Climate variability has increased in recent decades, and this trend is predicted to continue. Globally, there is a rise in extreme temperatures, such as heatwaves, which are strengthening in both intensity and frequency. As ectotherms, insects are especially affected by temperature variation. Insects are the most speciose group of animals and important members of most terrestrial ecosystems, providing critical ecosystem services. Most studies of global change have investigated only a single stressor, yet insects rarely experience one stressor at a time. Two stressors that co-occur are heatwaves and warming temperature regimes; thus, it is important that we study their interaction and whether diet may help mitigate negative effects from these stressors. We used a dietary generalist herbivore, the moth fall webworm Hyphantria cunea (Lepidoptera; hereafter, FW), and conducted a factorial experiment to test the separate and interactive effects of temperature warming, heatwave duration, and host plant on FW fitness. We found significant interactions between temperature regime, heatwave duration, and host plant. We also found that FW follow the temperature-size rule, but only when reared on one of the host plants, thereby supporting previous research that diet alters insect response to changes in temperature regime. Our results show that FW suffer reduced fitness when exposed to high temperatures and heatwaves, with more extreme temperatures and longer heatwaves resulting in increased negative effects on fitness. However, some of these negative effects may be attenuated by host plant identity, which could play an important role in how generalist herbivores respond to increasing extreme weather events as predicted by global change.