INSECT ENEMIES OF WESTERN FORESTS 7 



junipers have very few injurious insect enemies and none that 

 threaten the life of mature trees. Larch also is comparatively 

 free from insect pests. The broadleaved trees are the favored hosts 

 of many leaf -feeding species; but, since these trees can readily 

 replace their depleted foliage, such feeding is rarely fatal. Some 

 species, however, are much more subject to insect attack than 

 others. Certain oaks are reported to be hosts for more than 1,000 

 species of insects. Pines, spruces, firs, and hemlocks suffer much, 

 in the order named. 



Injurious forest insects are constantly at work, taking toll at 

 every stage in the development of the stand, and even after the 

 tree has been converted into lumber. Some insects feed on the 

 roots, others on the leaves, the terminal shoots, the branches, or 

 the phloem and bark of the main trunk. Still others feed on the 

 sapwood, and even on the heartwood. The fruits and the seeds also 

 are subject to attack by many insect species. 



In certain types of old-growth timber stands, particularly those 

 that are overmature, steady loss through insect activity is normal, 

 and this loss is for the most part counterbalanced by new growth. 

 On the other hand, epidemic insect outbreaks from time to time 

 definitely deplete the forest capital over large areas to such an 

 extent that long periods are required for replacement (fig. 2) . 

 Annual loss caused by forest insects in the Western States, in- 

 cluding depredations on standing timber and damage to logs, 

 sawed lumber, and finished wood products in use, is variously 

 estimated at 20 million to 100 million dollars. 



Much of this loss is as unavoidable as losses from lightning or 

 windstorms. On the other hand, much of it can be prevented 

 through silvicultural practices, proper forest management, and 

 direct control measures. 



DIRECT LOSSES 



In the mature timber stands of the West the largest single item 

 of insect loss results from activities of bark beetles. Surveys in- 

 dicate that these pests destroy annually 1 billion to 5 billion board 

 feet of mature timber in our western forests. 



A survey made in California in 1932 indicated that losses of 

 merchantable timber due to bark beetles in that year totaled about 

 1,500 million board feet, which represents a loss of over 3 million 

 dollars in stumpage values, aside from the regional asset repre- 

 sented by the manufacturing value of the lumber. 



In eastern Oregon and Washington, during the 20-year period 

 ending in 1940, the western pine beetle caused a gross loss of 

 15,480 million board feet of ponderosa pine (fig. 3). Typical high- 

 hazard sections in this region showed from 35 to 50 percent net 

 depletion in stand inventories. One area of the Modoc National 

 Forest in California lost 78 percent of its merchantable timber 

 from western pine beetle attack during this period. 



The lodgepole pine forests of Idaho, Montana, and Wyoming, 

 particularly those in and around Yellowstone National Park, have 

 suffered tremendous losses from mountain pine beetle attack dur- 

 ing the 30-year period following 1910. It has been conservatively 



