Acute and chronic toxicity of imidacloprid to the aquatic invertebrates Chironomous tentans and Hyalella azteca under constant- and pulse-exposure conditions

This experiment quantified the 96 hour toxicity of imidacloprid to Chironomous tentans and Hyalella azteca using both technical imidacloprid and the formulated product Admire (imidacloprid). The authors also examined the 28 day chronic toxicity of Admire (imidacloprid) under constant and pulse exposures. The two species were chosen because they are commonly used for toxicity testing and reflect differences in physiology and life history that make them more predictive of risk to aquatic systems. C. tentans were exposed to nominal acute concentrations of 1, 5, 29, 145, and 725 µg/L of imidacloprid both as pure active ingredient and formulated Admire. H. azteca were exposed to acute concentrations of 2, 11, 55, 275, and 1375 µg/L imidacloprid, also pure and formulated. The 96 hour LC50 for C. tentans was 5.75 µg/L for technical imidacloprid and 5.40 µg/L for Admire. For H. azteca though, the LC50 for technical imidacloprid was 65.43 µg/L and 17.44 µg/L for Admire, suggesting that they are more sensitive to a compound in the formulated product, or that other ingredients are enhancing the uptake of imidacloprid. For the 28 day chronic experiments, organisms were either exposed to Admire continuously, or to the compound for 96 hours before being transferred to clean beakers for the remaining 24 days to simulate a pulse from a runoff event. C. tentans were exposed to Admire with nominal concentrations of 0.1, 0.3, and 3 µg/L imidacloprid for 28 days, or to 0.3, 1, and 3 µg/L for the 96 hour pulse. H. azteca were exposed to Admire with nominal concentrations of 0.3, 1, 3, and 10 µg/L imidacloprid for 28 days, or to 1, 3, and 10 µg/L for the 96 hour pulse. The 28 day constant exposure LC50 for C. tentans was 0.91 µg/L and for H. azteca was 7.08 µg/L. They found that C. tentans was overall more sensitive to acute and chronic imidacloprid exposure, but was less sensitive to a single pulse than H. azteca. The authors compared their toxicity data to other published results, which showed that C. tentans and H. azteca may be among the most sensitive aquatic invertebrates to imidacloprid (with Epeorus spp. mayflies and Simulium vittatum black fly larvae more or equally sensitive). Their results show that imidacloprid toxicity differs based on exposure conditions, so toxicity testing should be carried out in a range of exposure regimes to better approximate environmental conditions.