The mountain pine beetle, Dendroctonus ponderosae, attacks all pine species but the most devastating outbreaks occur in lodgepole pine, Pinus contorta (Slide). There are two main reasons for this:

1. First, lodgepole pines often grow in extensive evenaged stands so that most of the trees become susceptible to beetle attack at about the same time.
2. Second, the mountain pine beetle is considered to be an aggressive bark beetle because it can kill fairly resistant trees when populations are large. Aggressive bark beetles have two important characteristics:
1. First, they are tolerant or even attracted to the defensive secretions of their host. In fact mountain pine beetle adults will almost swim in resin secreted by pines under attack and will not give up fighting the defense system of the tree until they die (Slide).
2. Second, attacking beetles convert host terpenes to aggregating pheromones (a smell that attracts others of the same species) that attract flying beetles to the tree under attack. Mass attack by large numbers of beetles can overcome the resistance of even healthy trees (Slide). Thus, when mountain pine beetles are able to build up large populations they can kill almost any tree in the stand. This gives rise to a +feedback loop because large beetle populations can kill more resistant trees, which produce more beetles, which can then kill even more resistant trees, and so on. As a result almost all the trees in a stand can be killed (Slide).

Mountain pine beetle outbreaks produce huge amounts of fuel (Slide) and increase the probability of forest fires (Slide). However, lodgepole pines have serotinous cones that require heat to open and release their seeds. In other words, lodgepole pine is a fire-adapted species that needs fire before it can spread the seeds of the next generation. Some have suggested that lodgepole pine is also adapted to the mountain pine beetle because it usually becomes susceptible to attack at an age (70-80 years) that optimizes its reproductive output. In other words, if lodgepole pine stands are killed by the mountain pine beetle around this age, sufficient cones will have accumulated to seed the next generation, provided a fire opens the cones and kills competing tree species. The mountain pine beetle increases the probability of forest fire and, thus, from the point of view of lodgepole pine, the beetle may be more friend than enemy!

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© 1998 Alan A. Berryman