TREE SELECTION 



Not all lodgepole pine trees in infested stands are likely to be attacked and 

 killed by the mountain pine beetle. The beetle first infests the larger diameter 

 trees which usually have thick phloem and are a better food supply. The number of 

 trees killed varies by environmental conditions as reflected in habitat types. 



Diameter 



Examination of three stands involved in the current epidemic show that the largest 

 and most vigorous trees are attacked first. As the numbers of trees are reduced by 

 mortality the beetles move into smaller trees until the epidemic subsides. Several 

 studies support these conclusions. 



Gibson^ shows the intensity of beetle infestation by diameter classes observed in 

 the Big Hole area of the Beaverhead National Forest from 1925 to 1940 (table 1). All 

 the lodgepole pine trees 12 inches and larger in diameter were killed. But the percent 

 of trees killed decreased rapidly in the smaller sizes below 12 inches in diameter. 

 These data were collected from a lodgepole pine stand; the majority of this stand was 

 in the Douglas-fir vegetational zone and was included in a severe outbreak that covered 

 about 20,000 square miles. 



Table 1 . -- Susceptibility of lodgepole pine trees to mountain pine 

 and secondary bark beetle attacks by 2-inch diameter classes 









d.b 



h. 



classes 







Trees killed by 



: 2 



: 4 



: 6 





: 8 : 



10 



12+ 













Percent- - - 







Mountain pine beetle 



0.1 



6.5 



27 



5 



56.5 



87 



100 



Secondary beetles 



1.5 



5.0 



9 







5.5 



1.5 



None 



Cole and Amman (1969) concluded from their studies of two stands in northwestern 

 Wyoming that the beetles strongly favor the larger diameter trees in the stand in any 

 given year as well as throughout the duration of the epidemic. Trees killed by the 

 beetles ranged from 1 percent of the 4-inch trees to 87.5 percent of the trees 16 inches 

 d.b.h. and larger. Furthermore, Cole and Amman pointed out that large infestations of 

 the mountain pine beetle are dependent upon the presence of large diameter trees (14 

 inches d.b.h. and greater) within a lodgepole pine stand. They also speculated that 

 this beetle is a food-limited insect within a given area because only trees 14 inches 

 d.b.h. and larger contribute sufficient numbers of beetles to maintain or cause an 

 increase in infestations. 



Studies by Hopping and Beall (1948) near Banff, Canada, revealed about a 5-percent 

 increase in infestation intensity for each inch increase in diameter; few trees under 

 6 inches d.b.h. were attacked. Our study shows an increase in percent of trees killed 

 of about 8.8 percent for each 1-inch increase in diameter (figure 1). In the areas 

 examined very few trees below 7 inches in diameter were killed. 



"^Gibson, Archie L. Status and effect of a mountain pine beetle infestation on 

 lodgepole pine stands. USDA Forest Insect Laboratory, Coeur d'Alene, Idaho, unpub. 

 typewritten office report, 34 pp. 1943. 



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