WESTERN YELLOW PINE IN OREGON. 13 
tonus brevicomis) and the mountain pine beetle (Dendroctonus monti- 
colx). These insects kill the trees which they infest by eating the 
soft inner bark and cambial layers, thereby girdling the tree. Colo- 
nies of the former are found here and there scattered through the 
yellow-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 three to ten or more dead trees in a 
clump, some of which 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 multiplication 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 chiefly in large, old trees, most frequently those 
which 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 -pine forests on the upper 
slopes of the Blue Mountains, particularly on the Wallowa 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 yellow-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. These 
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. 
