I 



Figure 1 . — Trees ki I led 

 by mountain pine 

 beetles as related 

 to diame-ter of host 

 trees . Confidence 

 limits at the 95 

 percent probability 

 level are shown by 

 the dash lines. 



6 8 10 12 14 16 18 



Diameter breast high (inches) 



Phloem Thickness 



Considerable work is in progress to determine the effect of phloem thickness on 

 beetle attack. Amman has shown in laboratory studies that successful brood development 

 is correlated with phloem thickness. Trees having phloem less than about 0.12 inch 

 thick do not produce enough brood per unit area of bark surface to sustain a successful 

 infestation . 



Phloem thickness among lodgepole pine trees is highly variable. However, we have 

 observed that the beetles tend to attack and kill the trees having thicker phloem and 

 pass up many trees of similar diameter that have thinner phloem. Observations show 

 that the thickness of the phloem determines whether the insect can maintain or increase 

 its numbers in the stand. During an epidemic Roe has observed beetles selecting trees 

 in the stand possessing the thickest phloem; and sometimes beetles choose the portion 

 of an individual tree having the thickest phloem. Hopefully, we will gain a greater 

 understanding of the relationship between thickness of phloem and diameter of tree and 

 tliis may help provide an index to tree susceptibility. 



Habitat 



Early work by Gibson^ pointed to the differences in beetle infestation intensity 

 that are related to elevation. He reported that the infestation appeared to be less 

 intensive on the upper end of his sample strips than on the lower. In the Beaverhead 

 National Forest data (table 2), the Elkhorn strip sample--located highest in elevation 

 and in the subalpine fir-Engelmann spruce vegetational zone--showed the fewest beetle- 

 killed trees. The Bitterroot Forest plot data in table 3 displayed the same trend 

 except in the plot at the lowest elevation. Amman (1969) found that brood production 

 in bark of a given thickness is inversely related to elevation. Differences in the 

 rate of tree stocking do not seem to be great enough to explain the variation in infes- 

 tation intensity in these studies. 



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