Intensive agricultural practices including the use of neonicotinoid insecticides and climate change are two potential drivers of the globally observed insect decline. As climate change progresses and average ambient temperatures increase, more intense and frequent temperature extremes, such as heatwaves, are also expected in the future. However, ecologically realistic field experiments that investigate these multiple stressor effects on emerging aquatic insects are scarce. To empirically test whether exposure to the neonicotinoid insecticide imidacloprid (1, 10 µg/L) and two different temperature-related climate change scenarios (elevated temperatures and reoccurring heatwaves) may cause a decline in insect emergence, we conducted an outdoor mesocosm study by means of experimental ponds. Aquatic insect communities were exposed in mesocosms to both single and combined chemical and thermal stressors while insect emergence was monitored during a 3-month period. We report significantly reduced insect biomass losses and abundances under the single and combined treatments. The combination of the high imidacloprid treatment (10 µg/L) and elevated temperatures caused a significant 47% decline in total insect biomass across the insect orders Diptera, Ephemeroptera, Coleoptera, Hymenoptera, Hemiptera, Odonata, and Trichoptera, marking the highest insect biomass loss amongst all treatments. Significant effects on insect community structure and population dynamics were noted, with Diptera and Ephemeroptera populations being most sensitive to the high and both imidacloprid treatments under ambient temperatures, respectively. Diptera populations dominated the emerging insect community and were significantly reduced in abundance by the high imidacloprid treatment and heatwaves. The significantly adverse effects of imidacloprid, warming, and thermal extremes, as well as, temperature-enhanced imidacloprid toxicity demonstrate the stressors’ risks in relation to globally observed insect declines.