72 MISC. PUBLICATION 273, U. 8S. 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 1f 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, however, 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 (55). 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 thrifty 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- 
