INSECT ENEMIES OF WESTERN FORESTS 219 



trees best adapted to it, to their proper spacing, and to the 

 regulation of drainage, temperature conditions, and stand density. 

 Mixed stands are also less susceptible to serious injury than 

 pure stands. These are just a few of the possibilities that suggest 

 themselves in which insect activity can be modified through 

 silvicultural practices. 



In the overmature virgin forests of ponderosa pine, bark 

 beetles are not indiscriminate in their attacks but make a selec- 

 tion of certain trees or groups of trees scattered through the 

 stand (133). A study of the types of trees selected has shown 

 that in general the more slowly growing trees, the codominants 

 and intermediates in the stand, and the older age classes are 

 selected in preference to the thrifty, dominant, young trees (91). 

 It also has been found that trees currently in poor health are of 

 highest risk to western pine beetle attack (1U1) and that beetle 

 control can be obtained over a period of at least 12 years through 

 sanitation-salvage logging, in which the high-risk trees, con- 

 stituting usually from 15 to 25 percent of the stand, are removed 

 and utilized. Instead of cutting heavily on small logging units, 

 forest managers are favoring a light selective system whereby 

 large areas will be opened up so that insect-killed and high-risk 

 trees can be quickly harvested and stands improved in both 

 growth and insect resistance (126, 127). 



Under management, the age at which certain stands become 

 susceptible to beetle attack will necessarily be taken into con- 

 sideration, and a cutting rotation adopted that will permit the 

 timber crop to be harvested before the beetle hazard becomes too 

 great. Lodgepole 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 

 regulated to avoid serious damage from insect attack. Pure 

 stands— 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 

 composed of a high percentage of hemlock. Where a heavy mix- 

 ture 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 per- 

 centage of true firs and Douglas-fir. 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 stands there is usually a natural balance between the 

 tree species, the vegetative ground cover, and the insects asso- 

 ciated in this type of forest. Stand density has an important 

 bearing on temperature and moisture conditions and often 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 that are adapted to the growing of trees, or growth 



