THE RELATIOX OF INSECTS TO SL-\SH DISPOSAL S 



The direct result of insect attack upon the slash itself is beneficial 

 because the insects aid in bringing about decomposition. In this 

 circular the direct influence of insects upon t^\'igs, limbs, and tops 

 can be ignored and only the role of slash need be considered in 

 (1) supplying breeding material for insects which emerge and kill 

 mature standing timber or seedlings, saplings, and poles, and (2) 

 attracting insects from the surrounding forest and concentrating them 

 in the vicinity of the slash, where they kill living trees. 



In general, the insects with which this discussion is concerned breed 

 in the same parts of a fallen tree as of a living tree. Insects which 

 attack the base of standing trees breed in stumps, those attacking 

 the main trunk breed in cull logs, and those attacking limbs and 

 twigs breed in the Hmbs and twigs composing the smaller portions of 

 slash. There are, however, some important exceptions to this rule, 

 such as certain species of Ips. 



Probably the heaviest losses of all are attributable to the effect of 

 slash in attracting beetles from the surrounding territory and con- 

 centrating them in the vicinit}', where they are able to do greater 

 damage to the standing timber than if their attacks had been scat- 

 tered over a wider area. 



This attractive influence has been noted many times. The felling 

 of trap trees ^ has often concentrated attacks on adjacent healthy 

 trees, even in some cases in which the felled trap tree has not been 

 attacked. The beetle population is usually attracted to fire-scorched 

 areas, even though the beetles are often unable to breed successfully 

 in the trees weakened by fire. It is also usually attracted to places 

 where heavy windfalls have occurred or to trees that have been 

 broken down by snow. 



It is evident that this class of damage can not be minimized through 

 slash disposal. In fact, burning the slash in the summer, which is 

 the flight period of the beetles, may even increase the attractive 

 influence and bring about a still greater concentration. 



FEW SPECIES OF TREES INVOLVED 



The problem can be further simplified by limiting the discussion 

 to such trees as are subject to primary damage from slash-breeding 

 bisects. Relatively few trees are thus involved. 



AU hardwoods can be eliminated from the discussion, as practically 

 no insects which breed in hardwood slash are of importance as tree 

 killers. Among the conifers, the slash of fir (Abies), larch (LarLx), 

 redwood (Sequoia), hemlock (Tsuga), cypress (Cupressus, Taxodium), 

 cedar (Thuja, Libocedrus, Chamaecyparis), and juniper (Juniperus) 

 either breeds insects of very little significance as tree killers, or the 

 trees killed are so few or of so little value as to be of small economic 

 importance. 



As a result of these eliminations the problem in the United States, 

 viewed from an economic standpoint under present forestry practi- 

 ces, resolves itself into a consideration of slash from pine (Pinus), 

 spruce (Picea), and Douglas fir (Pseudotsuga) . Each of these three 

 groups will be taken up in sequence. 



' Living trees girdled or felled at the proper time to attract the flying beetles to them and away from 

 healthy trees. 



