INSECT ENEMIES OF WESTERN FORESTS vil 
become available; or climatic factors may become especially favorable. 
Under any such ‘condition the injurious species will breed rapidly 
and in excessive numbers, and a sudden destructive outbreak soon 
develops. Within a few seasons a high percentage of a timber stand 
may be killed by bark beetles. Such epidemics may continue for 
years and spread over large areas. Defoliators may suddenly appear 
within an area and, after destroying the foliage of valuable timber 
over large acreages, "disappear with equal suddenness. Many factors 
come into play in bringing about these sudden changes, and it is 
often difficult to isolate the responsible causes. Outbreaks of such a 
character are called “epidemic” infestations and require immediate 
attention and drastic control. 
CONTROL OF INJURIOUS FOREST INSECTS 
The objective 1 in forest-insect control, in areas not under intensive 
management, is to prevent or suppress epidemic outbreaks of in- 
jurious insects and to prevent their spread. As has just been indi- 
cated, it is best not to disturb normal infestations of native insects, 
for there is small hope of exterminating them, and the complicated 
factors that hold the species in balance may be unfortunately dis- 
rupted. Control of native pests is therefore confined to the prevent- 
ing of threatened outbreaks and the suppression of those that have 
attained some proportions. In the case of defoliators, the object, 
is to hold down the injury to as low a point as possible until natural 
factors suppress the epidemic. In bark-beetle control the objective 
is to prevent or suppress the development of a large beetle popula- 
tion, at the same time giving every encour agement to the natural 
control agencies, so that the natural balance may be restored. 
The control of forest insects, scattered as they are over vast forest 
areas, May seem an insuperable undertaking, and yet certain methods 
are available that make this not so hopeless a task as it might appear. 
Control problems my be approached in at least three different ways: 
(1) Through such direct remedial methods as destroying the insects 
by burning, drowning, or poisoning; (2) by silvicultural methods 
that modify the physical or nutritional forest conditions so as to 
change temperature, moisture, or food supply; and (3) by biological 
methods that alter conditions so as to increase the numbers of 
natural parasitic or predacious enemies. Though these are distinct 
methods of approach, it is often necessary to utilize more than one 
of them in the solution of some forest-insect problems. 
Forest-insect control in the Western States is, at the present time, 
largely a matter of protecting mature timber stands from the ravages 
of insects through the application of direct remedial control meas- 
ures. But as mature timber stands are cut and brought under man- 
agement, there will be more and more opportunity “to apply silvi- 
cultural measures in the solution of forest-insect problems. The 
‘application of biological methods will depend largely upon what is 
discovered through ‘further research work as to the various inter- 
relationships of the insects concerned. 
The control of introduced or foreign pests presents quite different 
problems. In the first place, every ‘effort is made to prevent their 
introduction into this country through rigid quarantine inspections 
made at all ports of entry. Such pests as escape detection and become 
