INSECT ENEMIES OF WESTERN FORESTS 17: 
by large areas will be opened up so that insect-killed trees can be 
quickly salvaged and stands improved from the standpoints of both 
growth and insect resistance. 
Under management, the age at which certain stands become 
susceptible to beetle attack will necessarily be taken into considera- 
tion, and a cutting rotation adopted that will permit the timber crop 
to be harvested before the beetle hazard becomes too great. Lodge- 
pole pine is a good example of a tree whose short life is largely a 
result of periodic, devastating outbreaks of the mountain pine beetle. 
In some cases stand composition and density will have to be regu- 
lated to avoid serious damage from insect attack. Pure stands, 
which are those composed of a single tree species, are particularly 
susceptible to disastrous outbreaks. For instance, outbreaks of the 
hemlock looper have been especially destructive only in stands com- 
posed of a high percentage of hemlock. Where a heavy mixture of 
other species occurs the infestation soon thins out and loses its 
destructive power. Attacks of the spruce budworm also have been 
most destructive in stands composed of a high percentage of balsam 
and Douglas firs. It is particularly important that cuttings, in 
stands that normally grow as mixed types, should not favor the 
leaving of a single species. This is not so important in stands that 
normally occur in nature as pure types, for in such cases there is 
usually a natural balance between the tree species, the vegetative | 
ground cover, and the insects that are associated in this type of 
forest. : 
The stand density has an important bearing upon temperature 
and moisture conditions and in many cases must be regulated so as 
to improve growth rates and discourage the attacks of certain insects. 
Many of these problems of silvicultural control become increas- 
ingly important when new plantations are established. Sites must 
be selected which are adapted to the growing of trees, or growth 
will be so poor and the trees so weak that insects will have a fertile 
field for their activities. Here, too, there is the opportunity to select 
varieties of trees that are not only adapted to the particular site 
but are insect-resistant or capable of making such rapid growth as to’ 
overcome any set-backs from insect attack. Drainage, the mixture 
of species, and the spacing of the trees also must be given special 
consideration. 
BIOLOGICAL CONTROL 
