One chilly morning in October 2013, Diana Six parked her white Subaru at the edge of a pine forest in southwestern Montana’s Big Hole Valley. Beneath snow-tipped peaks, lodgepole pines in four different colors draped the hillside—a time line of carnage. The gray ones, now just trunks and branches, had died in 2009. Light red trees, still holding needles, had succumbed in 2011. Darker, auburn trees had perished in 2012. Even the seemingly healthy green trees, said Six, a ponytailed, bodybuilding, beer-brewing entomologist at the University of Montana, were not what they seemed. Roughly a quarter of them were already doomed.
Six zipped her jacket and ambled into the woods with an ax. She stopped at a mixed stand of emerald and burnt-orange lodgepoles. With the ax blade, she gently peeled a strip of bark from a green tree, exposing the pale wood beneath. There, wedged into narrow channels carved into the wood, were tiny black larvae the size of sesame seeds. They were dead, done in by an early hard frost—but it had come too late to save the lodgepole. Though the tree appeared to be thriving, its phloem, the fibrous layer under the bark that transports nutrients, was dry and brown.
Six moved to the next tree, another seemingly healthy one. Its phloem was greenish pink and pliant, clearly still hydrated. But it was laced with the same telltale channels. From their size and the lack of larvae, Six concluded that this tree had been invaded as recently as a week earlier. As she peeled back the bark with her ax, she accidentally squished a small black beetle.