INSECT ENEMIES OF WESTERN FORESTS 9 



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 approximately 

 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 balsam fir forests 

 of the northern Kocky Mountains and the Pacific Northwest. Out- 

 breaks of this insect, besides resulting in destruction of extensive 

 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 precautionary meas- 

 ure. The cost of controlling forest fires that have spread from burn- 

 ing snags within fire lines would alone justify large expenditures for 

 insect control. 



After the defoliation of large forest areas, the debris beneath the 

 stripped trees dries out quickly and becomes highly inflammable. A 

 flash of lightning, or a carelessly handled match or cigarette sets off 

 the mass, causing a widespread conflagration almost impossible to 

 control. Heavy defoliations in Douglas fir and hemlock stands and 

 epidemics of the mountain pine beetle in lodgepole pine have put 

 the forest in such a condition that, more often than not, forest fires 

 have followed. The increased fire hazard is an added reason why 

 forest-insect outbreaks should be controlled wherever possible. 



RELATION OF INSECTS TO FOREST MANAGEMENT 



Since the practice of forestry is concerned with the growth, pro- 

 tection, and perpetuation of timber resources, it must take into con- 

 sideration any agency having so important a bearing on the growth 

 and development of forests as insects. As has already been pointed 

 out, insects cause enormous losses in mature stands of timber which 

 are being held in reserve for future needs; they affect the rate of 

 growth of developing stands and lower the yields; frequently they 

 so change the composition of a forest that a complete reshaping of 

 the plan of management is necessary ; they take a varying toll from 

 crude and finished forest products; and they create serious fire haz- 

 ards. For these reasons insect problems enter into nearly every phase 

 of forest management and protection. 



Under virgin-forest conditions no checks were placed on the activi- 

 ties of destructive agencies other than those imposed by Nature her- 



