Comparative chronic toxicity of imidacloprid, clothianidin, and thiamethoxam to Chironomus dilutus and estimation of toxic equivalency factors

The authors compared the chronic toxicity of imidacloprid, clothianidin, and thiamethoxam throughout the life cycle of Chironomus dilutus. Lethal and sublethal endpoints were assessed at 14 and 40 days. They found 14 day LC50s of 1.52 µg/L for imidacloprid, 2.41 µg/L for clothianidin, and 23.60 µg/L for thiamethoxam. 40-day emergence inhibition EC50s were 0.39 µg/L for imidacloprid, 0.28 µg/L for clothianidin, and 4.13 µg/L for thiamethoxam. Changes in the sex ratios of emerged C. dilutus appeared at concentrations of 0.17µg/L imidacloprid, 0.46 µg/L clothianidin, and 3.60 µg/L thiamethoxam. While the EC50s for skewed sex ratio were not significantly different than for emergence, they were lower values which suggests that sex ratios are a more sensitive endpoint. Sex ratios are important because the gender balance in swarms can impact the fertility of the resulting egg masses and the population’s success over time. The authors noted developmental challenges during molting in treatments greater than 3.3 µg/L for all active ingredients that resulted in mortality. Based on their results, they calculated toxic equivalency factors for clothianidin and thiamethoxam relative to imidacloprid. Their toxic equivalency factors (TEF) for 14d LC50 were 1.05 (clothianidin) and 0.14 (thiamethoxam) and for 40d emergence EC50 were 1.62 (clothianidin) and 0.11 (thiamethoxam). While there is some uncertainty in the TEF calculations, they represent one of the first attempts to quantify the relative chronic toxicity of these neonicotinoids. Although thiamethoxam was an order of magnitude less toxic to C. dilutus than the other neonicotinoids, thiamethoxam degrades to clothianidin in field conditions, so its presence still poses a significant risk. Chronic studies on aquatic invertebrates are important because they observed a direct relationship between the exposure duration and the concentration that induced an adverse effect. Full life cycle tests are critical to ensure robust data about population-level risk.