Mountain pine beetles have a nasty reputation in the Intermountain West. Many Montanans are familiar with the term “beetle kill,” and have witnessed the damage a beetle infestation can cause. Efforts to control outbreaks have been aggressive. Yet the beetles continue their forward march, leaving North American forests decimated in their wake. But recent research is gaining insight into the resiliency of our forests – and how the trees are fighting back.
The mountain pine beetle lives the majority of its life within the bark of pine trees. When a female beetle finds a tree that is suitable for egg-laying (i.e. a tree that is relatively frail with weakened natural defenses), she will release an aggregating pheromone. This pheromone attracts other beetles to the site. The female beetles drill into the bark, piercing the inner layer to deposit their eggs in vertical egg galleries. After the larvae hatch, they mine further into the tissue of the tree, severing nutrient transport along the trunk. To aid their movement, the larvae employ “blue stain fungi,” a fungal symbiont introduced by the adult beetles. There, they overwinter and emerge as adults the following June.
Mountain pine beetles are minuscule. About the “size of a mouse turd,” according to Diana Six, a professor and forest entomologist at the University of Montana. So, how does such a small insect cause millions of acres of damage in the span of a couple of decades? Trees certainly aren’t without their own defenses: healthy trees are able to “flush” the beetles out with sap or resin. As Six puts it, “Trees can’t hide or flee, so they have to stand their ground and fight.”
So what’s happening to our forests, and why are they suddenly so susceptible to beetle attacks? While some blame poor forest management practices – too much fire suppression, not enough stand thinning, etc. – the most damning evidence points towards climate change.
There are multiple factors at play as temperatures rise. Not only are trees stressed from heat and drought, but beetles are thriving. Once upon a time, it took a full year for a mountain pine beetle to complete its life cycle. As temperatures climb, larval development goes into overdrive. Many scientists are reporting two generations of beetles in a season when we used to only have one. Additionally, warming temperatures allow the beetle to expand its range. Not only are they expanding northward into Canada and the Yukon (where it is considered an invasive species), but they are also moving physically upward. The whitebark pine, a high-elevation pine vulnerable to a blight known as “blister rust,” is now considered a threatened species thanks to beetle attacks.
Yet, as beetle populations grow, so does our insight into how trees can survive the devastating impacts of such minuscule marauders. Trees, like every living organism on the planet, have a genetic code unique to each individual. Some of those individuals have won the genetic lottery and are resistant to beetle attacks. In the same way that a person can find out if they are predisposed to a certain disease, researchers want to learn what makes a tree more or less susceptible in order to better inform forest management practices.