Surveys of local abundance of many taxa, including herbivorous insects, exhibit predictable species abundance distributions characterized by numerical dominance of a few species and a long “tail” of many scarce species. The pattern also holds for plant-insect associations in which a large number of polyphagous herbivores feed on similar host plants, such as temperate lepidopteran larvae feeding on forest trees. Whether ecological or life history characteristics can explain this variation in relative abundance is a question of importance to applied and theoretical ecology. We used an intensive seven-year survey dataset of caterpillars on a riparian tree (Acer negundo L.) to characterize abundance of 83 generalist moth species in central Maryland, and to classify species as core or transient herbivores. For the core group alone, and the entire assemblage, we tested the relationship between abundance and ecological traits such as fecundity, larval growth rate on the host plant, and parasitism rates on the host plant. Phylogenetically informed correlation analysis of field and laboratory data from species spanning the full abundance distribution shows a strong positive relationship of relative growth rate (mass added/day corrected for final pupal mass) with long-term abundance. Parasitism and fecundity data were more equivocal with respect to abundance and highly variable by species. These results suggest mechanistic population-level hypotheses for predictable abundance levels in this system, with implications for theoretical understanding of abundance distributions in general.