Field
Identification
Tree: Boring dust from the beetle's entry may be found
in bark crevices or at the base of the tree; it will be reddish
or yellowish. No pitch tube is found on the trunk, but resin streamers
may form from attacks on the upper trunk. Egg galleries are similar
to those of the mountain pine beetle in that they are long and
straight, running with the grain, and have the same 90 bend at
the bottom; they are typically 6 to 30 inches in length. Larval
galleries fan out from alternate sides of the egg gallery, moving
across the grain. These galleries are packed with frass (mixed
sawdust and insect droppings). A pupal chamber may be found at
the end of the larval mine. The foliage of attacked trees turns
yellow and then fades to reddish-brown by late summer or fall,
though it may hang on until the following spring.
Insect:
Adults are 4.4 to 7 mm in length, dark brown to black, and
rather hairy. Larvae are small, white, and resemble grains of
rice.
May
be confused with: Nothing else on Douglas-fir.
Life
cycle: Adult insects emerge and fly from April through August.
Normally, Douglas-fir beetle only breeds in felled, injured or
diseased trees, although epidemics occur where apparently healthy
trees are colonized. In Washington, this is generally the result
of a population explosion following drought, windstorm with extensive
windthrow, or large fire. Western larch is also attacked, but
brood is produced only in down trees. Females attack the tree,
bore through the bark and release pheromones that attract both
other females and males. A pair of beetles will work together
to construct the egg gallery, which will be mostly in the inner
bark but will slightly etch the sapwood. Eggs are laid in groups
on alternating sides of the gallery. Larvae hatch and tunnel individually
out from the egg gallery; as they get further from the egg gallery,
larval mines fan out. Both the egg gallery and the larval galleries
become packed with frass. After the larvae has developed, it hollows
out a pupal chamber at the end of the mine and pupates. The new
adults congregate together under the bark, sometimes for long
periods. Eventually they bore through the bark and fly off to
find another host. There is one generation every year, and larvae
and adults overwinter inside trees.
Predisposing
agents: Douglas-fir beetles successfully attack trees that
have been weakened by some stress. These agents of stress can
be abiotic (such as drought and fire) or biotic (overstocking
causing competition, diseases, insect attack, etc.). Root diseases
are heavily implicated in predisposing trees to bark beetle attack.
The presence of Armillaria root disease is indicative of advanced
stand degeneration, but laminated root rot is also a predisposing
factor. Dwarf mistletoe also weakens trees and reduces growth.
Douglas-fir beetles will breed in down trees that have been felled
by wind, as well as fire-injured trees. Insect defoliators such
as Douglas-fir tussock moth and western spruce budworm can severely
weaken trees and predispose them to attack. Lastly, logging damage
on residual trees can also predispose trees to attack.
Impact:
This is the most important bark beetle on Douglas-fir throughout
its range. Losses on larch are insignificant as brood are not
hatched in standing trees. According to the USFS, in 1990 Douglas-fir
bark beetles killed 23,200 trees over 29,300 acres in Washington,
causing a loss of 3.086 million cubic feet. In 1991, they killed
16,097 trees over 16,991 acres causing a loss of 2.067 million
cubic feet.
Management:
Direct control (e.g. felling, spraying, etc.) has not been attempted
often due to its poor success. In Washington, it is possible to
prevent damage by removing the windthrown trees that permit a
large population of beetles to develop. Trees can also be protected
by attaching capsules of MCH, a Douglas-fir beetle repellant,
to their boles.
Douglas-fir
beetles are a natural part of western ecosystems. Total eradication
is neither possible nor desirable, as the beetles normally remain
at a steady level and are responsible for thinning weak trees
and thus opening the stand for regeneration. The death of a few
trees on your property may tip you off about a root disease problem,
which will influence your choice of species and silvicultural
treatment used to regenerate. Information about root disease symptoms
is also available in WSU Cooperative Extension's "Forest
Health Notes." Or in the absence of root disease it may be
a hint that your stand is overstocked. It doesn't necessarily
mean the bark beetles are going to have an epidemic outbreak on
your property.
Many
site management techniques work well to prevent outbreaks. Attention
to root disease centers and overstocking are two big steps towards
a healthy, insect-resistant stand. 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 cuts such as seed tree or shelterwood
cuts (for Douglas-fir).
Particularly with shade-tolerant species like Douglas-fir, keep
in mind other stand health issues such as discouraging dwarf mistletoe
and regenerating other species on root disease centers.
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 could increase the severity of Armillaria,
Annosus and laminated rot (see WSU Cooperative Extension "Forest
Health Notes" on root rots for more thinning/salvage tips).
Practice "good housekeeping" in the forest by removing
windthrown and fire-damaged material before Douglas-fir beetles
breed in it and create an outbreak. Large numbers of cull stems
should not be left in the forest after a logging operation, especially
if they're shaded. Encourage diversity in species and age classes.
A mixed-species stand is much more resistant to insect pests and
diseases than is a pure stand. Use thinning, fertilization, prescribed
fire, etc. to maintain stand diversity and vigor.
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
bark 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.
