Invasive insects increasingly affect forested landscapes and have important ecological and economic impacts. My dissertation focuses on population dynamics of winter moth (Operophtera brumata L.), an invasive pest in the northeastern United States. Native to Europe, this is the species’ fourth accidental introduction to North America. The Elkinton lab established the biological control agent Cyzenis albicans across the range of winter moth in the northeastern U.S. Prior research indicates that C. albicans’ ability to control winter moth likely depends on additional mortality from native natural enemies. My dissertation research evaluates the identity and role of natural enemies already present in North America (predators, parasitoids, and pathogens) on winter moth, and their interactions with mortality from C. albicans. I found that in earlier years of the current North American introduction, predator communities were saturated due to the abundance of pupae to consume; however, as winter moth densities decreased to levels comparable to its native range (presumably due to mortality added to the system by C. albicans) pupal predation’s role on the population dynamics has changed; pupal predation is now density dependent and helps stabilize the winter moth populations. Furthermore, I detected parasitism on winter moth pupae by an ichenumonid wasp (Pimpla spp.), which causes additional mortality on winter moth pupae and rarely kills C. albicans puparia. While rates of mortality on winter moth from this parasitoid are lower than those from predation, parasitism is also density dependent and has a stabilizing effect on winter moth populations. Lastly, my research shows that mortality in larval and pupal stages is lower on winter moth than on the native congener, Bruce spanworm, Operophtera bruceata Hulst. Cadavers from both species had low rates of infection from nucleopolyhedroviruses. Each host had its own virus species and there were no cross-infectious between the two. Microsporidia were detected in Bruce spanworm, but not in winter moth. I conclude that our biological control efforts have reduced winter moth populations to non-pest levels and I expect native natural enemies, along with C. albicans, will regulate population densities indefinitely.