THE EELATION OF INSECTS TO SLASH DISPOSAL 7 



Numerous observations made by forest entomologists in the West- 

 ern States point conclusively to the fact that the western pine beetle 

 is strongly attracted to the vicinity of slash material, trees scorched 

 by fire, trap trees, and windfalls. As long ago as 1906, Webb {17, p. 

 80) pointed out that storm-felled and lightning-struck western yellow 

 pines became centers of infestation by this insect. If a concentra- 

 tion of the beetles is brought about through the presence of some 

 such material, they not only attack this material but go into the 

 standing living trees in the immediate vicinity. 



From these investigations the conclusion is reached that the destruc- 

 tion of standing trees by insects, consequent upon logging operations 

 in adjacent territory, is not caused by the breeding up of insects in 

 the slash, but by the concentration in the slash of beetles from the sur- 

 rounding territory, and the failure of the slash to absorb them all. 

 This strong attractive influence can not well be avoided. 



Since the western pine beetle breeds only in the slash material 

 more than 6 inches in diameter, such as cull logs and butts, the 

 present methods of slash disposal can be of little avail in reducing 

 the numbers of beetles. Neither can the very strong attractive 

 influence of the slash be avoided. The best way, therefore, to avoid 

 injury to neighboring standing trees is to supply continuously a 

 quantity of logs to absorb the broods of beetles, and, if necessary, to 

 reduce, through artificial control, the total population of beetles on 

 the area. 



Several species of Ips (/. emarginatus Lee, /. oregoni Eich.^ /. con- 

 fusus Lee, etc.) breed in the smaller pieces of the slash material as 

 well as in some of the larger pieces. When they become sufficiently 

 numerous, if there is a shortage of slash, they may kill young trees, 

 and the tops and limbs of larger trees. This damage is very sporadic, 

 and usually follows the interruption of logging operations in the sum- 

 mer months. Keen has reported three small areas on the North War- 

 ner division of the Modoc National Forest, in California, where species 

 of Ips have caused heavy losses of reproduction after the cessation of 

 logging operations in the summer months. The conditions were com- 

 plicated by extremely dry weather. Usually a supply of fresh slash, 

 continuously laid down during the summer months, will absorb all of 

 these beetles and prevent injury to neighboring living trees. 



In general it may be said that, in the light of present information, 

 no special methods of slash disposal need be recommended to avoid 

 insect damage to western yellow pine, except in the case of sporadic 

 cutting or when logging operations cease. In such cases infested 

 slash material should be destroyed before the broods of insects emerge. 

 The same general considerations apply to western yellow pine in the 

 northern Rocky Mountain region. 



In the southern Rocky Mountain and Colorado Plateau regions 

 the Black Hills beetle (Dendroctonus jjonderosae Hopk.) is the prin- 

 cipal insect enemy of the western yellow pine {Pinus ponderosa var. 

 scopulorum) . Edmonston has reported that in Colorado the forest 

 areas show^ing least infestation are always those adjacent to places 

 where logging operations are in progress, particularly where the oper- 

 ations have been continuous for several years. Several officials of 

 the Forest Service have reported similar observations. Recent studies 

 by Craighead, Blackman, and Keen in the Kaibab National Forest 

 in Arizona indicate that the felled logs and culls act as traps for the 



