title: Comparative chronic toxicity of imidacloprid, clothianidin, and thiamethoxam to Chironomus dilutus and estimation of toxic equivalency factors.
abstract: Nontarget aquatic insects are susceptible to chronic neonicotinoid insecticide exposure during the early stages of development from repeated runoff events and prolonged persistence of these chemicals. Investigations on the chronic toxicity of neonicotinoids to aquatic invertebrates have been limited to a few species and under different laboratory conditions that often preclude direct comparisons of the relative toxicity of different compounds. In the present study, full life-cycle toxicity tests using Chironomus dilutus were performed to compare the toxicity of 3 commonly used neonicotinoids: imidacloprid, clothianidin, and thiamethoxam. Test conditions followed a static-renewal exposure protocol in which lethal and sublethal endpoints were assessed on days 14 and 40. Reduced emergence success, advanced emergence timing, and male-biased sex ratios were sensitive responses to low-level neonicotinoid exposure. The 14-d median lethal concentrations for imidacloprid, clothianidin, and thiamethoxam were 1.52 μg/L, 2.41 μg/L, and 23.60 μg/L, respectively. The 40-d median effect concentrations (emergence) for imidacloprid, clothianidin, and thiamethoxam were 0.39 μg/L, 0.28 μg/L, and 4.13 μg/L, respectively. Toxic equivalence relative to imidacloprid was estimated through a 3-point response average of equivalencies calculated at 20%, 50%, and 90% lethal and effect concentrations. Relative to imidacloprid (toxic equivalency factor [TEF] = 1.0), chronic (lethality) 14-d TEFs for clothianidin and thiamethoxam were 1.05 and 0.14, respectively, and chronic (emergence inhibition) 40-d TEFs were 1.62 and 0.11, respectively. These population-relevant endpoints and TEFs suggest that imidacloprid and clothianidin exert comparable chronic toxicity to C. dilutus, whereas thiamethoxam induced comparable effects only at concentrations an order of magnitude higher. However, the authors caution that under field conditions, thiamethoxam readily degrades to clothianidin, thereby likely enhancing toxicity. Environ Toxicol Chem 2017;36:372-382. © 2016 SETAC.
authors.affiliation: School of Environment and Sustainability, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.
authors.forename: Michael C
authors.initials: MC
authors.lastname: Cavallaro
authors.affiliation: School of Environment and Sustainability, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.|Department of Biology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.
authors.forename: Christy A
authors.initials: CA
authors.lastname: Morrissey
authors.affiliation: Watershed Hydrology and Ecology Research Division, Water Science and Technology, Environment and Climate Change Canada, Saskatoon, Saskatchewan, Canada.
authors.forename: John V
authors.initials: JV
authors.lastname: Headley
authors.affiliation: Watershed Hydrology and Ecology Research Division, Water Science and Technology, Environment and Climate Change Canada, Saskatoon, Saskatchewan, Canada.
authors.forename: Kerry M
authors.initials: KM
authors.lastname: Peru
authors.affiliation: School of Environment and Sustainability, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.|Toxicology Centre, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.|Institute of Loess Plateau, Shanxi University, Taiyan, Shanxi, People's Republic of China.
authors.forename: Karsten
authors.initials: K
authors.lastname: Liber
mesh_terms: D000818:Animals; D002683:Chironomidae / Q000187:drug effects* / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D006146:Guanidines / Q000633:toxicity*; D007093:Imidazoles / Q000633:toxicity*; D007306:Insecticides / Q000633:toxicity*; D007448:Invertebrates / Q000378:metabolism; D000073943:Neonicotinoids; D009574:Nitro Compounds / Q000633:toxicity*; D010078:Oxazines / Q000633:toxicity*; D000077922:Thiamethoxam; D013844:Thiazoles / Q000633:toxicity*; D013997:Time Factors; D018675:Toxicity Tests; D014874:Water Pollutants, Chemical / Q000633:toxicity*
chemical_list: D006146:Guanidines; D007093:Imidazoles; D007306:Insecticides; D000073943:Neonicotinoids; D009574:Nitro Compounds; D010078:Oxazines; D013844:Thiazoles; D014874:Water Pollutants, Chemical; C480342:clothianidin; C082359:imidacloprid; D000077922:Thiamethoxam
keywords: Chronic toxicity; Macroinvertebrate; Neonicotinoid insecticides; Static-renewal test; Toxic equivalency factor