Interactions of Climate, Host Tree Defenses, White Pine Blister Rust and the Mountain Pine Beetle in Whitebark Pine

The dynamics of insect herbivore populations are seldom driven by a single dominant factor, but rather, by a combination of interacting factors. In this presentation, we discuss how climate, an exotic disease, and host tree defenses, may interact to affect mountain pine beetle population dynamics in a high elevation tree, whitebark pine. Warming trends have allowed mountain pine beetles to move higher in elevation where they are now well established and developing widespread outbreaks in whitebark pine. While increased temperatures are the proximate cause of this elevational expansion, other factors appear to exacerbate the beetle’s impact on the tree once it is present. Evidence is accumulating that whitebark pine may be a superior host for the beetle compared with its more common co-evolved host, lodgepole pine. In stands where lodgepole and whitebark pine co-exist, the beetle exhibits a strong preference for whitebark pine. This preference appears to be related to lower innate defenses and higher stored reserves in whitebark pine relative to lodgepole pine. Additionally, infection by Cronartium ribicola, the causal agent of white pine blister rust, strongly influences beetle preference for individual whitebark pine, with trees exhibiting higher levels of infection significantly more preferred than those exhibiting low levels of infection. Furthermore, sapwood moisture in whitebark pine decreases as infection severity increases; indicating that infection by the pathogen has an effect on water relations within the tree. Sapwood moisture also declines much more rapidly in whitebark than lodgepole pine under drought conditions, indicating this tree may become stressed (and subsequently susceptible to beetles) more rapidly in years when precipitation is low. The combination of low tree defenses, white pine blister rust, and altered precipitation patterns may, alone and in combination, enhance the ability of the beetle to kill trees in high elevations