WESTERN YELLOW PTFE IN OREGON. 18 



tonus hrevicomis) and the mountam pine beetle {Dendrodonus monti- 

 col^e). ^ These insects kill the trees which they infest by eating the 

 soft imier bark and cambial layers, thereby girdling the tree. Colo- 

 nies of the former are found here and there scattered through the 

 yeUow-pine region. Each colony kills a tree or two each year, but 

 ordinarily shows no tendency to spread. A group of infested trees is 

 usually characterized by from tlu-ee to ten or more dead trees in a 

 clmnp, some of wliich have apparently been dead several years, 

 some a shorter period; and if the colony is still at work, by one or 

 two reddish-topped trees. The bark of some of the trees commonly 

 shows holes where woodpeckers have worked to get at the insects. 



The inordinate multiphcation of the insects is prevented by their 

 natural enemies, so that usually they are found only in what may be 

 called the normal infestation. Under exceptionally favorable condi- 

 tions, possibly climatic, or as a result of a decrease in the number of 

 their enemies, these insects may become at any time manyfold more 

 plentiful; and if they do, they may kill an enormous amount of 

 timber. They work cliiefly in large, old trees, most frequently those 

 wliich have been damaged by fire, by lightning, or otherwise. The 

 damage that they do is inconspicuous because it is scattered, but in 

 the aggregate it amounts to a great deal. 



The mountain pine beetle has shown itself to be the worst insect 

 enemy of yellow pine in Oregon. Colonies occur in greater or less 

 abundance in the forests of the whole eastern part of the State, usu- 

 ally working in lodgepole pine. Recently it has been spreading with 

 alarming rapidity through the lodgepole-pme forests on the upper 

 slopes of the Blue Mountains, particularly on the WaUowa and 

 Powder River Mountains, so that within the past few years 500,000 

 to 600,000 acres in these mountains have been attacked and more 

 than half the lodgepole-pine trees on at least 300,000 acres have already 

 been killed. Four or five years ago the insects extended their ravages 

 to the yellow pine adjacent to the infested lodgepole pine and a good 

 deal of it was killed. Had the infestation continued to spread in 

 the yeUow-pine timber as it began, the damage would have been 

 enormous, but within the last two years it has subsided very greatly, 

 evidently having been regulated in time by natural causes. Tliese 

 infestations evidently have their ups and downs and through some 

 natural agency, imperfectly understood, subside and regain their 

 normal balance. This bark beetle prefers to work in the smaller 

 yellow pines, but at times attacks the largest and thriftiest old trees. 

 Extensive operations in felling and barking infested trees have been 

 conducted by the Forest Service and by individual owners in the Blue 

 Mountains under the direction of the Bureau of Entomology, with 

 the purpose of checking the spread of this pest; but definite con- 

 clusions as to the effectiveness of this work have not yet been 

 reached. 



