Induced plant defenses, host–pathogen interactions, and forest insect outbreaks

Significance Many forest insects undergo outbreaks, in which their densities rise from undetectable to extremely high. Outbreaks are widely assumed to be driven by specialist natural enemies such as infectious pathogens, but gypsy moth outbreaks show alternating severe and mild outbreaks in forests with a high percentage of oaks, a pattern that cannot be explained by host-pathogen models. We used an experiment to show that induced defenses in red oak reduce heterogeneity among gypsy moth larvae in the risk of virus infection, and extending standard models to allow for this effect produces alternating outbreaks, matching the data. The ability of our model to reproduce this complex pattern suggests that the role of induced defenses in insect outbreaks has been underestimated.