Interactions between native predators and an introduced parasitoid in the control of the invasive geometrid winter moth (Operophtera brumata) in northeastern U.S

In New England, winter moth (Operophtera brumata) is a non-native insect and for the last 15 years has caused widespread defoliation of deciduous trees A tachinid parasitoid (Cyzenis albicans) has been introduced to control the outbreak. The fly has now been established at 11 sites with parasitism levels of 30-40%. However, densities of winter moths at these sites have declined by 95%. In Nova Scotia in the 1950s and British Columbia in the 1970s, predation of winter moth pupae increased following parasitoid establishment. It appears that parasitism alone is not responsible for the declines in winter moth, but that it may be acting synergistically with predation to regulate the population. We tested three hypotheses as to why this may be: (1) Mortality due to parasitoids is enough to bring the winter moth population down to a level that can be effectively controlled by generalist ground predators. (2) Unarasitized pupae experience higher mortality rates than do the parasitized pupae. (3) The presence of C. albicans induces a numerical response in generalist ground predators because C. albicans is a longer-term food source than winter moth. Results suggest that (i) winter moth density reach a threshold over which generalist ground predators are saturated, (ii) early in the season winter moth was preferentially preyed upon, and (iii) invertebrate predators or small mammal numbers may correspond with C. albicans densities. The research emphasizes that it is vitally important to account for other sources of mortality when assessing the impact of an introduced biocontrol agent.