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WESTERN
PINE BEETLE
(Dendroctonus brevicomis)
Host: Ponderosa
pine, rarely lodgepole pine
Quick ID:
- Egg galleries
winding, branched, crossing
- Small yellow
pitch tubes
- Red boring
dust in bark crevices and at base of tree
- Larval galleries
short, entering bark
- Large patches
of bark often removed by woodpeckers
- Foliage
discolored
 Field
Identification
Tree:
Small yellow pitch tubes mark the point of entry of the beetle.
Red boring dust may be found in the bark crevices and at the base
of the tree. In the phloem (inner bark region), egg galleries wind
and branch randomly, crossing over the galleries of other beetles.
These galleries will be packed with frass (boring dust and insect
droppings). Sapwood markings from larval galleries off the egg gallery
will be short as the larvae quickly enter the bark to feed. On a
successfully attacked tree, the foliage will discolor first to yellowish-green,
then to reddish-brown. By that point, the trunk may have many small
round emergence holes indicating the beetles have gone.
Insect:
Adults are 3 to 5 mm in length and dark brown, with a cylindrical
shape. Larvae are white and resemble grains of rice with a yellow-brown
head. Larvae may be found in the outer bark of infested trees.
May be
confused with: Mountain pine beetle
Life cycle:
Western pine beetles attack and kill living trees. In an endemic
(normal) population, adults preferentially choose weakened trees,
especially those experiencing water stress. Once an epidemic outbreak
gets started vigorous trees may be attacked as well. Western pine
beetles prefer trees at least 10" in diameter. Adult insects
emerge and attack trees late in spring or in early summer. Females
attack the tree, bore through the bark to the phloem (inner bark),
and emit pheromones that attract both other females, who also colonize
the tree, and males. Attack
usually occurs on the midbole. "Mass attack" of the tree
by many beetles overcomes the tree's resistance. Trees resist colonization
by pitching out and immobilizing the beetles. Generally, trees that
are not under water stress can fend off an attack. During an epidemic,
however, all trees can be overwhelmed. Beetles mate inside the tree
and the females construct winding, "S-"shaped egg galleries.
Niches are cut into the sides of each gallery and eggs are laid
singly, about 1/4" apart.
These niches and the egg gallery itself are packed with frass behind
where the adults are working. Larvae hatch in 7 to 10 days and begin
mining the phloem at right angles to the egg gallery. After a quarter
of an inch or less, the larvae enter the bark where they develop
through a number of larval stages. At last, a pupal chamber is hollowed
into the outer bark, the larva pupates, and a new adult emerges.
Unlike mountain pine beetles, western pine beetles may have two
generations develop to maturity and a third overwinter as larvae
in the same year.
Predisposing
agents: Western pine beetles have greater success in attacking
trees that are under water stress. Root rot infections may predispose
trees to water stress during drought. Beetles selectively attack
trees that have been weakened by drought, lightning strike, or other
agents that interfere with the water balance of the tree. A tree
is in water balance when as much water is taken up by the roots
as is transpired. Trees out of water balance are preferentially
attacked as these trees cannot mobilize strong defenses to expel
beetles. Historically, epidemics occur as a result of prolonged
drought, as drought weakens trees across large areas. Warm temperature
aids development of larvae, hence a hot summer will permit faster
maturing of a generation and a greater likelihood of multiple generations
in a single summer.
Impact:
As it prefers ponderosa pine, western pine beetle is somewhat less
damaging than mountain pine beetle which preys upon all kinds of
pines. According to the USFS, in Washington in 1991 the western
pine beetle was responsible for the death of 21,679 trees over 28,159
acres resulting in a loss of 1.267 million cubic feet of lumber.
Management:
Methods of control have shifted away from direct control (felling,
burning, etc.) and towards indirect methods that discourage beetle
habitat and keep populations at endemic (normal) levels. 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.
Western pine
beetles are a natural part of western ecosystems; they evolved here
together with the pines they feed on. In the forest, they kill slow-growing,
drought-stressed trees and act as natural thinners, opening up the
stand for new trees and wildlife. Even outbreak conditions are "normal"
as cycles of drought have been predisposing factors for beetle epidemics
for thousands of years, as have root diseases. For this reason bark
beetle attacks will never be eliminated (nor should they be). The
death of a few trees on your property doesn't necessarily herald
the beginning of an outbreak; check the trees for signs of root
disease (there are other WSU Cooperative Extension "Forest
Health Notes" dealing with symptoms of root diseases).
To maintain
western pine beetles at their normal levels, predisposing factors
for outbreak must be removed. Environmental stresses are not controllable,
but many stress factors are associated with stand management practices
and therefore may be changed. One important situation that can be
addressed is having too many trees to the acre, or stand overstocking.
Overstocking creates inter-tree competition for water, light and
nutrients, and reduces their ability to resist attack by bark
beetles. A judicious thinning can show good results in ponderosa
pine stands even 90 years of age. 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 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
residual trees. Salvage logging of beetle-killed trees is fine
except in root disease areas where that may worsen the severity
of root diseases such as 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 western 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 dwarf
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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