Mountain Forests - Mountains and Plains: The Ecology of Wyoming Landscapes - page 184

the beetles. Less vigorous trees are unable to produce
enough resin or chemical defenses to repel the beetles. 35
Following a successful attack, one often sees fine saw-
dust around the holes made by the beetles and at the
base of the tree.
the mountain pine beetle is a tiny insect about 0.2
inch long, about the size of a grain of rice. the first few
female beetles to successfully burrow through the bark
promote mass attacks by emitting an aromatic chemi-
cal, known as an aggregating pheromone, which attracts
other mountain pine beetles to the same tree. 36 curi-
ously, once a sufficient number of beetles have been
attracted to the tree, the successful beetles emit a dis-
aggregating pheromone, which repels other beetles—an
amazing adaptation that helps ensure the survival of
the species. if too many beetles attack the same tree,
fewer would reproduce successfully. Such specific
responses to specific stimuli by both the beetles and the
tree are a reflection of the long history of co-evolution
of the insect and its host. 37
Following successful attack, the females lay eggs in
their galleries. At the same time, the spores of a fungus—
known as the blue-stain fungus—are introduced. 38 t he
fungal filaments grow into the sapwood, where they
restrict the flow of water and nutrients and kill the
tree in less than a year. 39 t he beetle eggs soon hatch,
and the larvae begin feeding on the inner bark, mov-
ing horizontally around the tree, cutting through the
phloem and stopping the flow of carbohydrates from
the leaves to the roots. Although this girdling surely con-
tributes to the death of the tree, trees stressed only by
girdling can live for 4-5 years . 40 it seems clear that the
plugging of the sapwood by the blue-stain fungus is the
primary factor causing tree death. notably, the larvae
also obtain important nutrients by feeding on the blue-
stain fungu s. 41
in the fall, the larvae begin to produce antifreeze-
like chemicals in their bodies. they also eliminate
substances that could serve as ice-nucleating agents.
this process of cold hardening greatly increases winter
survival with minimal protection from the cold—only
a thin layer of bark. nevertheless, long periods of very
cold temperatures at the right time of year can result in
high mortality of beetle larvae, and such cold-related
mortality may be an important mechanism preventing
or terminating a bark beetle outbreak. no single lethal

temperature threshold applies in all places and at all
times. Great genetic and physiological variations occur
among different beetle species—and among geographi-
cally segregated populations of the same species—in
the rate, timing, and effectiveness of cold hardening.
in general, however, bark beetle larvae are most vulner-
able to a spell of cold weather in late fall and early win-
ter, before the cold hardening process is completed, as
well as in late winter and early spring, when warmer
temperatures have stimulated the beginning of a break-
down of the antifreeze compounds. in the late spring or
summer, after further feeding and maturation, another
generation of adult beetles emerges to attack other live
trees, and the cycle continues.
this life cycle enables bark beetles to persist at low
numbers, killing the occasional weak or stressed tree
but having little impact on overall forest structure.
How, then, does a bark beetle population suddenly
explode into an outbreak in which the beetles kill so
many trees that everyone notices the effects? two con-
ditions are required: suitable climate conditions and
susceptible trees. Warmer temperatures affect both, by
enhancing survival of beetle larvae and contributing
to the development of tree water stress, thereby reduc-
ing the amount of resin produced and making the trees
more susceptible to beetle attack. ecologist teresa chap-
man and colleagues found that recent mountain pine
beetle outbreaks in northern colorado and southern
Wyoming closely followed a period of warming tem-
peratures, which included several years of reduced pre-
cipitation . 42 Large numbers of trees were vulnerable to
beetle attack, because they were stressed by drought and
were of sizes preferred by beetles.
As forests age, the trees experience increased compe-
tition from their neighbors and many become more sus-
ceptible to beetle epidemics. Defensive chemicals, such
as resin, are thought to have low priority in the alloca-
tion of limited carbon compounds by trees, compared
to the maintenance of leaf area and young roots, with
the result that less resin is produced and beetle inva-
sion is more likely. Also, damaged trees, such as those
scarred by lightning strikes, fire, or logging activity,
are more susceptible. Some research suggests that the
microclimate of closed, old-growth forests is a contrib-
uting factor to initiating an outbreak. According to this
hypothesis, closed canopies minimize air movement,