of ponderosa pine peeler logs has been exported 

 from eastern Oregon to a veneer plant in Portland. 



Insect Depletion 



Natural causes of depletion include forest in- 

 sects, disease, and wind throw. Of these, forest 

 insects is the only agency for which dependable 

 records of loss are available and these records are 

 limited to ponderosa pine. However, in the past, 

 forest insects have been by far the most destructive 

 of the natural-depletion agencies, and ponderosa 

 pine, because of its commercial value, is the only 

 species in which losses due to insects are of serious 

 economic concern. 



In Oregon and Washington ponderosa pine is 

 the host of two of the bark beetles — western pine 

 beetle (Dendroctonus brevicomis Lee.) and mountain 

 pine beetle (D. monticolae Hopk.); several pine 

 engraver beetles — species of Ips; and two defo- 

 liators — the pandora moth (Coloradia pandora 

 Blake) and the pine butterfly {Neophasia menapia 

 Felder). The western pine beetle has been by 

 far the most active and destructive. 



Although entomologists are practically certain 

 that the western pine beetle has always been 

 present in the pine forests of the region, contin- 

 uous records of infestations prior to 1911 are 

 lacking. Since that year there have been three 

 epidemic periods, each followed by declines in 

 activity resulting from the action of natural 

 agencies of control. Peaks of the epidemics were 

 reached in 1917, 1927, and 1932. 



Since 1921, the Bureau of Entomology and Plant 

 Quarantine has kept a continuous check of bark 

 beetle losses on a series of sample plots in the 

 Klamath Plateau unit. Beginning in 1931, these 

 annual surveys were extended to other parts of the 

 region and conducted cooperatively by the 

 Bureau of Entomology and Plant Quarantine, 

 Forest Service, Office of Indian Affairs, and 

 private timber protective agencies. The estimates 

 of depletion by forest insects are based upon the 

 surveys for 1931 to 1937, inclusive. Although 

 covering only 7 years, this period is fairly repre- 

 sentative of the trend of beetle activity during the 

 past two decades, since it included a severe epi- 

 demic followed by several years of declining sever- 

 ity of attack. However, losses that occurred 

 during 1932 were the greatest within the knowl- 



edge of entomologists and they may not be equaled 

 for many years. 



Increased activity of the western pine beetle 

 began in 1931, when climatic and host-resistance 

 conditions were especially favorable to it, and 

 resulted in a loss of over 5.7 billion board feet of 

 ponderosa pine during 1931 to 1937, inclusive. 

 The gross depletion during this period, as esti- 

 mated by the Bureau of Entomology and Plant 

 Quarantine, is shown by years in figure 21. The 

 peak of infestation was reached in 1932, when the 

 year's loss totaled nearly 1.7 billion board feet, 

 a record for the region and greater than the peak 

 volume of sawlog production during the period 

 1925-36. Although the epidemic was region- 

 wide, the loss was heaviest in parts of the region 

 which had been practically free of infestations for 

 many years. This was especially true in the South 

 Blue Mountain and Yakima River units. In 

 parts of the former unit a loss of the entire stand 

 occurred on tracts up to 10 acres and losses of 

 15 percent of the stand over large areas were com- 

 mon. Extremely low temperatures during the 

 winter of 1932-33 destroyed a large percentage 

 of the overwintering broods and brought about a 



Figure 21. — Estimated depletion of ponderosa pine by forest 

 insects in the ponderosa pine region during 1931-37, inclusive. 



37 



