Under normal population levels, spruce beetles live and feed in recently downed trees such as those blown down during wind events. If enough of this material is present and other conditions are right, populations can increase to high levels and beetles may attack standing trees.

When standing trees are attacked, larger diameter and slower growing trees are usually attacked first. Trees that are stressed by climate conditions, such as drought, and trees that are damaged by factors such as other insects or fire are typically more susceptible to spruce beetle than otherwise healthy trees.

Conditions that can contribute to population increases include:

Successive years of warmer than normal summers
Drought conditions
Abundance of host material available

spruce blowdown, William M. Ciesla, Forest Health Management International, Bugwood.org

Wind events can create an abundance of host material for spruce beetle. Photo by William M. Ciesla, Forest Health Management International, Bugwood.org.