findings support a 35-d no-observed-effect concentration (NOEC) of 0.3 mg thiamethoxam/L for mayflies (C. dipterum) under chronic conditions. Furthermore, because the 95th percentile of environmental concentrations has been reported to be 0.054 mg/L, these results indicate that populations of C. dipterum and similarly sensitive aquatic insects are unlikely to be significantly impacted by thiamethoxam exposure in natural systems represented by the conditions in our study.

Chronic toxicity of 6 neonicotinoid insecticides to Chironomus dilutus and Neocloeon triangulifer

“…Laboratorybased studies with C. dipterum reported 28-d EC50s for immobilization of 0.13, 0.195, and 0.68 mg L À1 for imidacloprid, 0.29 mg L À1 for thiacloprid, and 0.68 mg L À1 for thiamethoxam (Roessink et al 2013;Van den Brink et al 2016). A recent fieldbased study by Pickford et al (2018) that also studied C. dipterum reported a 35-d LOEC for larval abundance of 1.0 mg L À1 thiamethoxam. These literature values were lower than the most sensitive endpoint in the present study (emergence), but were within 1 order of magnitude.…”

Neonicotinoid insecticide residues in subsurface drainage and open ditch water around maize fields in southwestern Ontario

“…Mayflies are more sensitive to neonicotinoid insecticides than most other species [27, 28]. They were shown to have a chronic no-observed-effect-concentration (NOEC) of 0.3 ng/mL for thiamethoxam [27]. Assuming the toxicities of clothianidin and thiamethoxam to mayflies are broadly similar, these data support a 1.6 and 100-fold margin of safety to mayflies in most streams if fugitive dust on pneumatic planters were properly mitigated.…”

“…Mayflies are more sensitive to neonicotinoid insecticides than most other species [27, 28]. They were shown to have a chronic no-observed-effect-concentration (NOEC) of 0.3 ng/mL for thiamethoxam [27].…”

“…Some suggest neonicotinoid residues in surface water may affect aquatic invertebrates and ecosystem health [17, 23–26]. Of these invertebrates, mayflies (Ephemeroptera)[27, 28] and midges (Diptera: Chironomidae) [29] are among the most sensitive species and could be used as sensitivity benchmarks to neonicotinoid-contaminated surface waters. To understand exposure of these sensitive species, it is important to determine the pathways by which neonicotinoid residues reach the surface waters in question.…”

Uptake and Transfer of Polyamide Microplastics in a Freshwater Mesocosm Study

“…Each replicate system consisted of a 10 cm clay and sediment layer and approximately 1 m 3 water (Figure 1). Macrophyte flora (e.g., Chara globularis, Myriophyllum spicatum) in all mesocosms was harmonized thereafter [32].…”