Tropical invertebrates are currently living very close to their optimal temperature. Warming due to global climate change will likely exceed their physiological optima and have deleterious consequences for insects living at low latitudes. In this study, we assess the effects of various levels of summer warming predicted for the late 21st century (+1.2 and +3.7 °C, with the same diel oscillation as the current regime) on the physiology and demography of the aphid Myzus varians Davidson (Hemiptera: Aphididae) in subtropical and tropical Taiwan. Aphids subjected to a moderate (3.7 °C) increase in temperature did not reach adulthood and thus left no offspring, meaning that wild populations could go extinct during the summer. Slight (+1.2 °C) warming did not significantly affect development time and generation time. However, warming reduced nymphal survival, adult longevity, and reproduction, and, thus, reduced the fitness of aphid populations. Aphids have a number of adaptations for surviving or avoiding unfavorable conditions. However, predicted increases in global temperatures will likely decrease their survival and reproduction, which could increase the frequency of local extinction.