182 FORESTS AND MANKIND 



far the methods of doing this are based on two well-known 

 facts. First it has been observed that when a forest is kept 

 vigorous and healthy and as nearly approaching its natural 

 condition as possible, insects usually do not increase. Second, 

 when in spite of, or in the absence of, precautionary measures 

 these insects do multiply to the point that they become a 

 menace special means must be sought to reduce their numbers 

 to where their own natural enemies can keep them under nor- 

 mal control. Beyond this it seems impossible to go. Every forest 

 then has, and probably will always have, its insect enemies just 

 as the human body normally carries about the germs of many 

 diseases. 



Without the insects and birds that live on these tree enemies 

 and which usually keep their numbers down to the point 

 where damage is negligible, we should probably have no for- 

 ests. Normally they preserve a balance. It is only when condi- 

 tions cease to be normal and the balance between insect and 

 insect enemy is disturbed that these forest pests multiply until, 

 in the course of a few years, certain localities are infested with 

 their destructive hordes. Many things can make for such con- 

 ditions. Even very slight climatic changes may make it sud- 

 denly possible for destructive insects to increase enormously. 

 After lumbering, the ground may be left covered by a litter of 

 dead trees, brush and tops, all creating ideal conditions for the 

 breeding of certain destructive insects. Fire, too, may weaken 

 the resistance of the forest, opening scars in the bark and expos- 

 ing the trees to insect attacks. Something may occur to reduce 

 the natural enemies of the insects. Birds may be driven out by 

 fire or hunters and in their absence the forest enemies are left 

 free to breed unmolested until in a few years they reach appal- 



