<p>Studies of coevolution, ecosystem processes, and latitudinal diversity gradients are improved by understanding variation in resource specialization. Insect herbivory is one of the most ubiquitous terrestrial ecological associations, and is important for understanding the evolution of both plants and insects, yet the processes underlying global variation in diet breadth remain poorly understood. Here, we use global datasets of butterfly and plant distributions to investigate the patterns and drivers of butterfly larval diet breadth. Diet breadth showed a negative relationship with plant family richness, but this was offset by a direct effect of temperature acting in the opposite direction. Islands generally harbor species with broader diets, but islands with higher endemism had narrower diets than average. Our study provides a global baseline for understanding how plant and herbivore interactions structure ecological communities in the face of global environmental changes.</p>