14 MISC. PUBLICATION 657, U. S. DEPT. OF AGRICULTURE 
are of secondary importance, and only under special conditions does 
this sudden increase result in the destruction of green, standing timber. 
On the other hand, fire-scorched trees offer unsatisfactory breeding 
material for other destructive bark beetles and may serve as traps, 
actually reducing the numbers of beetles in the area. 
FOOD AND BREEDING MATERIAL 
The abundance or scarcity of the food supply is an important factor 
governing the distribution and successful development of insects. 
Most of the insects that prey upon living forest trees are limited in 
their distribution to that of their favorite host; those that feed on 
dying or dead trees are apt to be more widely distributed through 
forest regions, because they will often attack various species with 
little or no discrimination. 
Leaf-eating insects that attack healthy forest trees have an abundant 
food supply at their disposal, and their numbers are controlled pri- 
marily by biological and chmatic factors. At times, however, 
starvation checks: very effectively the progress of an outbreak of 
defoliators and renders the weakened, underfed caterpillars more 
susceptible to certain diseases. Outbreaks of many defoliators occur 
only in areas where there is an abundance of their favored host mate- 
rial or where it is dominant in the stand. Stripping of the foliage 
by the gypsy moth, the spruce budworm, or forest tent caterpillar 
coincides with the distribution of a high percentage of favored hosts 
in the forest. 
A great many insects, such as most of the bark beetles, can develop 
in large numbers only when a sufficient quantity of their host plant 
is available in a suitable condition for attack. Thus, the development 
of destructive bark-beetle outbreaks is dependent to a large degree 
on the supply of overmature or decadent trees, fire-weakened trees, 
slash, windfalls, snowbreaks, lightning-struck trees, or trees weakened 
by drought, high water, smelter smoke, filled-in earth, disease, or 
other causes. Such material is probably the natural habitat for 
many species which at times become excessively abundant and attack 
more healthy trees. 
SLASH 
The debris left from the cutting of trees in the forest is a suitable 
and attractive breeding material fora oreat many forest insects, some 
of them beneficial and some harmful species. When the slash is fresh, 
the dying inner bark is attractive to many species of bark beetles that 
are commonly found breeding in dying trees still standing. Usually 
these bark beetles select slash or stumps of a type and size similar to 
the parts of standing trees in which they would normally breed. 
Thus the limb- and twig- feeding bark beetles go into the brush and 
smaller pieces of slash; “trunk -breeding bark beetles go into the cull 
logs and butts, while those that nor mally work at the base of the tree 
attack the stumps. The abundance of the progeny depends a great 
deal on how the moisture and temperature conditions within the slash 
meet the requirements of the different species of beetles. 
The red turpentine beetle frequently develops in such numbers in 
pine stumps as to do serious injury to adjacent living trees. Large 
numbers of trunk-breeding pine bark beetles attack cull logs and 
butts, but they rarely find conditions suitable for dev elopine large 
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