Daily temperature extremes play an important role in predicting thermal effects

Organisms in natural environments experience diel temperature fluctuations rather than constant temperatures, including sporadic extreme conditions. Studies based mainly on model organisms have tended to focus on responses to average temperatures or short-term heat stress, which overlooks the potential impact of daily fluctuations including stressful daytime periods and milder nighttime periods. Here we focus on daily maximum temperatures, while holding nighttime temperatures constant, to specifically investigate high temperature effects on demographic parameters and fitness in the English grain aphid, Sitobion avenae (Fabricius). We then compared the observed effects of different daily maximum temperatures with predictions from constant temperature-performance expectations. Moderate daily maximum temperatures depressed aphid performance while extreme conditions had dramatic effects even when mean temperatures were below the critical maximum. Predictions based on daily average temperature underestimated negative effects of temperature on performance by ignoring daily maximum temperature, while predictions based on daytime maximum temperatures overestimated detrimental impacts by ignoring recovery under mild nighttime temperatures. Our findings suggest that daily maximum temperature will play an important role in regulating natural population dynamics and should be considered in predictions. These findings have implications for natural population dynamics particularly when considering the expected increase in extreme temperature events under climate change.