INSECT ENEMIES OF WESTERN FORESTS 



9 



50 to 150 board feet per acre per year are typical. In lodgepole 

 pine stands normal losses by bark beetles are practically negligible, 

 so any marked losses indicate abnormal conditions. 



Defoliating insects sometimes destroy extensive stands of ma- 

 ture timber. Major outbreaks, however, usually occur at rather 

 long intervals and are nearly always of short duration. In western 

 forests some of the worst defoliators are the pine butterfly, the 



Figure 3. — A commercial ponderosa pine stand in California badly damaged 



by bark beetles. 



