Habitat loss is often—but not always—correlated with diminishing levels of biodiversity. When species diversity does not vary across a gradient of patch sizes, species replacement may still occur in which some species are lost from smaller habitat patches and replaced by other species. In particular, matrix-dwelling species may become disproportionately abundant in smaller patches. In this study, I tested the hypothesis that the abundance of four crop and pasture grass consumers (Lepidoptera: Crambidae, Noctuidae) would be higher in smaller forest fragments compared with larger woodland sites in two ecoregions of Ohio. The two ecoregions differed in the dominant habitat type within the surrounding matrix and in the two most abundant matrix-dwelling moth species that were found in forest patches. In both ecoregions, smaller forests contained a significantly higher abundance of matrix-dwelling moth species, and patch area explained between 50 and 78% of the total variation in moth abundance. Because the total abundance of moths did not vary across the gradient of patch sizes in each ecoregion, the invasion of small forest stands by matrix-dwelling species may offset a reduction in the abundance of forest-interior moth species. If so, regional moth species diversity is predicted to become increasingly impoverished as small forest fragments and agricultural habitats converge in species composition.