8 INSECT DAMAGE IN THE NATIONAL PAKKS. 



Commercial cutting. — Commercial cutting of timber may contrib- 

 ute to the multiplication of certain species which breed in the stumps 

 and tops, but if the cutting is continuous the insects confine their 

 attack to the cut-over areas and do not invade the living timber. 

 Sporadic summer cutting, however, is dangerous. The odor of the 

 cut wood attracts the flying beetles to the locality. This contributes 

 to their concentration, and when the cutting is stopped they invade 

 the living timber. 



Secondary energies. — The secondary enemies of the trees consist of 

 numerous species which attack the bark and wood as soon as the 

 trees become weakened and are dying from other causes. The Den- 

 droctonus beetles are the primary enemies or leaders in the attack. 

 The secondary enemies are, to a certain extent, their allies, and 

 when very abundant may contribute to favorable conditions for 

 rapid advance in the destructive movement, but more often they 

 are dependents and scavengers, merely utilizing the dead and waste 

 material. "With rare exceptions these secondary enemies are not 

 capable of killing trees on their own account. 



UNFAVORABLE CONDITIONS FOR THE BEETLES. 



The unfavorable conditions for the destructive work of these 

 Dendroctonus beetles are to be found in administered forests where 

 the ripe or matured timber is utilized and where the young timber 

 is protected by the prompt disposal, during the fall, winter, and 

 spring months, of any clumps of dying trees. 



In other words, systematic forest management based on a knowl- 

 edge of the principles of silviculture and forest entomolog^^ will 

 soon present conditions so unfavorable for the Dendroctonus beetles 

 that^ they can no longer exist as agents of destruction and waste. 



The natural enemies of the beetles serve as a repelling force against 

 the progressive development of an invasion. Indeed, they are among 

 ihe i^rincipal factors which have prevented the extermination of 

 certain of the more important forest-tree species. These natural 

 enemies consist of parasites and predatory insects, which feed on 

 all stages of the barkbeetles, and birds, which feed on the adults 

 ?.nd young of the barkbeetles. Were it not for the fact that birds 

 also feed on the predatory and parasitic insect enemies of the bark- 

 beetles, and that such birds are limited in numbers, they might render 

 the great service that is so commonly credited to them. 



Insect diseases in the form of epidemics sometimes serve to bring 

 an invasion under complete control, and unfavorable climatic con- 

 ditions have been known to exterminate a species of Dendroctonus 

 beetles within an area of thousands of square miles. 



Therefore, although under natural conditions successive genera- 

 tions of the older trees are killed by their insect enemies, these in 



