Spruce beetles can take between one and two years to complete their life cycle in Alaska. Temperature plays a major role in how fast they develop. Adult beetles emerge from infested trees in mid-May or when temperature reach ~60F and continue flying through mid-July. Female beetles seek a suitable host, such as wind thrown trees or those affected by another stressor, and then use chemical signals called pheromones which attract both males and additional females to help colonize the tree. Once the tree is successfully colonized a second pheromone is released to stop too many beetles from coming to the tree, thereby preventing competition within the host. The female beetle bores into the bark and constructs an egg gallery in the phloem parallel to the wood grain where she lays eggs in clusters on either side. The eggs hatch into larvae which feed in the phloem cross-wise to the egg gallery. Larvae do not enter the wood but may score the outer surface. The larvae go through four instars before pupation.

Spruce beetle galleries with larvae (bottom) and pupae (top). Photo by: E. Graham, USDA Forest Service-Region 10.

One-year life cycle beetles develop from egg to pupae the first summer. New adults spend the winter under the bark at the base of the infested tree and emerge the following spring. Two-year life cycle beetles spend the first winter as larvae beneath the bark. In spring they resume development, pupate, and become new adult beetles that migrate to the base of the dead or dying tree where they spend a second winter and emerge the following spring.