INSECT ENEMIES OF WESTERN FORESTS 217 



infestations which continue under these conditions are called 

 endemic. This is the normal condition in nature and it is a 

 hopeless and unwise undertaking- to try to exterminate native 

 insects under such conditions. The result of an effort in this 

 direction would be more likely to disrupt the delicate balance than 

 to accomplish the objective. 



Under certain conditions, the natural balance may be broken by 

 any one of a number of factors. For example, the beneficial in- 

 sects or other enemies of harmful species may become reduced in 

 numbers; the resistance of the trees may be lowered through 

 drought, fire, or stagnation; large quantities of slash or other 

 breeding material may become available ; or climatic factors may 

 become especially favorable. Under any such condition the in- 

 jurious species will breed rapidly and a sudden destructive out- 

 break will soon develop. Within a few seasons a large proportion 

 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 in forest-insect control, in areas not under inten- 

 sive management, is to prevent or suppress epidemic outbreaks 

 of injurious insects and to prevent their spread. As has just been 

 indicated, 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 disrupted. Control of native pests is therefore 

 confined to the preventing of threatened outbreaks and the sup- 

 pression of those that have attained some proportions. With 

 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 develop- 

 ment of a large beetle population, at the same time giving every 

 encouragement to the natural control agencies, so that the natural 

 balance may be restored (62). 



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 may 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 



