INSECT ENEMIES OF WESTERN FORESTS 173 



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, 

 Avhich 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 



As has been previously mentioned, parasitic and predacious insect 

 enemies and insectivorous birds and manmials are often of great value 

 in holding destructive forest insects in check. To increase the effec- 

 tiveness of these natural control agencies through artificial means 

 offers hopeful possibilities but is beset with many difficulties. 



Where native insects which already have a full complement of nat- 

 ural enemies are concerned, an effort can be made to create favorable 

 conditions for the multiplication of these beneficial agencies. Di- 

 rect control methods, such as burning or sun-curing, can be modified 

 so that the beneficial insects will not be destroyed in as large num- 

 bers as the harmful species. Sometimes only a slight change in this 

 direction will give the beneficial insects the upper hand, and they 

 will quickly bring an epidemic back to normal balance. To increase 



