INSECT ENEMIES OF WESTERN FORESTS 11 



forest growth and altering the stand from valuable to inferior 

 kinds of trees. 



In some forest types insects often are one of the chief limiting 

 factors in successful management. They frequently upset well- 

 organized plans aimed at the continuous production of forest 

 crops. In the western white pine and lodgepole pine forests of the 

 northern Rocky Mountain region bark beetles so affect the propor- 

 tion of tree species as to convert many stands to entirely different 

 composition. In Modoc County, Calif., a bark-beetle epidemic in a 

 mixed second-growth stand of ponderosa pine and white fir killed 

 out all the pine and converted the stand into pure fir. 



Much less frequently the effect of insect activity on stand com- 

 position is beneficial. In the Yosemite and Crater Lake National 

 Parks, for instance, lodgepole pine stands completely destroyed by 

 bark beetles have been succeeded by stands of the hemlock-fir type, 

 which, for park purposes at least, is far superior to the lodgepole 

 pine type. 



Certain defoliators, even though they do not kill the timber, may 

 cause a cessation or reduction of growth, which may increase the 

 rotation period of the stand by 5 to 10 or more years, or they may 

 so weaken the trees as to make them easy prey for tree-killing 

 bark beetles. Such defoliation may be local and confined to a single 

 tree species, or may spread over an enormous area and involve sev- 

 eral species. For instance, an outbreak of the pandora moth in the 

 ponderosa pine stands of southern Oregon, between 1918 and 1925, 

 covered approximately 400,000 acres. Growth measurements on 

 plots on this area showed that over a period of 11 years the normal 

 forest increment was reduced by an average of 32 percent, or ap- 

 proximately 100,000,000 board feet. The weakening of these trees 

 was followed by heavy bark-beetle killing, as much as 30 percent 

 of some stands having been killed by the beetles. 



The spruce budworm, which is so destructive in the Northeast 

 and in Canada, is present also in the Douglas-fir and true fir for- 

 ests of the northern Rocky Mountains and the Pacific Northwest. 

 Outbreaks of this insect, besides resulting in destruction of exten- 

 sive stands of Douglas-fir in the Rocky Mountain region, have left 

 many trees in a weakened condition that renders them susceptible 

 to bark-beetle attack. Many other defoliators, by partially reducing 

 the leaf surface of trees, adversely affect their growth; and in 

 most cases the forester has little opportunity to prevent this 

 damage. 



Another indirect result of bark-beetle and defoliator damage is 

 increase in forest-fire hazard. The old snags of insect-killed trees 

 scattered throughout mature forests, averaging on some ponderosa 

 pine areas as many as 10 per acre, stand for many years and 

 greatly increase the cost, difficulty, and danger in fire control. The 

 felling of snags is now required in many sales of national-forest 

 timber, and many private operators have adopted this precaution- 

 ary measure. The cost of controlling forest fires that have spread 

 from burning snags within fire lines would alone justify large 

 expenditures for insect control. 



After the defoliation of large forest areas, the debris beneath 



