As a biologist, I find the interaction between the blue stain fungi and the mountain pine beetle really quite amazing. We have a mutualistic symbiosis between two completely different organisms which happen to have the same host, the pine tree. As I have an inquisitive mind, I took a slightly deeper look into this relationship. What I found is that there are still aspects of the beetle-fungal relationship currently being debated. Here are some paradigms being suggested.

Let’s first start by looking at the relationship from the fungal perspective. One fact I came across was that some MPB-associated fungal species are capable of killing pine trees on their own. Why then might they enter this relationship with the beetle? (note that I’m anthropomorphizing here as we really can’t determine whether these organisms have ‘actively’ done anything).  At a glance, there are at least two major benefits. First, fungi are stationary and can’t move to new hosts without help. The MPB serve as transportation to new potential host trees. Second, MPB provide access to the phloem, the fungal food source, when they penetrate the bark in their colonization attempts. Penetrating the bark would be difficult for the fungi without help. From these aspects alone, the fungi benefit greatly from associating with the MPB.

So what benefits to the MPB appear to get from this relationship? This is where the waters turn a little murky. The fact that MPB benefit from associating with the fungi is not under discussion. However, the debate revolves around the way in which MPB benefits. One paradigm suggests that the fungi act primarily as plant pathogens and aid the beetle in overwhelming the tree’s defences. The trees must now defend against two separate types of attack, beetle and fungal. Fungi that are more pathogenic would be desirable early on in attack. However, fungi that are too pathogenic could be detrimental to the beetles in addition to the trees. An alternate paradigm suggests that fungi play a primary role as a nutrition source for beetles throughout development. Adult fungi ‘seed’ new galleries with fungal spores providing developing beetles with another food source besides phloem. Fungal species with increased nutritive value would then be highly desirable after the tree’s defences have been overwhelmed.

Given these paradigms, it is interesting to observe that individual MPB are often associated with more than one fungal species.  By associating with multiple fungi simultaneously, the beetles may get the best of both strategies. This is not always the case though and it remains to be seen whether differences in the fungal species complement results in differences in attack success or development of beetles. Many other interesting questions can also be asked. For example, does the mix of fungal associates change from year to year, or from location to location? Research on this is currently underway. How do MPB differentiate between fungal associates present if they do so at all? How do the different fungal species interact with one another within a beetle gallery or tree? To me, answers to these questions will only enhance the interesting nature of this symbiotic relationship.

Thank you again to Dr Adrianne Rice, Mycologist with Natural Resources Canada for providing fungal information used in this posting.