The neonicotinoid imidacloprid shows high chronic toxicity to mayfly nymphs

This study aimed to fill the data gap on chronic non-standard freshwater macroinvertebrate imidacloprid toxicity. Test organisms were Ephemeroptera (Cloeon dipterum and Caenis horaria), Diptera (Chaoborus obscuripes), Hemiptera (Plea minutissima, Micronecta spp. and Notonecta spp.), Trichoptera (Limnephilidae), Megaloptera (Sialis lutaria), Isopoda (Asellus aquaticus), and Amphipoda (Gammarus pulex). Acute tests consisted of a 4 day exposure, after which immobility and mortality were evaluated. The chronic tests were refreshed with new test medium weekly, and emerged individuals were removed. In acute tests, mayflies (C. dipterum 96h immobilization EC50 1.02 µg/L) and caddisflies were the most sensitive species. During chronic tests, C. dipterum (28d immobilization EC50 0.123 µg/L) and C. horaria (28d immobilization EC50 0.126 µg/L) were most sensitive. The acute to chronic ratios were all greater than 10, and the large values for some arthropod species suggest that acute data are not accurate to estimate the chronic risks from imidacloprid exposure.