The yearly emergence of beetles and subsequent attack of trees has occurred for millions of years. It is easy to see the effect this phenomenon has on individual trees. In the midst of an outbreak many thousands of trees can be killed from one year to the next. But the beetles are not always in an outbreak mode. During most years the mountain pine beetle can be considered a rare creature, only occasionally killing single isolated trees and not having a noticeable impact upon the forest.
The coming and going of these beetle "waves" is primarily dependent on two factors: forests stand conditions and weather patterns. Mountain pine beetle must have hosts of a certain condition; they need pine trees of sufficient size in which to produce their young. Despite the apparent devastation seen in a beetle outbreak, they cannot attack seedlings and saplings. Under dire circumstances they will attack pole-sized trees, but this is an act of desperation. To have a bark beetle outbreak there must be a sufficient quantity of suitable trees.
The second major factor is recent weather patterns. We have seen how a tree's resin system acts as a defense against beetle attack. Strong, vigorously growing trees naturally have a stronger defense system and can fend off beetle attack. During years of drought the trees' defenses are weakened and they become more susceptible to beetle attack. If the trees are growing in a dense stand and are competing for moisture and nutrients, this can also adversely affect the trees' abilities to defend themselves. Thus, a combination of recent weather (primarily moisture) and stand conditions (primarily density and age) determine whether an outbreak can occur in a given stand.
The fact that beetles respond to large numbers of susceptible host trees illustrates the symbiotic relationship between the pine trees and the bark beetles. An analogy would be the relationship between predators such as wolves and their prey such as a herd of elk. While at first glance it appears that the wolves feeding on the elk would be a one-way relationship, we now know the benefit that wolves provide to the herd. By their action of removing the old, the weak, and the sick elk, the wolves actually make the herd stronger by making more resources available to the rest of the herd. The same can be said of the relationship between the pine trees and the bark beetles. As bark beetles kill trees under stress due to overcrowding, old age or disease, they free resources for the residual trees. As trees are killed and decay they release nutrients that are incorporated into the soil, openings in the stand allow the in-growth of young trees and there is now moisture available for the new growth.

(top) A pitch tube, showing the beetle entry canal and boring dust. Such tubes usually indicate a successful attack, but not always. (right) A ponderosa pine after it has completely faded. Dead trees can be left for habitat.

Predisposing factors have been evident for some time and they have been combined into techniques by which the risk of a stand being attacked can be rated. These "risk rating systems" take various stand factors into account such as tree density, average age, rate of growth, etc. and combine them with other biological factors such as elevation or measures of beetle activity. These risk ratings can then be used to prioritize treatments to alleviate high risk.
If trees are under stress due to high stocking levels, (ie. the trees are competing for moisture and have low defenses), thinning the stand can reduce stress levels. Thinning of at-risk stands is one way to reduce the potential for bark beetle outbreaks. The biology involved is relatively simple, but the politics are not.
To be effective, stand thinning should occur when beetle populations are low. Thinning during an outbreak doesn't always work and sometimes it may even backfire. Cutting during an outbreak releases large amounts of tree odors that the beetles are attracted to. In addition, the stress caused by harvesting a stand (such as mechanical wounding and the sudden increase in sunlight) can actually stress a tree. The trees are weakened in the short term and become more susceptible to beetle attack. But thinning when no beetles are present is problematic on a political basis; it is difficult to convince forest users that a given stand is at risk to bark beetle attack when to external appearances the stand appears vigorous and healthy. In addition, thinning stands to levels which can withstand beetle attack may be unacceptable to forest users. Spacing on the order of one tree every 20 feet often appears "unnatural" to forest users, although much evidence points to the fact that such spacing was the norm in pre-settlement times.
It is clear that the reduction of living biomass in pine stands can reduce the risk of mountain pine beetle outbreaks. Biomass reduction can be accomplished through the use of prescribed fire or by mechanical harvesting. However, it is not possible to establish stocking control over all stands within the forest. Steep slopes make any sort of manipulation difficult. The encroachment of homes and development into the forest environment greatly reduces the ability of managers to use fire as a management tool.
Wise forest management does not seek the elimination of the influence of pine beetles, fire and other disturbance agents in the forest environment. The prudent course of action is to make conscious decisions to prioritize treatments based upon land use patterns. In certain portions of the forest it makes sense to use prescribed fire to encourage the development of a beetle and wildfire resistant forest. Other portions of the forest will require the direct removal of biomass by means of mechanical harvesting. Mechanical harvesting includes an entire range of techniques including horse logging, tractor skidding and helicopter logging.
Due to the factors of economy, topography, and scale, the vast majority of forestlands will need to be left unregulated. It is unreasonable to expect that the ancient relationships between pine beetles and pine trees can be eliminated as a result of fleeting human sentiment. Judicious forest management requires long-term planning and the political will to follow through. Local residents must educate themselves in the complicated biological and ecological facts of life. Managers must constantly remind their constituents that forest planning is an endeavor that occurs on a scale of hundreds of years.
Crisis management is not the most favorable method to deal with conflicts between humans and natural systems. In the long run, a thoughtful attempt to work within the tenants of the nature will pay great dividends. We all must choose our battles and make difficult decisions, always keeping in mind that these forests will be here long after we have gone.

Q & A

Where did the beetles come from that killed my trees?

Beetles normally fly upwind toward a source of attractant pheromones. If prevailing winds are strong enough, it is conceivable flying beetles could be carried downwind; regardless of pheromone sources they would prefer to locate.
It is not possible to determine where beetles came from, short of experimental application of chemical markers prior to flight and retrieval following flight.

Q & A

Are pitch tubes reliable indicators of a successful attack?

Not all pitch tubes are created equal. The presence of pitch tubes should be combined with other external and internal symptoms to identify if the tree has been attacked.
Professionals can say with 75% certainty if an attack has occurred.