I have had the Mountain Pine Beetle (MPB) on my mind recently for many reasons.
First and foremost, my field work planning is highly complicated by trying to choose sites that will have the least amount of MPB kill. Second, I recently watched an episode of The Nature of Things on CBC about MPB (here) which was an excellent intro to the beetle and the current situation, and also featured UBC forestry professor Dr. Allan Carroll. And third, I have just returned from attending the annual CSEE conference which was in Kelowna, BC this year, where I saw a few talks on the topic. Jasmine Janes from the University of Alberta gave a talk on the genetic signature of MPB range expansion, and I also attended a talk by Mathias Kaiser from the University of Calgary who talked about beetle stridulation, and the different noises the MPB makes to communicate with other beetles.
There is so much that can be said about the beetles, so I just want to highlight some of the things I found interesting after what will be a short introduction. Mountain pine beetle (Dendroctonus ponderosae) is a bark beetle native to western North America. One beetle is about the size of a grain of rice. In the past few years, these beetles, which inhabit many pine tree species in the west, have exploded in population size and caused devastation to the lodgepole pine.
Entire forests of lodgepole pine have been killed across BC. MPB has been around in the past and has co-evolved with western pine species, but their increased numbers leading to this explosive destruction seem to be due to two main causes: warmer winters from climate change and reduction of natural fire regimes in forests.
When beetles attack an individual tree, they overwinter under the bark. With warmer winters due to climate change, more beetles started to survive than in the past, allowing populations to expand much more than they had historically. And larger population sizes have more of an implication than one might think initially, a point I will come back to momentarily. Historically, lodgepole pine is a fire-associated species, too. Serotinous cones open during extreme heat and reseed forests after fire in a natural situation. This also used to help control the beetles and keep populations down, however human efforts to suppress fires have decreased this effect.
So what are the cool things about MPB that make its larger populations so deadly? Allan Carroll was kind enough to have a chat with me and explain some of the finer details.
It takes many beetles to kill one tree, and the tree doesn’t go down without a fight. An individual beetle may land on a tree, find a good spot and chew in through the bark and into the phloem. The pine, in an effort to stop the beetle, has a constitutive defense where its resin canals kick in to try to flush the beetle out just with physical force. Once the beetle gets far enough into the tree to start chewing through live tissue, the tree has an induced response where it starts releasing toxins within the resin as a chemical defense against the beetle. (You can see this in action if you watch the CBC show; there is very cool footage.) The next neat thing that happens though from the beetle’s point of view, is that while this toxic resin is being produced, it really has no choice if it wants to continue other than to just ingest it. And once it starts ingesting the toxins, it begins to release what is called an aggregation pheromone. This aggregation pheromone is picked up by other nearby beetles and acts as a signal that “hey this tree is under attack right now”. If nothing further happened at this point, the individual beetle would be hopeless and die alone, and the tree would prevail. But, with other beetles in the area, they come to the tree like moths to a flame when they smell the pheromone. With many beetles attacking at once, the tree has a much harder time defending, and will eventually lose and be killed. The beetles will eat to their heart’s desire, lay their eggs, and come next spring, the next generation of beetles will go off and do this again.
Another cool MPB fact I learned was that one female can have around 60 offspring in a generation. Not only this though, but they also do not have 50:50 sex ratios! As an evolutionary biologist, this is such an interesting point. With an altered sex ratio (more females than males) the population can grow at a much faster rate than otherwise. And a larger population means higher likelihood of beetles picking up the pheromone and coming in for the attack. All of which is bad news for the pines.
As the beetles have expanded their range northward and up in elevation, it also seems like the “naive” trees that did not have to deal with MPB in the past are less able to defend themselves. There is strong selection for the trees to be able to make the toxins (monoterpenes) in their resin, but this seems to be less strong or less common in naive populations.
And lastly, to return to the point about the fire regime, MPB prefers older trees of about 60-80 years in age or greater. These trees are bigger and thus a better food source and better for surviving through the winter. As the trees age, they also have decreased ability to fight off the beetles. When fire was more common for lodgepole, stands of trees would not reach this age as often; forests would regenerate more frequently and on average have a younger age structure. Lack of fire has created an important change in the landscape that is another token of misfortune for the trees in the face of mountain pine beetle.
There are multitudes of other things that can be said about the mountain pine beetle, but I personally found the above facts most interesting as well as new for me to learn. And here’s hoping that the beetles have held off from the sites I am visiting this summer to collect DNA samples!