WESTERN PINE BEETLE
16 , 2003
Stages and Development
Western pine beetles pass through the egg, larval, pupal,
and adult stages during a life cycle that varies in length
from about 2 months in warm weather to 10 months in cool weather.
All stages are completed beneath or in the bark of infested
trees, except for a period when the adults fly to find new
trees to attack.
During an attack period, which may last 3 weeks, each female
lays about 60 tiny pearl white eggs individually in niches
cut into the sides of the egg gallery. Some of these parent
females may emerge and reattack to establish additional galleries
elsewhere in the same tree or other host trees. After incubating
from 1 to 2 weeks, the eggs hatch.
The larvae are small white grubs that feed first in the phloem,
where they construct a short gallery. They then mine into
the middle bark where most of their development takes place.
After completing four larval stages, they transform into pupae
and then into adults. As these brood adults feed on the middle
and outer bark, fungal spores collect in their mycangia. Once
the adult insects emerge, they are ready to renew the attack-infestation
cycle in living trees.
Several conditions often work together to influence the number
of beetles and the beetle caused tree mortality in a given
area. The significant conditions follow.
-- the availability of suitable host material - phloem and
inner bark - is a key condition influencing western pine beetle
outbreaks. Most trees are either healthy or too weak to provide
material in which beetle numbers can increase. Healthy trees
can withstand many attacks before the beetles are successful,
the brood is established, and new adult beetles are produced.
Weak trees, such as those that have been smog damaged, diseased
or suppressed by competition, although easily killed, also
produce relatively few beetles.
The thick, nutritious phloem and inner bark of healthy trees
become host material for attacking western pine beetles when
these trees undergo sudden and severe moisture stress. Healthy
trees ordinarily produce abundant amounts of resin, which
pitch out or eject attacking beetles. But, when suddenly deprived
of moisture, stressed trees cannot produce sufficient resin
flow to resist attack, and their nutritious food supply suddenly
becomes available to beetles. In these trees, almost all attacking
beetles can succeed and reproduce many times their number
of offspring, increasing the beetle population to outbreak
Moisture stress results when the water balance between the
foliage and the roots changes dramatically. An imbalance may
result from increased water loss from the needles, decreased
water uptake by the roots or from a combination of the two.
Any condition that results in excessive demand for moisture,
such as tree crowding, competing vegetation, or sudden exposure
to severe sunlight: or any condition that reduces that ability
of the roots to supply water to the tree, such as mechanical
root damage, root disease, soil compaction, or drought, can
cause moisture stress and increase susceptibility to attack
by the western pine beetle.
Landowners have two basic alternatives when choosing the control
strategy most appropriate for their needs: beetle population
suppression and damage prevention.
-- Over the years, several suppression methods have been tried
to help reduce beetle populations enough to lower tree mortality
significantly. These methods have included the removal of
infested trees and applying toxic residual sprays to kill
emerging beetles. Because adult beetles can fly many miles
and produce many offspring, effective suppression methods
required the location (spotting) and treatment of all, or
nearly all, infested trees over extensive areas in a short
period of time.
Timely spotting and treatment are difficult and expensive
tasks that require cooperation among many landowners. Consequently,
the results have often been unsatisfactory. Also, these projects
have failed because the basic underlying cause for the population
outbreak - and abundance of stressed trees - has not changed.
Typically, if a habitat favorable to high-level western pine
beetle populations persists, suppression - by whatever means
- will probably fail to reduce tree mortality significantly.
In urban forests, preventing tree killing by the western pine
beetle is often more appropriate than attempting to suppress
beetle populations. Landowners can prevent unacceptable damage
on their land by maintaining thrifty, vigorous trees or stands
that do not afford a suitable food supply for the beetle.
Trees with a high risk of damage by beetles characteristically
have poor vigor and can be recognized by crown symptoms such
as dead tops, branches and twigs, and short, sparse, poorly
colored foliage. Also, they may be older slow-growing trees
that are heavily infected with dwarf mistletoe, that are root
disease, or that have been struck by lightning.
Prevention can also take the form of minimizing injury or
disturbance to individual trees or sites. Careful cutting
practices and care in developing urban forest land are simple,
yet effective, ways to prevent damage by western pine beetles.
Information obtained from USDA Forest Service.