Susceptibility of Fire-Injured Douglas-Fir to Bark Beetle Attack in Southern Idaho

Abstract

Abstract Of 328 Douglas-fir trees examined on a burn in southern Idaho, 228 (70 percent) had been attacked by the Douglas-fir bark beetle one year after the fire. Even small or lightly burned trees attracted bark beetles; the incidence of attack increased with size of tree and severity of fire injury to crown and cambium up to the point of tree death, but decreased sharply with outright fire kill. The number of attacking beetles per tree was low, apparently because attractive, fire-damaged trees were plentiful. Beetles established broods in 88 percent of the trees attacked, but brood populations were small because of adverse conditions—pitch invasion of brood galleries and "sour sap." In one-half of the attacked trees, the vertical extent of attack was only one-eighth of that of trees in unburned areas. This study indicates that fire injury subjects Douglas-fir to high incidence of bark beetle attack and salvage cutting should include all large trees, fire-killed trees, and severely defoliated trees.