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Dietary generalist insects are important to ecological communities because they are commonly found in many environments and play important roles in ecosystem services like pollination and decomposition. Although dietary generalist herbivores eat a broad range of plant species, regional populations of these species may have significantly narrower or specialized diet breadths. Fall webworm (Hyphantria cunea, hereafter FW) is a dietary generalist at the species level, but we do not know if there is dietary generalism at the population level or how generalism varies across populations. In Colorado, FW larvae feed on only a few plant species, but many plant species are available that are used by FW elsewhere and not locally. We investigated if FW may be an example of a species that is a dietary generalist when considered over a large geographic range but is composed of populations with narrower diets regionally.We reared FW larvae from fifteen maternal lines in Colorado on a local high-quality host plant and compared their performance (survival, development time, and pupal mass) with larvae reared on plants that are not used locally. We found that FW performance was significantly reduced on plant species that Colorado FW does not use. Our findings demonstrate that Colorado FW cannot eat the same plants as FW in the eastern United States and thus lack the physiological ability to feed on these plants. Our research also suggests that FW are a generalist species with narrower diets that vary regionally at the population level.