INSECTS AND FOREST MANAGEMENT — CRAIGHEAD 369 



of all in respect to extent and completeness of kill, although this tree species 

 is not so valuable as several other species of pine. In the northern Rocky 

 Mountains an outbreak originating in the Bitterroot Valley in 1922 swept down 

 the Continental Divide and laid waste lodgepole pine forests over thousands 

 of square miles. Estimates placed the total destruction at 7,250 million board 

 feet, with more than 36,000,000 trees having been killed in one national forest 

 alone. [PI. 5, fig. 2.] 



Taking the pine forests of the western States as a whole, recent estimates 

 place the annual mortality associated with bark beetles at 2.8 billion board 

 feet. The magnitude of this yearly figure can best be visualized by comparing 

 it with the 4.4 billion board feet of lumber produced by all western pine saw- 

 mills and the 1.4 billion board feet of saw timber killed by forest fires 

 throughout the entire United States in 1936. 



DEFOLIATING INSECTS 



Defoliating insects have been even more destructive than bark 

 beetles. The larch sawfly {Lygaeonematus erichsonii (Hartig) ) had 

 practically wiped out larch as a commercial species in the eastern 

 States and Canada by 1910 (Hewitt, 1912) and is now attacking 

 stands west of the prairies. The spruce budworm {Cacoecia fumi- 

 ferana (Clem.)) has repeatedly invaded our forests. (PI. 9, fig. 2.) 

 An exceptional outbreak in the northeastern States and Canada 

 (Swaine and Craighead, 1924) ravaged the spruce and fir forests for a 

 period of 10 years (from 1910 to 1920), and it has been estimated that 

 in spruce-fir types of Maine, Ontario, Quebec, and New Brunswick 

 from 40 to 70 percent of this timber was destroyed, i. e., the equiva- 

 lent of more than 25 years' pulpwood supply for current annual 

 American paper requirements (Senate Document 19). Certain 

 species of defoliators, even if they do not kill the timber, cause a ces- 

 sation or reduction of growth which may increase the rotation period 

 of the stand from 5 to 10 years or more. Such defoliations may be 

 local and confined to a single species of tree or they may spread over 

 enormous areas involving several species. The most recent outbreak 

 of the Pandora moth {Coloradia 'pandora (Blake)) in the ponderosa 

 pine stands of southern Oregon (pi. 7, fig. 2; pi. 9, fig. 1) occurred be- 

 tween 1918 and 1925 and covered approximately 400,000 acres (Pat- 

 terson, 1929). Growth measurements from plots in this area showed 

 that for a period of 11 years the normal forest growth was reduced an 

 average of 32 percent, or suffered a loss of increment of approximately 

 100 million board feet. The spruce sawfly {Diprion polytomum 

 (Htg.) (pi. 10, fig. 2) ) has more recently threatened the spruce forests 

 of the New England States and Canada. In the Gaspe Peninsula in 

 Canada it has already (1940) killed 10,000,000 cords of spruce (Balch, 

 1941). Late in 1940 it was suddenly checked by a disease which 

 decimated its number throughout its range. 



