3^72 MISC. PUBLICATION 273, U. S. DEPT. OF AGRICULTURE 



established in the country are hedged about with domestic quarantines 

 for the prevention of their further spread, while every effort is 

 made to exterminate them before they have become so firmly en- 

 trenched as to make such efforts impractical. If these efforts fail, a 

 search is made in their native homes for the specific parasites and 

 predators which normally hold them in check there, and these are in- 

 troduced and their establishment attempted in this country. Work 

 of this sort has been highly successful in controlling many foreign 

 pests that have been inadvertently introduced. 



SILVICULTURAL CONTROL 



When timber stands are brought under management, it becomes 

 possible so to regulate conditions as to make forests less vulnerable 

 to insect attack ; or if insect damage does occur, to salvage the timber 

 without undue loss. The underlying aim is to maintain a biological 

 balance throughout the period of rotation. This task is hardly as 

 simple as it sounds and cannot be accomplished without a thorough 

 understanding of all the factors contributing to insect abundance 

 and the resistance of forest stands. The possibilities in this direc- 

 tion have not as yet been fully investigated, and there is still much 

 to be learned about the management of western forest types before 

 thoroughly sound methods of procedure can be recommended with 

 complete assurance of success. 



It is apparent, hoAvever, that this field of silvicultural control 

 offers almost unlimited possibilities. In the older forests much can 

 be done to lessen insect damage by avoiding injury to the trees from 

 forest fires and other weakening influences; by keeping forests in a 

 healthful condition through disposal of windfalls, slash, and other 

 insect-breeding places ; and by selective cutting operations to remove 

 the trees most susceptible to insect attack, and through these cuttings 

 to regulate forest composition and density. In new plantations con- 

 sideration should be given to the selection of the site and the plant- 

 ing of species and varieties of 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 selection of certain 

 trees or groups of trees scattered through the stand (70). 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 (SS). The damage in these pine forests also 

 becomes more acute when the stands are suffering from stagnation 

 or are subjected to periods of drought, and the soil moisture is not 

 sufficient to keep all of the trees in a thrifty, growing condition. A 

 selective cutting to remove the more beetle-susceptible trees, to release 

 the stand from stagnation, and to give the more thrift}^ dominant 

 trees first chance for such moisture as may be available is the most 

 obvious solution. Instead of cutting heavily on small logging units 

 forest management is looking toward a light selective system where- 



