214 MISC. PUBLICATION 273, U.S. DEPT. OF AGRICULTURE 



resistance to beetle attack is lessened in other ways during periods 

 of drought. Such conditions present a highly favorable field for 

 bark-beetle activity, and the heavy losses that have been sustained 

 in the pine regions during the past few years indicate very clearly 

 that the bark beetles have not neglected this opportunity. 



Trees that have been felled by wind or snow, injured in logging 

 operations or in land clearing, or struck by lightning furnish 

 favorable breeding places for many destructive bark and wood 

 borers. Material of this kind is probably the natural habitat for 

 many species that at times become excessively abundant and 

 attack living trees. Many severe bark-beetle epidemics are known 

 to have originated in areas of wind-blown timber. 



SLASH 



The debris left from the cutting of trees in the forest is a 

 suitable and attractive breeding ground for a great many forest 

 insects, some beneficial and some harmful (61, 130, 155). When 

 slash is freshly produced, the dying inner bark is attractive to 

 many species of bark beetles that commonly breed in standing 

 trees. Usually these bark beetles select in the slash or stumps the 

 type and size in which they normally breed. Thus the limb- and 

 twig-feeding bark beetles go into the brush and smaller pieces 

 of slash, trunk-breeding bark beetles go into the cull logs and 

 butts, and those that normally work at the base of the tree attack 

 the stumps. The abundance of the progeny depends a great deal 

 on the moisture and temperature within the slash and the re- 

 quirements of the different species of beetles. The red turpentine 

 beetle, which breeds readily in pine stumps, frequently develops 

 in such numbers as to do serious injury to adjacent forest trees. 

 The trunk-breeding pine beetles rarely find suitable conditions 

 in the cull logs and butts, and the progeny which they produce 

 under such circumstances seldom cause any trouble in neigh- 

 boring forests or to the reserve stand, especially where logging 

 operations are continuous. The engraver and twig beetles, which 

 breed in the smaller pieces of slash, frequently emerge in such 

 enormous numbers as to kill patches of young trees and some- 

 times the tops of older trees. 



The wood-boring species that breed in slash must be considered 

 generally beneficial, in that they help to decompose the wood and 

 reduce the slash with its accompanying fire hazard. They may 

 become injurious, however, and in order to avoid or reduce a 

 possible menace from slash-breeding insects special considera- 

 tions in slash disposal are frequently necessary. When a logging 

 operation is continuous, and a fresh supply of slash is furnished 

 throughout the flight period, the emerging progeny is repeatedly 

 absorbed in the slash and in the logs removed to the mill, and no 

 special precautions need be taken. But if a cutting operation 

 ceases or is intermittent, as in road and power-line developments, 

 then some damage from slash-breeding insects may be expected 

 and should be avoided if possible. Burning the slash is beneficial, 

 provided the large limbs, cull logs, and stumps are included and 

 the burning is done before the insects emerge. In many cases 



