Native generalist natural enemies and an introduced specialist parasitoid together control an invasive forest insect

Specialized natural enemies have long been considered a major force driving the population dynamics of outbreaking forest insects. While research has traditionally focused on the role of specialist parasitoids, recent studies and reviews reflect an appreciation of complex interactions among many regulatory factors. The sources suggest that specialist parasitoids and generalist predators can each inflict strong top‐down effects and that specialists and generalists can interact to regulate insect herbivore populations. Here we use the model study organism winter moth (Operophtera brumata) in its invasive range in the northeast United States to investigate interactions between the introduced, host-specific tachinid parasitoid Cyzenis albicans and native, generalist pupal predators. Prior research in Canada showed that predation of winter moth pupae increased after C. albicans establishment. To explain this phenomenon, the following hypotheses have been suggested: (1) parasitoids suppress the winter moth population to a density that can be maintained by generalist predators, (2) unparasitized pupae are preferred by predators and thus experience higher mortality rates, or (3) C. albicans sustain higher predator populations throughout the year more effectively than winter moth alone. We tested these hypotheses by deploying winter moth pupae over six years spanning 2005 to 2017 and by modeling pupal predation rates as a function of winter moth density and C. albicans establishment. We also compared predation rates of unparasitized and parasitized pupae and considered additional mortality by a native pupal parasitoid. We found support for the first hypothesis; we detected both temporal and spatial density dependence, but only in the latter years of the study when winter moth densities were lower. We found no evidence for the latter two hypotheses. Our findings suggest that pupal predators have a regulatory effect on winter moth populations only after populations have been reduced, presumably by the introduction of the host-specific parasitoid C. albicans.