INSECT ENEMIES OF WESTERN FORESTS O7 
and harden on the bark in various forms of pitch or resin tubes. 
Some species construct ventilation tunnels at intervals along the ege 
galleries. These are perpendicular to the egg galleries and extend 
through the bark to the surface or may end before the surface is 
reached. Boring dust is pushed out of those that are open at the 
surface of the bark, and they all are probably used as turning 
niches as well as for ventilation of egg tunnels. Later, as the mine 
progresses, these are sometimes plugged with boring dust. 
The eggs, which are very small, are oval, round, or slightly elon- 
gate, and clear or chalky white. They are deposited in small cup- 
shaped cavities along the sides of the egg galleries. Usually a single 
ege@ is placed in each cavity or egg niche, which is closed with a plug 
of “boring dust in such a way that the smooth cylindrical egg gallery 
is but little altered. Some species cut larger cavities or egg pockets 
and deposit from two to eight eggs in each. Others cut an elon- 
gated groove on one or both ‘sides of the ege gallery and deposit the 
egos in layers or rows. 
“The larvae or grubs are thick-bodied, always legless, cylindrical 
and curved, white or cream colored, with a distinct head and promi- 
nent dark-colored mandibles. At first the larvae and their mines are 
very small, but both increase in size as feeding progresses. The 
larval mines start away from the egg gallery more or less at right 
angles and may continue nearly straight or turn and run parallel to 
the egg tunnel. They are always packed with excrement and boring 
dust. 
Transformation to the pupal stage takes place at the end of the 
larval mine in a specially constructed pupal cell. The pupae are 
soft, white, and unprotected. The antennae, mandibles, legs, and 
wing pads are clearly visible, and hairs and spines are often ‘present 
on the various regions of the body. Gradually the pupae darken, 
turning light yellow and then brown, as the adult form is reached. 
The adults, after a short hardening period, emerge and fly t 
attack new host trees, or congregate in cavities under the bark of the 
old host tree, or drop to the ground to hibernate. Some adults do 
a certain amount of feeding under the bark before emerging, and 
food tunnels made in this way are quite distinct in character from 
the regular egg galleries. Others upon emerging feed upon twigs 
or buds of other trees before again attacking the bark of a new 
host. 
Normal or endemic infestations of bark beetles are present in 
practicaliy all mature forests (S/), causing an annual ioss of a frac- 
tion of 1 percent of the timber on the area. Under conditions favor- 
able to the insects, serious epidemics develop from these normal 
infestations in a very few years. Such outbreaks may be of short 
duration, or they may continue for many years, destroying large 
volumes of merchantable timber over extensive acreages. 
If bark-beetle attacks are to be successful, the attacking insects 
must be present in suificient numbers to overcome the resistance of 
the tree. Dead and dying trees offer little resistance to attack and 
for this reason they are usually chosen by the secondary species that 
are not capable of coping with a vigorous pitch flow. Light attacks 
by primary species on living trees often fail because the flow of 
pitch is so copious that the attacking beetles are overcome or driven 
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