INSECT ENEMIES OF WESTERN FORESTS 167 
many of the Temperate Zone species appears to be about 77°. Tem- 
peratures either above or below this optimum range limit activity 
and extremes cause death (62, 63). Few insects can withstand tem- 
peratures above 120° and this makes possible the control of many 
species of bark- and wood-boring insects by raising the temperature 
of their environment to fatal heights. Low temperatures also are 
fatal. It has been found that larvae of the western pine beetle are 
killed when exposed to a temperature of —10° and extremely cold 
winters with air temperatures below —20° have proved fatal to a 
high percentage of this and other bark-beetle broods (4a) (45a). 
Moreover, elevation and exposure modify temperature and limit the 
distribution of insect species, regulate their activity, and govern the 
number of generations per year. _ 
MOISTURE 
Moisture has an important bearing on insect abundance, both 
through its direct effect upon the insects and indirectly through its 
influence upon the host. Some insects require very moist conditions 
under which to work to best advantage and are killed by dryness; 
others require very dry conditions and are killed by moisture. More- 
over, moisture through precipitation has an important influence upon 
the growth and resistance of trees to bark-beetle attack; and periods 
of drought are frequently followed by serious consequences in 
supplying insects with an abundance of weakened host material. 
FOOD SUPPLY 
The abundance or scarcity of the food supply is an important 
consideration in determining the distribution and successful develop- 
ment of insect progeny. Most of the insects which prey upon living 
forest trees are limited in their distribution to that of their favorite 
host, whereas those which feed upon dying or dead trees are apt to 
be widely distributed throughout forest regions. 
Insects, such as the defoliators, that attack healthy forest trees 
always have an abundant food supply at their disposal, and their 
numbers are controlled primarily by biological and climatic factors. 
On the other hand, a great many insects, such as most of the bark 
beetles, can develop in large numbers only when a sufficient quantity 
of their food material in a suitable condition for attack is available. 
Thus the development of certain destructive bark-beetle outbreaks 
is dependent to a large degree upon a supply of overmature or deca- 
dent trees, fire-weakened trees, slash, windfalls (67), snow-broken 
or lightning-struck trees, or trees weakened by drought, smelter 
smoke, disease, or other causes. Under favorable conditions, however, 
a few species of bark beetles can develop in epidemic numbers 
without such contributing factors. 
In the virgin pine forests of the West, a high percentage of the 
trees are mature or decadent. They grow more slowly, and resistance 
to beetle attack is lessened in other ways during periods of drought. 
Such conditions present a highly favorable field for bark-beetle 
activity, and the heavy losses that have been sustained in the pine 
regions during the past few years indicate very clearly that the bark 
beetles have not neglected this opportunity. 
