Abstract: Mountain pine beetle (MPB) is a serious source of tree mortality, having killed up to 20% of lodgepole pines in the United States during a recent outbreak (2000-2015). In contrast to many forest insects, MPB outbreaks are highly synchronized across space, with correlated infestations across hundreds of kilometers. Here, we use field data and spatially-explicit models to show that the spatial synchrony of MPB outbreaks is a consequence of idiosyncratic life-history traits.
Unlike other bark beetles, MPB must kill its host trees in order to reproduce. To overcome a single tree’s defenses (i.e. resin flow), thousands of beetles must simultaneously bore into the tree’s phloem. Thus, MPB experiences a strong Allee effect. MPB also experiences effective overcompensatory density dependence, due to a preference for large-diameter trees and density-dependent larval survival. MPB will extinguish all large-diameter trees, suffer from high mortality within small-diameter trees, and be unable to recover due to the Allee effect. This sequence of events explains why MPB outbreaks collapse quickly.
Using two mechanistic models of MPB dynamics (calibrated with ground surveys and aerial surveys respectively) we show that outbreaks collapse before all suitable host trees are killed, leaving a contiguous path of resources along which future MPB outbreaks might spread. Indeed, spatially-explicit variants of our models show that in the absence of either overcompensatory density dependence or the Allee effect, MPB kills nearly all suitable trees, leading to greatly reduced spatial synchrony. Thus, inhibitory processes at the population scale (i.e. overcompensation and an Allee effect) are beneficial for MPB at the metapopulation scale. Our models explain the synchrony of MPB outbreaks up to 100km. Longer-scale correlations are likely caused by declining tree vigor as a response to widespread drought. Our work suggests that silvicultural practice --- thinning and prescribed burns --- could reduce the traversability of the landscape and “flatten the curve” of mountain pine beetle epidemics.
