Mountain Pine Beetle in the Black Hills

Beetle Biology:  

 

The mountain pine beetle is one of several common bark beetles native to North America.  The beetle kills its host by disrupting the movement of food from the needles to the roots through the construction of tunnels known as galleries throughout the stem of the tree.  Each beetle also carries a fungus disease known as blue stain from tree to tree; blue stain fungi plug the water-conducting tissue of the tree, blocking the flow of water from the roots to the needles.  This combined attack is generally sufficient to kill an infected tree within one year.  Once a tree is attacked, it is too late to do anything to save it.  The beetles generally attack trees in such large numbers – by the hundreds of thousands – that there is no doubt they can overcome the tree’s only defense, heavy resin flow.

 

Beetles like these are omnipresent in nearly all forested systems.  Most times, they exist in what are called endemic populations, killing only a few already weakened trees per year.  Occasionally, in accordance with what has been documented to be a fairly regular 10-year cycle in the Black Hills, beetle populations skyrocket to epidemic proportions.  Under normal (“natural”) forest conditions, the outbreak would begin in an especially dense area of forest, spreading outward until it reached a forest type that was inhospitable for the beetles.  There, the outbreak would stop and have been confined to a relatively small area.  On the Black Hills National Forest today, however, years of fire suppression and insufficiently intensive management have produced a forest that is literally “infested” with trees.  This phenomenon equates to large expanses of “prime beetle habitat” (overly dense stands comprised of 8” and larger trees) and therefore little exists in the way of natural barriers to the beetles’ progression.

     

 

Mountain pine beetle vital stats:

Primary hosts:               ponderosa and lodgepole pine
Tree size preference:      greater than 10” diameter at breast-height
Forest stand preference: low light conditions, low wind speed, high individual tree stress; primarily related to high stand density
Generations/year:          one
Adults emerge/fly:        late-June through mid-September (ponderosa pine)
Preliminary signs:         “pitch masses” in mid to late summer
Symptoms:                browning needles followed by tree death (external symptoms not apparent until the year following attack)
Natural controls:            woodpeckers, predatory insects, viruses – not effective in controlling epidemic beetle populations
Management options:
           Outbreak Control (i.e., less than ¼ acre affected area):
                                     ·   insecticide spraying before June 30
                                     ·   cut and burn host (green) trees in winter
                                     ·   cut and debark or chip host trees before May 15 or after August 15
                                     ·   solar treatment (pile and cover with tarp)
                                     ·   pheromone trapping
          Prevention:
                                              ·   thinning (most effective form of treatment)

 

 Common pine beetle myths and fallacies:

 

    There is not an outbreak of mountain pine beetle; it’s all snow damage.  This is a half-truth. There has been a lot of snow damage in the Black Hills during the past couple years.  The early and late season snow storms that have occurred resulted in a lot of branch breakage and snapped tops. However, these are also conditions that favorable for rapid expansion of mountain pine beetle populations.  Trees that have a snapped top are especially vulnerable to mountain pine beetles, as their primary defense against such attacks – resin flow – is much reduced; yet the inner bark, the primary food source for the developing beetle, is still fresh.  Beetle populations increase dramatically in such trees and the large broods that hatch from them are capable of successfully attacking nearby healthy trees.  If you walk in the Forbes Gulch area, or any one of a number of areas in the northern Black Hills, you’ll notice a lot of snow damage.  But if you look closely, you’ll find these same trees riddled with emergent holes of the mountain pine beetle.  You’ll also find that many of the apparently healthy nearby trees are covered with pitch masses, evidence of a successful attack.  These trees will be killed before spring and the pocket of dead trees surrounding the snapped top trees will continue to expand.  And expand, they have.  Across the Black Hills, the number of beetle-killed trees increased 790 percent between 2000 and 2001 according to Forest Service data; almost 300,000 trees were killed in 2001.

    There is nothing to worry about; our winter will kill the beetles.  Winter temperatures in the Black Hills, while often blustery, are rarely cold enough to result in significant beetle mortality.  To kill beetles, we would need to experience sustained temperatures below -40 F during midwinter or temperatures below 0 F in the fall or spring.  We did not experience these temperatures this fall, nor are we likely to see them this spring.  Rarely can we rely on winter to solve a beetle problem.

    The beetles will not be a problem; the summer rains (or woodpeckers) will kill them.  Rain does not stop the beetles from flying.  We would need something of Biblical proportions to have any influence on flight behavior.  Woodpeckers as well as a number of other bird and insect species feed on mountain pine beetles.  Unfortunately, once the beetle populations start increasing rapidly, their natural enemies can’t eat them fast enough to have much of an effect.

    Well, if the beetles are a problem, it is because of logging.  In fact, silvicultural control measures are the most effective means of managing the mountain pine beetle; by this we mean regulating stand density.  The beetle tends to prefer trees in dense stands.  These trees are usually weaker due to the intense competition for light, moisture and nutrients, and so are easier to attack and kill.  Beetle behavior is influenced by stand density in another way – it likes cooler, more shaded environments found in very dense forest.  If a stand is held below a certain density (identified through research as around 80 square feet of basal are per acre), the beetle is rarely a problem.  Below are three images illustrating the resistance of a thinned area against severe beetle infestation in surrounding dense stands.

    But the increase in smaller diameter trees – since they cut all the big ones – is increasing the beetle population, so logging is the problem.  No, while it is true there has been an increase in smaller diameter trees (i.e., less than 8 inches in diameter) from 120 years ago and a decrease in larger trees (i.e., those over 16 inches in diameter), the beetle tends to attack large diameter trees, not small ones.  Stands with an average tree diameter of more than 10 inches and extremely dense stocking (> 150 square feet per acre) are at highest risk of attack.  As the average diameter decreases and density decreases, the potential risk also decreases.  It is the bigger trees, not the smaller ones that are vulnerable to attack.

    The beetles are a natural part of the Black Hills, there is nothing we can do about it.  Again, this one is half-correct.  The beetle is native to the Black Hills and probably has been here as long as there have been pines.  Outbreaks have been reported in the Black Hills many times during the past one hundred years.  Perhaps the most famous was the outbreak that occurred about 100 years ago.  That one resulted in approximately 150 to 200 square miles of beetle-killed forest.  While none of the outbreaks that have followed have even come close to the same losses, they can still result in significant tree losses.  The statement, “we cannot do anything about it”, is dead wrong.  No, we can’t eliminate the beetle, but we can manage it.  Maintaining healthy stands is the best, but not the only means of managing the beetle.  As an example, we can also employ such tactics as using the beetles’ own complex chemical communication system against them through pheromone trapping.

  Information collected from:
John Ball, professor of forestry, SD State University
2001 US Forest Service mountain pine beetle aerial surveys
Images courtesy of the Greater Arkansas River Nature Association (www.garna.com), Pope & Talbot, Inc, and the Colorado State University Cooperative Extension (http://www.ext.colostate.edu/pubs/insect/05528.html).