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FIR
ENGRAVER
(Scolytus ventralis)
Hosts:
Grand fir, rarely Douglas-fir, Engelmann spruce, mountain hemlock,
western larch
Quick ID:
- Reddish-brown
or white boring dust in bark crevices or at base of tree
- Egg galleries
straight, across the grain (horizontal)
- Larval galleries
at 90 from the egg gallery
- Galleries
deeply etch sapwood
- Galleries
free of frass
- Foliage
may thin, yellow, or redden
 Field
Identification
Tree: In
Washington this beetle primarily attacks grand fir, although it
has been observed on occasion attacking Douglas-fir, Engelmann spruce,
mountain hemlock and western larch.
Reddish-brown or white boring dust may be found in bark crevices,
caught in spiderwebs, or at the base of the attacked tree. No pitch
tube is found as true firs are nonresinous. Under
the bark, the egg gallery is straight and perpendicular to the grain
(horizontal). Midway along its length a larger nuptial chamber is
found. Larval galleries are formed at right angles to the egg
gallery. The egg gallery and the larval galleries deeply etch the
sapwood, leaving marks that can easily be identified well after
the death of the tree. Both egg and larval galleries are free of
frass (tightly packed boring dust and insect droppings). On successfully
attacked trees, the foliage may thin, yellow, or redden. On unsuccessfully
attacked trees, the attack may heal over leaving an area of rough
bark 1/4 to 1/2 inch across.
Insect:
Adult fir engravers average about 4 mm in length. They are black
and the posterior of the abdomen is prominently concave on the underside.
Larvae are small and white.
May be confused
with: No other bark beetle has straight, across-the-grain galleries.
Life cycle:
Adult insects emerge and fly in search of new hosts from June to
September, with the greatest emergence in mid-July. Attacks seem
to be made at random on both resistant and susceptible trees. Females
attack trees first, tunnel into the inner bark, and await the males
in the nuptial chamber. Other females will attack the same tree
after a first attack, particularly if the first attack was successful.
After mating, the female will tunnel horizontally from the nuptial
chamber, depositing eggs in niches cut in the walls. Resistant reactions
by the host tree may cause the female to abandon the attack at this
point; in this case, the attack heals over, leaving a brown
pitch pocket in the wood that will degrade the value. In contrast
to Douglas-fir and mountain pine beetles, fir engravers are not
very tolerant of the resins produced by resistant trees, and will
readily abandon the attack if they incur too much resistance. Furthermore,
eggs laid in resistant trees will be poisoned by these resins. However,
in a susceptible tree, eggs will hatch within two weeks. Larvae
tunnel away from the egg gallery to feed, and pupate at the end
of the larval mine. Larvae mostly overwinter before pupating and
developing into adults in the spring. New adults bore through the
bark in summer and seek out new hosts. Normally there is one generation
per year.
Predisposing
agents: Fir engravers are a secondary pest, meaning that they
cannot successfully attack and kill healthy vigorous trees (in contrast
to other bark beetles such as Douglas-fir, mountain pine and western
pine beetles). They prey upon weakened, dying, or recently killed
fir trees, and are well-adapted to finding this normally scattered
food source. Certain agents of stress have been shown to predispose
firs to engraver attack. These include drought; overstocking, especially
if a high percentage of the stand is true fir; root disease, particularly
Annosus root disease; defoliation, particularly by Douglas-fir tussock
moth; and the presence of abundant dead
material such as slash and windthrow nearby.
Impact:
Fir engraver is considered to be a major pest of true fir in western
forests, but beetle-induced mortality can mask mortality due to
predisposing factors such as drought and root disease. Nevertheless,
in 1990 the USFS reported the fir engraver had killed 238,000 trees
over 245,000 acres in Washington, with a loss of 12.744 million
cubic feet of lumber. In 1991, it was responsible for the death
of 68,674 trees over 146,542 acres with a volume loss of 3.684 million
cubic feet of lumber.
Management:
Direct control measures (e.g. felling, burning, spraying) for fir
engravers are impractical. Therefore, minimizing stand stresses
to keep trees healthy and vigorous is the only real option for control.
Fir engravers
evolved here together with grand fir, and as such will never be
completely eradicated. Under normal conditions, they serve to thin
out weak trees and open up the stand for regeneration. The species
shift in Eastern Washington in the last century away from ponderosa
pine and western larch and towards grand fir and Douglas-fir, coupled
with the suppression of fire, has created hundreds of thousands
of acres of true firsunder stress from overstocking and served to
create a habitat for fir engravers that is very favorable. Gradual
correction of that situation by good stand management practices
will bring down the incidence of fir engraver attack.
Many stand
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 or making a seed tree/shelterwood
cut to mimic a fire. Particularly with shade-tolerant species
such as grand fir, keep in mind other stand health issues such
as planting other, more resistant species on root disease centers.
- Remove diseased
and unhealthy trees and logging debris; minimize soil compaction
and damage to residual trees. However, salvage logging of fir
engraver-killed trees should only be undertaken with caution:
in Washington fir engravers generally indicate the presence of
root diseases, and salvage logging on root disease centers has
been shown to worsen their severity (see WSU Cooperative Extension
leaflets on root diseases).
- Practice
"good housekeeping" in the forest by removing windthrown
and fire-damaged material before fir engraver beetles breed in
it. Large numbers of cull stems should not be left in the forest
after a logging operation.
- 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.
Adapted from
Berryman, A.A. Forest insects and Population dynamics of forest
insects.
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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