Multiple Stressor Effects of a Neonicotinoid, Heatwaves, and Elevated Temperatures on Aquatic Insect Emergence

Intensive agricultural practices, including neonicotinoid insecticides, and climate change are two potential drivers of global insect decline, contributing to biodiversity loss. However, ecologically realistic field experiments investigating these multiple stressor effects on emerging aquatic insects are scarce. To empirically test whether exposure to imidacloprid (1, 10 μg/L) and two different climate change scenarios (i) elevated temperatures (+4 °C vs. ambient temperatures) and (ii) reoccurring heatwaves (+0 to 8 °C) may cause a decline in insect emergence, we conducted an outdoor mesocosm study. Aquatic insect communities were exposed to single and combined stressors, while emergence was monitored during a 3-month period. We report significant losses in insect biomass and abundance under single and combined treatments. The high imidacloprid treatment and elevated temperatures combined caused a significant 47% decline in total insect biomass across the insect orders Diptera, Ephemeroptera, Coleoptera, Hymenoptera, Hemiptera, Odonata, and Trichoptera. Community structure and population dynamics were significantly affected, with Diptera and Ephemeroptera being most sensitive to the high and both imidacloprid treatments, respectively. Diptera dominated but was significantly reduced by the high imidacloprid and heatwave combination. Temperature-enhanced imidacloprid toxicity and the significant threat these stressors pose to aquatic insect communities highlight the need for effective climate change mitigation strategies to conserve aquatic insect biodiversity.