Defending the castle – a pine tree response to MPB attack
December 14, 2008 9:30 PM
Filed Under: Mountain Pine Beetle
Looking at images of pine forests in BC and Alberta, the effects of the MPB epidemic are easy to see. In face of this pictorial assault, it is easy to forget that MPB attacks may not always result in the death of the tree. Pine trees have their own array of defence responses giving them a chance to repulse invading insects (such as MPB) and pathogens. I thought I’d touch on one of the primary responses by pines (and other conifers) in this blog, and that is the production of oleoresin, a viscous and odoriferous mixture of chemical compounds.
Oleoresin flows following insect attack and serves a couple of purposes for the tree (see picture below of an oleoresin response to an insect attack). Resin flow can push invading MPB out of the entry wound (a process called ‘pitching out’) or to entomb them so they cannot cause further damage. The resin flow also cleans the wound. In addition, some chemical components of the oleoresin are toxic to the MPB and fungi. I think of this as being similar to boiling pitch which medieval castle defenders would pour on attackers. If you’re not pushed off the scaling ladder, you get scalded by the pitch. Oleoresin serves both purposes, if the MPB are not pitched out, toxic chemicals contained within it can kill the beetle or fungal associates. The oleoresin also serves to seal the entry wound once invaders have been dealt with. Upon exposure to air, volatile chemicals in the resin evaporate leaving non-volatile diterpene acids to harden (polymerize) and seal the wound. This minimizes the chance of further microorganism or insect attack through the entry wound.
Oleoresin (pitch) response to an insect attack.
So why doesn’t this always protect the tree? Even the pitch response picture shows an excavated entry area. Many other factors influence the tree defence of an MPB attack. For example, the chemical composition of the oleoresin and the speed and amount of resin that flows may differ depending on the health of the tree. Oleoresin properties (chemistry, physical attributes) can be affected by environmental factors such as air pollution or drought stress. Also, the severity of MPB attack plays a role as well. Taking the last comment to heart, it is not hard to see how the sheer number of MPB in the current epidemic could overwhelm tree defences. Even the strongest castles can be overwhelmed by superior forces.
Posted by Matt Bryman at December 14, 2008 9:30 PM