Can’t take the heat: Temperature-enhanced toxicity in the mayfly Isonychia bicolor exposed to the neonicotinoid insecticide imidacloprid

This study explored the role of temperature in imidacloprid toxicity for the mayfly Isonychia bicolor. Temperature-enhanced toxicity has been observed across many aquatic organisms, but the physical basis for it is poorly understood. The authors examined the degree to which increasing temperature accelerated the onset of toxicity and how temperature affected uptake rates and respiration. Nominal imidacloprid concentrations were 1, 2, 4, 8, 10, 20, 40, 80, and 100 µg/L. Toxicological endpoints were immobility and mortality, which were assessed throughout the 96h exposure. The 96h EC50 (cumulative immobility and mortality) was 5.88 µg/L. The time to effect for impairment and immobility were evaluated across four temperatures, 15, 18, 21, and 24oC and monitored every 6h. Increasing temperatures decreased the amount of time before effects were seen. At 15oC with an exposure of 5.75 µg/L, impairment was evident at 60h and immobility at 76h, while at 24oC impairment occurred at 6h and immobility at 26h. Larvae tissue was sampled throughout the exposures, showing that there was a significant positive relationship between tissue concentration and temperature. The positive relationship between temperature and toxicity could be due to increased respiration rates at higher temperatures or differences in imidacloprid uptake rate. The authors noted that impairment, which can have dire consequences for aquatic invertebrates seeking to avoid predators, occurred at a much lower concentration and sooner than immobility or lethality. They recommend that more sensitive endpoints should be evaluated to determine toxicity. The temperature-dependent response times suggest that additional abiotic factors need to be considered in toxicity testing, especially in light of climate change. Given the range of sensitivities to imidacloprid, the authors suggest that common surrogate species like Daphnia magna may not appropriately reflect toxicity hazards.