Evaluation of factors affecting tree and stand susceptibility to the Douglas-fir beetle (Coleoptera: Scolytidae)

Tree and site characteristics were compared between 19 groups of Douglas-fir, Pseudotsuga menziesii (Mirb.) France (Pinaceae), infested by Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins, and 19 uninfested groups to identify individual or combinations of characteristics associated with Douglas-fir beetle attacks. Of the tree characteristics, diameter, height, age, phloem thickness, bark thickness, and a standardized variable consisting of diameter divided by the last 10 years growth rate (DBHTYG) showed significant differences. None of the site characteristics showed significant differences between infested and uninfested areas. Two models, identified through discriminant analysis, placed 84 and 87% of the data in the correct infestation groups. These models included the variables diameter, height, phloem thickness, aspect, and DBHTYG in various combinations. The results were confirmed using a jackknifed cross-validation approach. These variables have potential for use in the development of a biological process model of tree and stand susceptibility to the Douglas-fir beetle.