Time-cumulative effects of neonicotinoid exposure, heatwaves and food limitation on stream mayfly nymphs: A multiple-stressor experiment Samuel J. Macaulay a, ⁎, Kimberly J. Hageman b , Jeremy J. Piggott c , Christoph D. Matthaei a a Department of Zoology, University of Otago, 340 Great King Street, Dunedin 9016, New Zealand b Department of Chemistry and Biochemistry, Utah State University, 0300 Old Main Hill, Logan, UT 84322-0305, USA c School of Natural Sciences, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland HIGHLIGHTS • Delayed interactions between environ- mental stressors and contaminants are possible. • Six-week exposure to field-realistic levels of imidacloprid causes severe effects. • Delayed, time-cumulative toxicity of imidacloprid to mayfly nymphs observed. • Time-dependent synergistic and antago- nistic heatwave-by-pesticide interactions. • High imidacloprid tolerance among heatwave-exposed nymphs evidence of selection. GRAPHICAL ABSTRACT abstract article info Article history: Received 22 June 2020 Received in revised form 19 August 2020 Accepted 22 August 2020 Available online 29 August 2020 Editor: Henner Hollert Keywords: Pesticides Imidacloprid Toxicity Selection Synergism Antagonism The global intensification of agriculture has resulted in pesticides playing an increasingly important role as an- thropogenic stressors and drivers of environmental change. There is also a growing need to determine if other environmental stressors, especially those predicted to worsen with climate change, interact with pesticides to alter their effects on non-target biota. Two such stressors are increased extreme temperature events and periods of food limitation. This study is the first to investigate the combined effects of the world's most widely used in- secticide, imidacloprid, with heatwaves and food limitation on a freshwater animal. A 6-week, full-factorial lab- oratory experiment with Deleatidium spp. mayfly nymphs was performed to investigate the potential for direct and delayed interactive effects of simulated heatwaves and starvation with chronic exposure to a field-realistic concentration of imidacloprid (0.4 μg/L). The experiment included two 6-day simulated heatwaves, one during a starvation period prior to imidacloprid addition, and one during the first 6 days of imidacloprid exposure. The simulated heatwaves alone caused such drastic negative effects on Deleatidium survival and mobility that mainly antagonistic interactions were observed with the other stressors, though delayed synergisms between imidacloprid and the second heatwave also affected mayfly mobility. Time-cumulative toxicity of imidacloprid was evident, with imidacloprid first affecting mayfly mobility after 12 days but eventually causing the strongest effects of all manipulated stressors. However, lethal effects of imidacloprid could only be detected in the absence of heatwaves and starvation, possibly as a result of selection for stronger individuals due to prior exposure to these stressors. Our findings demonstrate that heatwaves of increasing severity will critically affect sensitive freshwater organisms such as mayflies, and that the impacts of widespread pesticide use on freshwater ecosys- tems under global climate change cannot be ignored. © 2020 Elsevier B.V. All rights reserved. Science of the Total Environment 754 (2021) 141941 ⁎ Corresponding author. E-mail address: sam.macaulay@otago.ac.nz (S.J. Macaulay). https://doi.org/10.1016/j.scitotenv.2020.141941 0048-9697/© 2020 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv