Meteorological versus spatial drivers of the spatial synchrony of forest insect pest outbreaks in North America

Population spatial synchrony has major consequences for the impacts of forest insect pest outbreaks at regional scales. We tested the predictions that the strength and drivers of this synchrony would differ among species according to their dispersal abilities and feeding guild. Using a matrix regression approach, we statistically partitioned the importance of spatial and environmental drivers of outbreak synchrony in six species of phloem-feeding bark beetles and six species of defoliating Lepidoptera in North America. Potential drivers included in the regressions were synchrony of weather conditions and spatial proximity. Overall, model selection operations on the matrix regressions indicated that synchrony in the outbreaks of forest insect pests arises from a combination of spatial drivers such as dispersal and synchrony of weather, also known as Moran effects. Moran effects appeared to be more important in driving the synchrony of bark beetles than defoliators, possibly because weather (e.g., drought) has stronger impacts on bark beetle outbreak dynamics. Nonparametric spatial correlation functions showed that defoliators exhibited stronger synchrony over short distances than bark beetles, possibly because the cyclical nature of defoliator populations allows them to be more easily synchronized. The greater influence of Moran effects on the synchrony of bark beetles compared to defoliators, coupled with climate-change-driven increases in synchrony of weather, may lead to more widespread events of high tree mortality due to bark beetle epidemics.