INSECT ENEMIES OF WESTERN FORESTS 171 



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 

 3^ears 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 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 encouragement 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 



