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MOUNTAIN
PINE BEETLE
(Dendroctonus ponderosae)
Hosts: Lodgepole
pine, ponderosa pine, western white pine, whitebark pine
Quick ID:
- Egg galleries
long and straight, with the grain (vertical)
- Larval galleries
at 90 angle from egg gallery
- Yellow pitch
tubes on trunk
- Red boring
dust in bark crevices
- Pitch streamers
on trunk
- Foliage
discolored
 Field
Identification
Tree:
Trees from 4 to 5" in diameter and up may be attacked. Attacks
usually occur in the middle of the trunk. Yellow pitch tubes are
found on the trunk at the point of entry, and red boring dust may
be found in bark crevices and at the base of the tree. Pitch streamers
on the trunk may also indicate point of entry. Egg galleries within
the inner bark are long and straight, and follow the grain. These
galleries will be packed with frass (boring dust and insect droppings).
Generally there is a bend at the bottom of the egg gallery. Although
there may be several egg galleries on the same tree, they do not
cross. Larval galleries branch off of the egg gallery at a 90 angle
and are 1 to 2" in length. As the successful attack proceeds,
foliage discolors from dark green to light greenish yellow and then
to reddish brown. The wood of successfully attacked trees becomes
discolored by bluestain fungi.
Insect:
Adults are about 1/5" in length, stout, black, cylindrical
beetles. Larvae are about 1/4" long, white and resemble grains
of rice. They are found in separate tunnels off the egg gallery;
pupal cells with pupae may be found at the end of these tunnels.
May be confused
with: Red turpentine beetle, western pine beetle.
Life cycle:
Mountain pine beetles are both a primary and secondary pest, infecting
both apparently healthy and disease-weakened trees. In the endemic
(constantly maintained) population, beetles preferentially attack
trees that are weakened by root diseases, but once an epidemic gets
started beetles attack healthy trees as well. Mountain pine beetles
show a strong preference for pines of large diameter more than 80
years in age. In July and August, when trees are most likely to
be under stress from water deficiency, adults emerge, females find
a new host, and bore through the bark to the cambial area. There,
they emit pheromones that attract males and other females. The other
females that are attracted will also colonize the tree; this "mass
attack" strategy overcomes the tree's defenses. Mating occurs
under the bark, then the female constructs an egg gallery and deposits
eggs in niches on either side. The gallery becomes packed with frass.
When the larvae hatch in 7-10 days, they mine separate feeding tunnels.
When fully grown, the larvae hollow out a pupal cell at the end
of the tunnel and pupate. New adults bore their way out and the
cycle is repeated. One generation per year is the rule in Washington.
Insects overwinter in trees as larvae and adults.
Predisposing
agents: A strong association has been found between root diseases
and bark beetles, where disease weakens trees and makes them targets
for bark beetle attack. Agents of stress such as drought, overcrowding,
overtopping, and defoliation can also weaken trees. Overmature stands
are ripe for insect attack, as they are not vigorous and have a
thick phloem layer, which beetles prefer. Low-elevation stands are
harder hit than high-elevation stands as the warmer temperatures
at low elevations are more favorable for insect survival; likewise,
unusually warm summers aid bark beetle development. Outbreaks of
mountain pine beetle typically get
started in overstocked stands or in stands on poor sites, then spread
to healthy timber. Essentially, the success or non-success of bark
beetle attack has to do with the physiological condition of the
tree. Healthy, vigorous trees are able to overwhelm attacking beetles
with pitch, while trees that have been stressed may not have the
available resources for massive pitch production. However, during
outbreak conditions an otherwise healthy tree may simply be overwhelmed
by sheer numbers.
Impact:
Mountain pine beetle has historically been the most damaging of
the bark beetles. According to the USFS, in 1990 they were responsible
for the death of 289,800 trees totaling 6.5 million cubic feet over
186,600 acres in the state of Washington. In 1991, 298,400 trees
were lost totaling 5.6 million cubic feet over 155,422 acres. Outbreaks
when mountain pine beetle attacks reach epidemic levels have been
recorded in the west since 1894. Outbreaks can last for more than
ten years, and mountain pine beetles are generally in an epidemic
condition on at least one of their hosts somewhere in the west.
Management:
Control methods have shifted away from direct control (e.g. spraying,
felling, burning) and towards prevention of outbreaks. This course
of action was chosen after thoroughly exploring direct control measures
for nearly a century and arriving at a simple conclusion: They don't
work. It is possible to prevent infestation with penetrating sprays
on individual, high value trees such as those in campgrounds and
near houses, but they need to be applied before the tree is infected
and the cost of such treatments is prohibitive for any large-scale
application.
Once a mountain
pine beetle outbreak begins to spread, it can be stopped by thinning
the stand ahead of the edge of the outbreak. This is because outbreaks
expand on a tree to tree basis where the incoming beetles switch
their attacks from a recently attacked-stem to the next largest
tree. More importantly, infestations can be prevented by thinning
stands before crown closure, an operation that not only increases
the vigor of the residual stand, but also prevents the spread of
an outbreak if individual trees have been attacked.
Mountain pine
beetles are a natural part of western ecosystems, and for this reason
will never be completely eradicated (nor should they be, as they
serve to create small stand openings which are important for biodiversity
of both flora and fauna). As such, the death of a few trees on your
property doesn't necessarily mean an epidemic is getting started;
check your trees for root disease symptoms. To maintain mountain
pine beetles at their normal levels, predisposing factors for outbreak
must be removed. Some of these, such as environmental stresses,
are not possible to control. However, many stresses are related
to stand management practices. First and foremost, two situations
need to be addressed: root disease centers and overstocked stands.
More details about treatment for root disease centers have been
given in other WSU Cooperative Extension "Forest Health Notes;"
in summary, they need to be identified and planted with resistant
species. Overstocking causes trees to compete for water, light and
nutrients, and thus weakens their defenses against bark beetle attack.
To minimize stand stresses and maintain vigorous growing conditions,
stand managers should: (adapted from Berryman: Forest Insects, 1986).
- Choose
tree species that are adapted to the area on which they'll be
planted
- Harvest
trees in a way that mimics natural processes, such as cutting
small patches to mimic a low-intensity fire (for pine and larch
management) or selective or shelterwood cutting (for grand fir
and
- Douglas-fir
management)
- Remove diseased
and unhealthy trees and logging debris, and minimize damage to
standing trees.
- Salvage
logging is fine for beetle-killed trees except in root disease
areas where that could increase the severity of Armillaria and
Annosus.
- Encourage
diversity in species and age classes
- Use thinning,
fertilization, prescribed fire, etc. to maintain stand diversity
and vigor
- Prevent
trees from becoming overmature by harvesting on time
Patch cutting
6 to 10 acre blocks every few years and managing these as small
even-aged stands helps keep the total number of older trees low
and creates a variety of age classes that discourages mountain pine
beetle attack. It has additional benefits for wildlife by creating
small openings and edges. This may not, however, be a good strategy
if trees at the edges of the cut are heavily infested with mistletoe
and the species to be planted or naturally regenerated is the same
species.
Note:
Use pesticides with care. Apply them only to plants, animals, or
sites listed on label. When mixing and applying pesticides, follow
all label precautions to protect yourself and others around you.
It is a violation of the law to disregard label directions. If pesticides
are spilled on skin or clothing, remove clothing and wash skin thoroughly.
Store pesticides in their original containers and keep them out
of the reach of children, pets and livestock.
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