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Forest Insect
& Disease
Leaflet 127
U.S. Department
of Agriculture
Forest Service
The Spruce Beetle
Edward H. Holsten,’ R.W. Thier,” and J.M. Schmid?
The spruce beetle, Dendroctonus Figure 1— Yellowish orange and reddish
rufipennis (Kirby), is the most signif- colors in the tops of trees are evidence of
spruce beetle infestation in Arizona.
icant natural mortality agent of ma-
ture spruce. Outbreaks of this beetle
have caused extensive spruce mortal- tial spruce stands. Spruce beetle dam-
ity from Alaska to Arizona and have age results in the loss of 333 to 500
occurred in every forest with substan- million board feet of spruce sawtim-
ber annually. In the past 25 years,
outbreaks have resulted in estimated
‘Entomologist, U.S. Department of Agricul-
losses of more than 25 million board
ture, Forest Service, Alaska Region, Anchor-
feet in Montana, 31 million in Idaho,
age, AK.
7Entomologist, U.S. Department of Agricul- over 100 million in Arizona, 2 billion
ture, Forest Service, Intermountain Region, in Alaska, and 3 billion in British Co-
Boise, ID. lumbia (fig. 1).
7Entomologist, U.S. Department of Agricul-
Spruce beetle outbreaks cause
ture, Forest Service, Rocky Mountain Forest
extensive tree mortality and modify
and Range Experiment Station, Fort Collins,
CO; stand structure by reducing the aver-
age tree diameter, height, and stand Hosts
density, leaving small, slow-growing The spruce beetle infests all spe-
trees and intermediate-sized trees to cies of spruce within its geographical
become dominant. range (fig. 2). The more important
As mature spruce are killed, for- commercial tree species attacked in-
age may increase, benefiting some clude white, Lutz, Sitka, and Engel-
wildlife species. But species that de- mann spruce.
pend on the mature spruce for habitat
may be adversely affected. Evidence of Infestation
Indirectly, extensive spruce mor- On standing trees, the first sign
tality can also affect water yields and of spruce beetle infestation is reddish-
result in water gains in rivers, lakes, brown boring dust accumulating at
and streams because of reduced tran- the beetle’s entrance holes, in bark
spiration from dead and dying trees. crevices, and on the ground around
the trunk of infested trees. Masses of
pitch may accumulate around the en-
Figure 2—The geographic range of the
spruce beetle.
No
trance sites. These signs are most vis-
ible the summer following infestation
and become less noticeable months
later.
On windthrown trees and log-
ging residuals, spruce beetle attacks
are readily detected on the lower
surfaces of the material and should
not be confused with Jps beetle at-
tacks more commonly found on the
upper surfaces.
Some standing trees may be at-
tacked on only one side of the bole,
creating a “‘strip attack.” The infested
area may die, but the tree usually re-
mains alive, so the foliage does not
discolor. Trees with “strip attacks”
frequently are infested by subse-
quent spruce beetle generations and
may host two or more generations
simultaneously.
During the first fall and winter
following spruce beetle infestation,
one should look for trees “debarked”’
Figure 3—/Jnfested spruce debarked by
by woodpeckers (fig. 3). Partially de-
woodpeckers.
barked, green trees are easily noticed.
However, on trees without significant Identification of the Life Stages
debarking, one must be relatively Adult beetles are blackish brown
close to see sawdust in bark crevices to black with reddish-brown or black
and around the tree base. wing covers. The beetles are cylindri-
The needles of infested trees do cal, approximately 1/4 inch (6 mm)
not usually fade or discolor within long and 1/8 inch (3 mm) wide
the first year following attack. How- (fig. 4).
ever, during the second summer fol- Spruce beetles look similar to
lowing attack most needles turn yel- other Dendroctonus beetles and, if no
lowish. Some needles even remain host material is present, can be distin-
green until the third summer, or up to guished from them only by micro-
2 years after the initial infestation. scopic examination. At first glance,
The needles on separate branches of spruce beetles may also be confused
the same tree discolor at different with Jps beetles in spruce. It is impor-
times. Needles are removed periodi- tant to remember that the posterior
cally from the trees by wind or thun- margins of the wing covers on spruce
derstorms, leaving the upper crowns
beetles are evenly rounded, while Jps
of exposed twigs with a yellowish-
beetles have wing covers with concave
orange to reddish hue.
margins and teethlike projections.
Figure 4—An adult spruce beetle.
The eggs of the spruce beetle are Life Cycle
oblong, pearly white, and 1/16-inch Spruce beetles may complete
(1.5 mm) long. The larvae are stout, their life cycle in 1 year on warm sites
cylindrical, legless grubs that pass at lower elevations or take up to 3
through 4 larval stages (instars) and years on cool, well-shaded locations
reach a length of 1/4 inch (6 mm) at on north slopes.
maturity (fig. 5). The pupae are However, it generally requires 2
Opaque white, inactive, and some- years for the spruce beetle to com-
what similar in size and shape to plete its life cycle. Adults may emerge
adults. any time from May to October, de-
pending on temperature. The beetles
attack host material soon after emerg-
ing. Adults that appear in August to
October may represent a reemergence
of parent adults or a movement of
maturing brood adults to hibernation
sites.
F igure 5— Spruce beetle larvae.
Figure 6— Spruce beetle egg gallery and larval mines
To deposit eggs, female beetles Stand Conditions Conducive to
bore through the outer bark of the Infestations
host tree and create egg galleries in Endemic spruce beetle popula-
the underlying phloem tissue. Eggs tions usually live in windthrown trees
are laid on either side of the egg gal- (fig. 7). When beetle populations in-
lery ( fig. 6). Egg galleries are slightly crease to high levels in downed trees,
wider than the beetle and, except for beetles may enter susceptible, large-
the terminal portion, are packed with diameter, standing timber. Most out-
frass and boring dust. Egg gallery breaks in standing timber originate in
length ranges from about 2.5 to 12 windthrown trees.
inches (6 to 30 cm). Eggs are usually In mature stands, large-diameter
deposited in short rows along alter- trees (218") usually are attacked
nate sides of the gallery in numbers first, an obvious characteristic denot-
ranging from 4 to 14 eggs per centi- ing susceptibility to spruce beetle at-
meter of gallery. tack. If an infestation persists in a
Most of the eggs hatch by Au- stand, smaller diameter trees are at-
gust. The larvae bore outward from tacked. Recent evidence from Alaska
the egg gallery and feed as a group for indicates that tree diameter is impor-
the first and second instars. Third tant in determining susceptibility only
and fourth instars construct individ- when coupled with less-than-average
ual feeding galleries. The larval stage radial growth in the preceding 5 years.
predominates during the first winter, The proximity of uninfested standing
although adults and eggs may also be spruce trees to infested hosts also de-
present. During the 2-year life cycle, notes vulnerability to attack.
most larvae pupate approximately 1 In the Rocky Mountain area,
year after attack. Pupation lasts 10 to susceptibility of a stand to spruce
15 days and usually takes place in pu- beetle attack is based on the physio-
graphic location, tree diameter, basal
pal chambers at the end of the larval
area, and percentage of spruce in the
galleries.
canopy. Spruce stands are highly sus-
During the second winter of the
ceptible if they grow on well-drained
2-year cycle in standing trees, some
sites in creek bottoms, have an aver-
beetles overwinter in their pupal
age diameter (d.b.h.) of 16 inches or
sites. Other beetles—from 5 to 88
more, have a basal area greater than
percent—emerge, move to the base
150 square feet per acre, and have
of the tree, and bore into the bark
more than 65 percent spruce in the
near the litter line to hibernate. In
windthrown trees, most adults over- canopy.
winter in place. Approximately 2 In Alaska, the susceptibility of a
years after attack, adults emerge spruce stand is based on average tree
from overwintering sites and attack
new host material.
Figure 7— Windthrown trees and logging residuals—prime habitat for beetle populations.
diameter, age of the stand, condition Management Strategies
of the stand, and proportion of white Forest managers can develop
spruce in the canopy. A spruce stand various strategies to avoid or reduce
of old-growth or damaged sawtimber resource losses to spruce beetles. Be-
is very susceptible to spruce beetle fore developing a strategy, the forest
attack if the larger diameter spruce manager must evaluate the resource
trees have a_ slower-than-average values and economics of manage-
growth rate, have an average diame- ment actions for each stand in light
ter (d.b.h.) greater than 12 inches, of management objectives. The bee-
and if the stand has more than 70 tle population level must also be
percent white spruce. considered because population lev-
Susceptibility of a spruce stand els will determine the priority of
to spruce beetle attack in British Co- management actions and the type of
lumbia and the Northeastern United strategy to be invoked.
States is based on criteria similar to
that used in the Rocky Mountains
and Alaska.
Hazard rating systems based on
the stand and site conditions dis-
cussed above have been developed so
that managers can identify stand sus-
ceptibility to spruce beetle attack.
The primary strategy should be _ The primary strategy assumes, in
silvicultural treatments of potentially general, beetle populations are not €
susceptible stands in order to main- immediately threatening resource
tain their health with a moderate values. If beetle populations are
growth rate. The first step in this threatening, then strategies involv-
strategy is to hazard-rate spruce ing suppression methods are more
stands, which will indicate the most appropriate. Suppression methods
susceptible stands. The stands can including silvicultural, physical, and
then be treated with harvesting di- chemical measures are available to
rected at the most susceptible stands.
forest managers for reducing spruce
Infested logging residuals need
beetle populations. Some methods
never become a significant contrib-
are suitable only for populations in
utor to spruce beetle populations if
windthrown host material; other
stump height is kept below 18 inches
methods are better suited for infes-
(45 cm) and cull logs and tops are
tations in standing trees. Most sup-
limbed, cut into short lengths, and pression methods are short-term
left unshaded, unpiled, and exposed
responses to existing beetle popula-
to sunlight. Silvicultural treatments
tions and, therefore, correct only
have greater long-term effectiveness,
the immediate situation.
because these treatments modify
stand conditions.
Le ae Sie
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Figure 8—Green trees felled to capture emerging spruce beetles.
