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Forest Insect
& Disease
Leaflet 127
United States
Department of
Agriculture
Forest
Service
The Spruce Beetle
Ee Holsten!, R.W. Thier2, AS. Munson, and K.E. Gibson
rh
Figure 1 — Yellowish orange and reddish colors of faded spruce
he spruce beetle, Dendroctonus more than 122,000 acres and killed more
rufipennis (Kirby), is the most than 3,000,000 spruce trees. In the past
significant natural mortality 25 years, outbreaks have resulted in es-
agent of mature spruce. Outbreaks of this timated losses of more than 25 million
beetle have caused extensive spruce board feet in Montana, 31 million in
mortality from Alaska to Arizona and Idaho, over 100 million in Arizona, 2
have occurred in every forest with sub- billion in Alaska, and 3 billion in Brit-
stantial spruce stands. Spruce beetle ish Columbia (fig. 1).
damage results in the loss of 333 to 500
million board feet of spruce saw timber
l Entomologist, U.S. Department of Agriculture,
annually. More than 2.3 million acres of
Forest Service, Anchorage, AK.
spruce forests have been infested in 2 Entomologist, U.S. Department of Agriculture,
Alaska in the last 7 years with an esti- Forest Service, Boise, ID.
mated 30 million trees killed per year at 3 Entomologist, U.S. Department of Agriculture,
Forest Service, Ogden, UT.
the peak of the outbreak. In the 1990’s,
3 Entomologist, U.S. Department of Agriculture,
spruce beetle outbreaks in Utah infested
Forest Service, Missoula, MT.
Spruce beetle outbreaks cause extensive Hosts
tree mortality and modify stand struc-
ture by reducing the average tree diam-
The spruce beetle infests all species
eter, height, and stand density. Residual
of spruce within its geographic range
trees are often slow-growing, small and
intermediate-sized trees which eventu- (fig. 2). The more important commer-
ally become dominant. cial species of spruce attacked include
Spruce beetle outbreaks can affect white (Picea glauca [Moench] Voss),
non-timber resources as well. For ex- PUtZAC Pee LUiciileitle),-oltkaace
ample, as mature spruce are killed, for- sitchensis [Bong.] Carr.), and Engel-
age may increase benefiting some wild-
mann (P. engelmannii Parry ex
life species. But species that depend on
Engelm.).
mature spruce or clumps of spruce to
meet habitat requirements may be ad-
versely affected. There has been a sig-
nificant change in fuel type and an in- Evidence of Infestation
crease in large woody debris accumu-
lating on the forest floor following
On standing trees, the first sign of
spruce beetle outbreaks in Alaska. Un-
spruce beetle infestation is reddish-
characteristic, stand-replacing fires oc-
brown boring dust accumulating at the
curred in central Idaho spruce stands
following 10 years of spruce beetle out- beetles’ entrance holes, in bark crevices,
breaks. Extensive spruce mortality can and on the ground around the trunk of
also affect water yields resulting in wa- infested trees. Masses of pitch may ac-
ter increases in rivers, lakes, and streams cumulate around entrance sites. These
because of reduced transpiration from signs are most visible during the sum-
dead and dying trees. Scenic quality may
mer of attack and become less notice-
also be diminished throughout affected
able the following season.
landscapes.
Figure 2 — The geographic
range of the spruce beetle.
i)
On windthrown trees and logging re-
siduals, spruce beetle attacks are readily
detected on the lower surfaces of the
material and should not be confused
with engraver beetle (/ps spp.) attacks
more commonly found on the upper
surfaces.
Some standing trees may be attacked
on only one side of the bole, creating a
“strip attack." The infested area may
die, but the tree usually remains alive
so the foliage does not discolor. Trees
with “strip attacks” frequently are in-
fested by subsequent spruce beetle gen-
erations and may host two or more gen-
erations simultaneously.
During the first fall and winter follow-
ing spruce beetle infestation, look for trees
“debarked” by woodpeckers (fig. 3). Par-
tially debarked, green trees are easily no-
ticed. However, on trees without signifi-
cant debarking, one must be relatively
close to see boring dust in bark crevices
and around the tree base. Figure 3 — Infested spruce debarked
The needles of infested trees do not by woodpeckers.
usually fade or discolor within the first
year following attack. However, during
the second summer most needles turn Spruce beetles are similar to other
yellowish-green or orange-red (Alaska). Dendroctonus beetles and, if no host
Some trees remain green until the third material is present, can only be distin-
summer or up to 2 years after the initial guished by microscopic examination. At
infestation. The needles on separate first glance, spruce beetles may also be
branches of the same tree discolor at dif- confused with /ps beetles in spruce. It
ferent times. Needles on infested trees is important to remember that the pos-
commonly drop as a result of wind or terior margins of the wing covers on
thunderstorms the second summer af- spruce beetles are evenly rounded,
ter the tree was attacked. Upper crowns while /ps beetles have concave margins
first turn yellow to reddish-orange. En- with teeth like projections.
tire crowns later turn grey. Eggs of the spruce beetle are oblong,
pearly white, and 1/16 inch long. The
cream-colored larvae are stout, cylin-
Identification of Life Stages
drical, legless grubs that pass through
four larval stages (instars) and reach a
Adult beetles are dark brown to black length of 1/4 inch at maturity. The pu-
with reddish-brown or black wing
pae are opaque, inactive, and somewhat
covers. The beetles are cylindrical,
similar in size and shape to adults.
approximately 1/4 inch long, and 1/8
inch wide (fig. 4).
Figure 4 — An adult spruce beetle.
Life Cycle
Spruce beetles may complete their life ment of maturing brood adults to hiber-
cycle in | year on warm sites at lower nation sites.
elevations or take up to 3 years in cool, To deposit eggs, females bore through
well-shaded locations on north slopes. the outer bark of host trees and create
However, throughout most of its egg galleries in the underlying phloem
range and in most seasons, 2 years are tissue. Eggs are laid on either side of
generally required for the spruce beetle the egg gallery (fig. 5). Egg galleries
to complete its life cycle. Although are slightly wider than the beetle and,
adults may emerge any time from May except for the terminal portion, are
to October, depending on temperature, packed with frass and boring dust. Egg
most attacks occur in early summer. gallery length ranges from about 2.5 to
Adult beetles attack host material soon 12 inches (fig. 6). Eggs are usually de-
after emerging. Adults that appear in posited in short rows along alternate
August to October may represent a re- sides of the gallery in numbers ranging
emergence of parent adults or a move- from 10 to 30 eggs per inch of gallery.
Figure 5 — Spruce beetle eggs along side of egg gallery.
Figure 6 — Spruce beetle larval galleries.
Most eggs hatch by August. Larvae In mature stands, larger diameter (>
bore outward from the egg gallery and 18 inches d.b.h.) trees usually are at-
feed as a group for the first and second tacked first, an obvious characteristic
instars. Third and fourth instars con- denoting susceptibility to spruce beetle
struct individual feeding galleries. The attack. If an infestation persists in a
larval stage dominates during the first stand, smaller diameter trees are at-
winter, although parent adults and eggs tacked. Recent evidence from Alaska in-
may also be present. During the 2-year dicates that tree diameter is important
life cycle, most larvae pupate approxi- in determining susceptibility only when
mately | year after attack. coupled with less-than-average radial
Pupation lasts 10 to 15 days and usu- growth in the preceding 5 years.
ally takes place in pupal chambers at In the Rocky Mountain area, suscep-
the end of larval galleries. tibility, or hazard of a stand to spruce
During the second winter of the 2- beetle attack is based on physiographic
year life cycle in standing trees, some location, tree diameter, basal area, and
beetles overwinter in their pupal sites percentage of spruce in the canopy.
but the majority, often as high as 95 Spruce stands are highly susceptible if
percent of the new adults emerge, move they grow on well-drained sites in creek
to the base of the tree, and bore into the bottoms, have an average d.b.h. of 16
bark near the litter line to overwinter. inches or more, have a basal area greater
Overwintering at the base of the infested than 150 square feet per acre, and have
tree reduces predation by woodpeckers more than 65 percent spruce in the
and reduces winter mortality due to ex- canopy.
tremely cold temperatures. In In Alaska, susceptibility of a spruce
windthrown trees, most adults overwin- stand is based on average tree diameter,
ter in place. age of stand, condition of the stand, and
Approximately 2 years after attack, proportion of spruce in the canopy. A
adults emerge from overwintering sites spruce stand of old-growth or damaged
and attack new host material. host trees is susceptible to spruce beetle
attack, particularly if larger diameter
trees have a slower-than-average
growth rate, have an average d.b.h.
Stand Conditions greater than 12 inches, and if the stand
Conducive To Infestations is more than 70 percent spruce.
Susceptibility of a spruce stand to
spruce beetle attack in British Colum-
Endemic spruce beetle populations
bia and the northeastern United States
usually live in windthrown trees (fig.
is based on criteria similar to that used
7). When populations increase to high
in the Rocky Mountains and Alaska.
levels in downed trees, beetles may en-
Hazard and risk (= probability of an
ter susceptible, large-diameter standing
trees. Most outbreaks in standing tim- outbreak) rating systems based on the
stand and site conditions discussed
ber originate in windthrown trees.
above have been developed which en-
able managers to identify stand suscep-
tibility to spruce beetle attack.
6
Figure 7 — Windthrown trees and large diameter logging residuals — prime habitats for
beetle populations.
susceptibility. Infested logging residu-
Management Strategies
als will not become a significant con-
tributor to spruce beetle populations if
Forest managers can develop various stump height is kept below 18 inches
Strategies to avoid or reduce resource and cull logs and tops are limbed, cut
losses to spruce beetles. Before devel-
into short lengths, and left unshaded,
oping a strategy, the forest manager unpiled, and exposed to sunlight. Silvi-
must evaluate resource values and eco- cultural treatments have greater long-
nomics of management actions for each term effectiveness because treatments
stand combined with overall manage- modify stand conditions that contribute
ment objectives. The beetle population to spruce beetle population increases.
level must also be considered because Silvicultural strategies may be more
population levels will determine the effective if beetle populations are not
priority of management actions and the immediately threatening resource
type of strategy to be invoked. values. If beetle populations are
A principal strategy that should be
threatening, then strategies that include
considered for susceptible sites would suppression methods are more
consist of silvicultural treatments of appropriate. Suppression methods,
moderate-to-high hazard stands that re- including silvicultural, physical (fire
sult in maintaining their health with a
and solar heat), and chemical measures,
moderate growth rate. The first step in are available to forest managers for
this strategy is to hazard rate spruce reducing spruce beetle populations.
stands, which will designate the most Some methods are suitable only for
susceptible stands. Moderate-to-high populations in windthrown host
hazard stands can then be treated using
material; other methods are better suited
silvicultural strategies that reduce stand for infestations in standing trees. Most
Figure 8 — Green trees (“trap trees”) felled to capture emerging spruce beetles.
suppression methods are short-term
responses to existing beetle populations
and, therefore, address only an @ Trap trees are green trees with a
immediate need. d.b.h. greater than 12 inches that are
felled before beetle flight. Trap trees can
absorb up to 10 times the number of
spruce beetles that a standing tree will
Silvicultural Methods absorb. Once infested, trap trees must be
removed.
Sanitation overstory removal entails Trap trees shaded from direct sunlight
removal of all infested and susceptible attract the most beetles. Spruce beetles
spruce to encourage regeneration of a attack cool, shaded portions of the trap
new vigorous stand. Sanitation partial cut tree boles (fig. 8). Felled trees should not
includes the removal of infested and sus- be delimbed. Limbs on the upper side of
ceptible spruce to improve the growth the bole provide shade while limbs on
of the residual stand. Sanitation partial the underside permit the beetles to colo-
cuts remove most of the larger trees but nize the underside of the bole by keep-
may leave a residual stand that 1s below ing it off the ground.
the recommended level of basal area. Past ratios of trap trees to infested
This residual stand may be more suscep- standing trees have ranged from 1:2 to
tible to windthrow. Pruning the lower 1:10. Current ratios vary with the size of
one-third of the live crown of smaller di- the green trees to be felled as traps com-
ameter trees significantly reduces the pared to the number and size of infested
susceptibility of spruce to beetle attack trees and the existing beetle population.
in Alaska.
