50 percent reduction of loss in 1933. This decline 

 in activity continued in subsequent years, princi- 

 pally because more moisture resulted in increased 

 tree vigor. The low for the period was 377 mil- 

 lion board feet in 1937. However, in 1938 

 and 1939 there were increases in the beetle losses 

 in portions of the region that presage an upward 

 trend considerably above the 1937 low. 



Recently the mountain pine beetle has been 

 active both in virgin and dense second-growth 

 stands of ponderosa pine, although volume of 

 timber killed has been small. 



Damage caused by Ips beetles has been confined 

 principally to sapling and pole stands of ponder- 

 osa pine and cannot be readily measured. How- 

 ever, a considerable acreage of small second-growth 

 pine has been killed in recent years, particularly 

 in 1934. 



Table 17. — Estimated annual gross depletion, log scale, of 

 ponderosa pine by forest insects in the ponderosa pine region in 

 1931—37, inclusive, by forest-survey unit 



[In million board feet — i. e. 000.000 omitted] 





Region 

 total 



Eastern Washington 



Year 



Chelan- 

 Colville 



Yakima 

 River 



North 

 Blue 



Moun- 

 tain 



Total 



1931 



1932 



1933 



1934 



1935 



1,025 

 1,688 

 844 

 759 

 574 

 457 

 377 



47 

 93 

 58 

 23 

 32 

 36 

 32 



76 

 164 

 148 

 89 

 21 

 20 

 27 



2 

 3 

 2 

 3 

 2 

 1 

 1 



125 

 260 

 208 

 115 

 55 



1936 



57 



1937 



60 







Total 



5,724 



321 



545 



14 



880 







Annual average- 



818 



46 



78 



2 



126 





Eastern Oregon 



Year 



North 

 Blue 



Moun- 

 tain 



Des- 

 chutes 

 River 



South 

 Blue 



Moun- 

 tain 



Klamath 

 Plateau 



Total 



1931 



45 

 63 

 26 

 46 

 26 

 18 

 21 



200 

 360 

 187 

 164 

 128 

 101 

 80 



225 

 480 

 175 

 114 

 107 

 111 

 99 



430 

 525 

 248 

 320 

 258 

 170 

 117 



900 



1932 -.- --- 



1,428 



1933 -.. - - 



636 



1934 



644 



1935 



519 



1936 



400 



1937 . - 



317 







Total - 



245 



1,220 



1,311 



2, 068 



1.844 







Annual average. . 



35 



174 



187 296 



692 



Of the two defoliators present that attack 

 ponderosa pine, the pine butterfly has been the 

 most destructive and is potentially one of the 

 worst insect enemies in the region. No outbreaks 

 have occurred in recent years, but during the 

 period 1893-95 the larvae of this butterfly killed 

 from 20 to 80 percent of stands on 150,000 acres 

 in the Yakima Indian Reservation. 



The pandora moth, the other defoliator, has 

 been active in parts of Oregon in recent years, 

 but the last epidemic occurred during 1920-25 

 in Klamath County. Although the trees defoliated 

 by this insect are not always killed, their rate of 

 growth is so greatly retarded as to reduce volume 

 production seriously. 



Practically all the 818 million board feet of 

 ponderosa pine lost annually through insect activity 

 (table 17) is attributed to the western pine beetle. 

 This is approximately 82 percent of the average 

 annual ponderosa pine sawlog production from 

 1925 to 1936. Nearly 85 percent of this deple- 

 tion occurred in Oregon, and the greatest loss 

 was in the Klamath Plateau unit where annual 

 depletion averaged 296 million board feet. In 

 Washington, where the beetles were much less 

 active, the greatest loss occurred in the Yakima 

 River unit. 



Although nearly all other species in the region 

 have been subjected at one time or another to 

 insect attacks, these attacks have seldom reached 

 an epidemic stage and the volume of any one 

 species killed annually has not been great. In 

 recent years the mountain pine beetle has infested 

 lodgepole pine stands in several parts of the region 

 but reached an epidemic stage only in Crater 

 Lake National Park and adjoining national-forest 

 areas. Control work and low temperatures of the 

 winter of 1932-33 brought about the cessation of 

 the epidemic. Western white pine is also the host 

 of the mountain pine beetle. 



The Douglas-fir beetle {Dendroctonus pseudotsuga 

 Hopk.) has been active in parts of the region re- 

 cently and killed a comparatively small amount of 

 Douglas-fir in the Chelan-Colville unit. Some 

 western larch in this unit has been killed by the 

 fir flatheaded borer {Melanophila drummondi Kby.) 

 and western larch roundheaded borer (Tetropium 

 oclutinum Lee). 



The annual loss of volume due to insects in 



38 





