title: Mixture toxicity of imidacloprid and cyfluthrin to two non-target species, the fathead minnow Pimephales promelas and the amphipod Hyalella azteca.
abstract: Two species, the fathead minnow Pimephales promelas and the amphipod Hyalella azteca, were tested to examine acute toxicity to two insecticides, cyfluthrin and imidacloprid individually and as a mixture. Cyfluthrin was acutely toxic to P. promelas and H. azteca with EC50 values and 95 % confidence intervals of 0.31 µg L(-1) (0.26-0.35 µg L(-1)) and 0.0015 µg L(-1) (0.0011-0.0018 µg L(-1)), respectively. Imidacloprid was not acutely toxic to P. promelas at water concentrations ranging from 1 to 5000 µg L(-1), whereas it was toxic to H. azteca with a EC50 value of 33.5 µg L(-1) (23.3-47.4 µg L(-1)). For the P. promelas mixture test, imidacloprid was added at a single concentration to a geometric series of cyfluthrin concentrations bracketing the EC50 value. A synergistic ratio (SR) of 1.9 was found for P. promelas, which was calculated using the cyfluthrin-only exposure and mixture-exposure data. Because cyfluthrin and imidacloprid were toxic to H. azteca, the mixture test was designed based on an equipotent toxic unit method. Results from the mixture test indicated a model deviation ratio (MDR) of 1.7 or 2.7 depending on the model. Mixture test results from the simultaneous exposure to cyfluthrin and imidacloprid with both species indicated a greater than expected toxic response because the SR or MDR values were >1. Because these two insecticides are commonly used together in the same product formulations, nontarget species could be more affected due to their greater-than-additive toxicity observed in the current study.
authors.affiliation: Center for Fisheries, Aquaculture, and Aquatic Sciences and Department of Zoology, Southern Illinois University, 1125 Lincoln Dr. Mailcode 6511, Carbondale, IL, 62901, USA, lanteigne@siu.edu.
authors.forename: Michelle
authors.initials: M
authors.lastname: Lanteigne
authors.forename: Sara A
authors.initials: SA
authors.lastname: Whiting
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Lydy
mesh_terms: D033304:Amphipoda; D000818:Animals; D003530:Cyprinidae; D007093:Imidazoles / Q000633:toxicity*; D007306:Insecticides / Q000633:toxicity*; D007928:Lethal Dose 50; D000073943:Neonicotinoids; D009570:Nitriles / Q000633:toxicity*; D009574:Nitro Compounds / Q000633:toxicity*; D011722:Pyrethrins / Q000633:toxicity*; D023382:Toxicity Tests, Acute; D014874:Water Pollutants, Chemical / Q000633:toxicity*
chemical_list: D007093:Imidazoles; D007306:Insecticides; D000073943:Neonicotinoids; D009570:Nitriles; D009574:Nitro Compounds; D011722:Pyrethrins; D014874:Water Pollutants, Chemical; C082359:imidacloprid; C052570:cyfluthrin