When’s the best time to study a bark beetle outbreak? If you ask a scientist like Dr. Diana Six, Professor of Forest Entomology/Pathology at the University of Montana, she’s likely to say, “After it’s over.” She argues that post outbreak is one of the times we can learn the most from our forest in order to know how to manage them for the future. Unfortunately, that’s precisely when public interest in the outbreak and doing anything about it has dwindled because the outbreak has collapsed. She said when an outbreak begins to build people respond and throw money and effort into it, but when it goes down, all that stops, and everybody goes and does something else.
But Six and other research scientists have stuck around after these catastrophic infestations are done and over to take a look at what was left. Her argument that the best time to study the phenomena is after its’s over, is based on the interesting results that have emerged from post outbreak studies.
A severe bark beetle infestation will generally kill most of a host species, in lodgepole pine this means that 90% to 99% will not survive the attack. But what Six noticed and thought somewhat remarkable was that, no matter how complete the decimation, some trees survived, and she wondered why. Her studies have already produced some preliminary results that could help guide forest management practices and the promise of some new tools based on that knowledge for making better management decisions.
The primary species of bark beetle involved in the local infestations on the Bitterroot National Forest is the mountain pine beetle, the scientific name for which is Dendroctonus ponderosae or as it translates from the Latin: ‘Ponderosa tree-destroyer’.
According to Six, it is somewhat of a misnomer, however. Ponderosa pine trees are so much more resistant to pine beetle attacks than lodgepole pines.
A Ponderosa pine forest creates its own park-like spacing of large, slow growing trees covered with a thick armor of heavy bark. They are also very heavy producers of sap which is used to resist and expel bark beetles that are trying to bore in. The Ponderosa is also a relatively drought resistant tree and recovers from drought quickly.
Lodgepole pines, according to Six, are not able to muster the same level of defense. They grow close together in crowded stands, are faster growing, a characteristic the beetles seem to prefer, and it has a very thin bark. Neither can the Lodgepole muster the same level of sap expulsion as a big old Ponderosa can.
Maybe they would be better named Dendroctonus Pinus contorta or ‘Lodgepole Pine tree-killer.’
According to Six, bark beetle infestations begin by a few beetles feeding on the weakest trees. But once their numbers get large enough they are able to infest and “destroy” even a very healthy tree.
Research has shown that when the beetles attack a tree it triggers a chemical defense on the part of the tree, and it begins to use sap combined with chemical toxins to fight the bug and push it out. The beetles, however, have a way of turning the tree’s own chemicals into an attractant for other beetles. So while they fight the tree, at the same time they manufacture a Pheromone out of the trees toxins that serves to attract nearby beetles. When enough have arrived the tree is simply overwhelmed.
What can tip the overall situation in favor of the beetles are the environmental conditions, especially the key factors of temperature and moisture. When a whole forest is under stress due to drought and high temperatures it is set up for suffering a major defeat. Once an infestation begins in a lodgepole pine stand under hot, drought conditions there is not much that will stop it, according to Six, except running out of food. The infestation can readily grow to the point that it will kill pretty much every host tree on the whole forest if unchecked. That happens by either wiping out its host species or by the weather.
Well, almost the whole forest, and that’s a very important point. What Six noticed, when visiting sites on several forests in which the bugs had already swept through, was that no matter how bad the decimation was, there were always a few, even if only a very few, healthy green trees left behind, and she began to wonder why.
The survivor trees looked just like all the others. So why would the bugs leave them when their food supply was dwindling to such a point that their population was collapsing? All similar trees in proximity growing under the same conditions and one is spared.
Six intuitively suspected that there was some basic difference that they couldn’t see. After some core sampling, and DNA analysis they discovered what the difference was.
Oddly enough, what they found in the survivor trees that distinguishes them from their neighbors was very low levels of defense, that is they had low levels of the sap and toxins used to resist the bugs. But this low level of sap and toxins, however, leaves the beetles with less substance to manufacture their “attractant”. So while these trees are short on defenses they also don’t end up drawing a crowd. Six believes in this fashion they fall under the radar and are not mobbed by a lethal number of bugs.
Two key factors involved in the conditions for a massive bark beetle infestation are temperature and moisture. Some small number of bark beetles are generally found in most of our forests all the time if you look hard enough. But when things get dry and hot the trees become stressed and the battle between these little bugs and the tree can tip in favor of the bugs and the bugs can overwhelm the tree.
Climate change is exacerbating the bark beetle infestation that has recently been sweeping northward in the Rocky Mountains. Our own infestation pretty much collapsed in 2016. But across the U.S. and Canada it has killed over 70 million trees. In Canada it has jumped the Rocky Mountains and is moving inland already having made it across Alberta.
Compared to past berk beetle infestations the latest one is 10 times bigger than anything we’ve seen before.
With little possibility of stopping such monstrous infestations, focusing on how to manage what is left becomes crucial.
The discovery of this chemically based dynamic and how it works to leave “survivors”, according to Six, could help us in our management after these outbreaks. She said this new understanding suggests that it may be important to re-plant the areas decimated by a beetle infestation with seeds from the surviving trees and not with imported trees that may not have the same genetic make-up.
Six said that by developing a handheld device that could detect the chemical make-up of a living tree could help forest managers in managing stands to ensure that a certain number of “survivor” trees are left when harvesting an area.
“Our forests are facing very different futures and we need to think adaptation and not just spacing and size for relieving stress we have to do more.”
For more information about bark beetles you can visit Six’s web site at https://www.cfc.umt.edu/personnel/details.php?ID=1140
Walter Buck says
Very interesting article. I live part of the year in southwest Montana and have seen the impact of this pest. Dr. Six’s research sounds promising.