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.

An update of the Worldwide Integrated Assessment (WIA) on systemic insecticides. Part 2: impacts on organisms and ecosystems

“…Exposure of Chironomus riparius larvae to various mixtures of pyrethroid (deltamethrin and esfenvalerate) and neonicotinoid insecticides (imidacloprid and thiacloprid) at 50% of their known LC50s showed sometimes additive and other times antagonistic effects on survival (Kunce et al 2015). In the case of the amphipod Hyalella azteca, combined exposure to imidacloprid and cyfluthrin resulted in mortality ratios of 1.7 to 2.7 higher than either insecticide alone, indicating greater than additive toxicity (Lanteigne et al 2015).…”

Mixture toxicity of phostebupirim and cyfluthrin: Species‐specific responses

“…This higher toxicity noted for aquatic invertebrates compared with fish was expected, because insecticides are usually selected for their low toxicity to vertebrates. In addition, H. azteca is known to be more sensitive to pyrethroids than other invertebrate species . The different organisms selected in the present study greatly differ in physiology, and the lower toxicity of both insecticides to fish larvae may suggest either a more potent toxicity mechanism in aquatic invertebrates or better defense mechanisms in fish, especially for phostebupirim.…”

“…Few ecotoxicological data are available for cyfluthrin and phostebupirim , and to our knowledge no data exist for mixtures of the 2 compounds. Phostebupirim has been shown to pose an acute risk above the concern levels of the US Environmental Protection Agency (USEPA) to small birds, amphibians, reptiles, mammals, fish, and crustaceans .…”

Non-target toxicity of novel insecticides

“…Imidacloprid was also shown to be toxic to earthworm Eisenia fetida (Savigny) (108,109) and Hyalella azteca (Saussure) (110). Eisenia andrei (Bouché) showed bioaccumulation of various pesticides, which correlated with DNA damage (111).…”