Douglas-Fir Beetle Infestations Are Associated with Certain Rock and Stand Types in the Inland Northwestern United States

Landscape-level geographic analysis was used to examine the occurrence of Dendroctonus pseudoteugae Hopkins (Coleoptera: Scolytidae) in association with geology (rock type) and forest stand types on the Panhandle National Forests of northern Idaho and northeastern Washington from 1996 to 1999. Beetle infestations occurred most often in mature stands comprised primarily of trees ≥23 cm in diameter at breast height (≈1.4 m), in habitat types likely to contain Douglas-fir. Douglas-fir stands growing on Prichard and lower Wallace formation metasedimentary rocks and intrusive dikes and sills were significantly more likely than Douglas-fir stands growing on other rock types to harbor beetle populations during nonoutbreak years early in the study period. By 1999 (when outbreak populations of Douglas-fir beetle were present), beetle infestations occurred across all rock types at rates proportional to the amount of stand area on those rocks, indicating a decrease in infestation rates for stands on particular rock types when beetle populations were large. Although the biological mechanisms influencing beetle preferences were not examined, information derived in this study could be useful in developing quantitative hazard-rating models for Douglas-fir beetle, and, subsequently, in designing silvicultural practices to reduce the amount of beetle-susceptible Douglas-fir stands on particular rock types.