210 



TIMBER RESOURCES FOR AMERICA'S FUTURE 



Periodically, insect epidemics kill large quantities 

 of timber. Bark beetles, by girdling trees and by 

 introducing lethal fungi, are especially serious 

 agents. 



Next to killing trees, the most important effect 

 of insects on the timber resources is growth reduc- 

 tion. Cone and seed insects may deplete seed 

 crops. Insects may wipe out young stands or 

 seriously injure plantations. Twig and terminal 

 insects may impair growth rates or ruin the form 

 of trees. Defoliators, by destroying the needles 

 or leaves, may devitalize trees and seriously re- 

 duce growth and productivity. Insects also de- 

 stroy usable wood by boring into the sapwood or 

 heartwood and by introducing stains and decay 

 which result in ciill and degrade. 



Since 1900 many major forest insect outbreaks 

 have killed timber over vast areas. Six catas- 

 trophic outbreaks are shown in table 133, p. 217. 

 These accounted for over 52 bdlion board-feet of 

 softwood timber. An additional 12 billion board- 

 feet of timber are kno^vn to have been killed during 

 this period in other outbreaks of lesser size, and 

 probably twice this much has been kUled in small 

 outbreaks which were not recorded. 



Insect Impact on Timber Growth 



Insects kUled more timber than any other 

 agency in 1952. They were responsible for 28 

 percent of the growing-stock mortality and 10 

 percent of the national growth loss. The total 

 growth impact to growing stock was 16 percent 

 of that from all destructive agencies, or 1,778 

 million cubic feet (table 113, p. 189). Growth 

 impact on sawtimber was 8,617 million board-feet, 

 20 percent of the national total. 



The West led with 55 percent of the total 

 insect losses to growing stock and 65 percent of the 

 sawtimber losses for all regions (table 114, p. 190). 

 The North and South suffered almost equally. 



Unlike the losses from all other agencies except 

 weather and miscellaneous causes, the mortality 

 from insects exceeded the growth loss from insects 

 for the country as a whole. This was due to 

 heavy mortality in the West (table 130). On 

 growing stock in the North and the South, growth 

 loss exceeded mortality by five and two times, 

 respectively. 



Important Timber Losses Caused By 

 Many Types of Insects 



Bark beetles, the most important single group of 

 forest insects, killed 4}^ billion board-feet of saw- 

 timber in 1952, accounting for 90 percent of the 

 insect-caused mortality of sawtimber and 63 per- 

 cent (table 131) of the growth impact. In the 

 West, bark beetles attack mostly mature and over- 

 mature timber. Nationally, their damage is 

 measured largely in terms of mortality rather than 



Table 130. — Mortality from insects compared with 

 growth loss, by section of the United States and 

 Coastal Alaska, 1952 



GROWING STOCK 



Section 



Mor- 

 tality 



Growth 

 loss 



Growth 

 impact 



North 



South 



West 



Coastal Alaska 



Million 



cu. ft. 



65 



112 



796 



27 



Million 



cu. ft. 



333 



251 



180 



14 



Million 



cu. ft. 



398 



363 



976 



41 







Total, United States 

 and Coastal Alaska- 



1,000 



778 



1,778 



SAWTIMBER 



North 



South 



West 



Coastal Alaska . _ 



Million 



bd.-ft. 



99 



412 



4,432 



98 



Million 

 bd.-ft. 

 1,315 

 1,049 

 1, 137 

 75 



Million 



bd.-ft. 



1,414 



1,461 



5,569 



173 







Total, United States 

 and Coastal Alaska, 



5,041 



3,576 



8,617 



growth loss; in 1952, 84 percent of their damage 

 was mortality. 



The western pine beetle, during a 25-year period 

 from 1921 to 1945, probably killed 25 billion 

 board-feet. The mountain pine beetle decimated 

 lodgepole pine stands for hundreds of mUes along 

 the Continental Divide in Idaho and Montana 

 between 1911 and 1935. The amount of timber 

 kUled has been estimated at 15 to 25 billion board- 

 feet. The mountain pine beetle is also estimated 

 to have killed 10 billion board -feet of ponderosa 

 pine, western white pine, and sugar pine between 

 1910 and 1950. 



One of the most spectacular outbreaks was that 

 of the Engelmann spruce beetle, which destroyed 

 nearly all of the spruce and some lodgepole pine 

 over hundreds of square miles of western Colorado 

 between 1940 and 1951. About 5 billion board- 

 feet were destroyed, verj^ little of which has been 

 salvaged as yet. A new outbreak of this beetle 

 started in 1950, and threatens to kill all of the 

 mature spruce on seven national forests in Idaho 

 and Montana. 



Another recent major outbreak is that of the 

 Douglas-fir beetle. Following a serious blowdown 

 in western Oregon and Washington, this insect 

 killed 3 billion board-feet of Douglas-fir. Currently, 

 epidemics of this beetle are prevalent throughout 

 the range of Douglas-fir. 



Bark beetles have killed large volumes of 

 southern pine timber over the years. Several 



