Entomologists have used mortality ratios as a device to determine the relative 

 susceptibility to beetle infestation of ponderosa pine trees in different tree classi- 

 fications or risk ratings (Keen 1943; Keen and Miller 1960). The ratios are derived by 

 dividing the percentage of trees killed in a given risk rating by the percentage of 

 occurrence of all trees, living and dead, in this rating. Using data from 22 undisturbed 

 study plots in table 5 and from table 10, mortality ratios indicate that Risk 3 trees 

 are likely to be killed by bark beetles 2.77 times as often as the average tree in the 

 plot stands and Risk 4 trees 22.47 times as often (table 12). 



Table 12 .--Relative susceptibility of risk rated ponderosa 

 pine trees to hark beetle infestation in western Montana 

 as indicated by mortality ratios based on tree volvmes for 

 the period 1948-2968 



Risk : 



Percentage 



of 



occurrence 



Mortality 



ratings : 



Living and 





Beetle-killed 



ratios ^ 





dead trees 





trees 





1 (Low) 



11.7 





0.2 



0.02 



2 (Moderate) 



71.5 





23.7 



.33 



3 (High) 



15. 3 





42.4 



2.77 



4 (Very high) 



1.5 





33.7 



22.47 



3 and 4 











combined 



16.8 





76. 1 



4.53 



Values 



greater than 



1. 



00 indicate susceptibility 



to beetle attack; values 



less than 1.00 indicate resistance 



to beetle attack. 









27 



