Hosts:
White spruce (Piceae glauca), Lutz spruce (P. glauca x sitchensis), Sitka spruce (P. sitchensis) and at times black spruce (P. mariana). Rare non-spruce hosts: Scots pines (Pinus sylvestris) and Siberian larch (Larix sibirica) have been observed in Alaska
Life Cycle:
Spruce beetles can take one or two years to complete their life cycle in Alaska, with temperature playing a major role in how fast they develop. Adult beetles emerge from infested trees in mid-May or when temperatures reach ~60°F 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 to attract males and additional females to mass attack 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 that 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.
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.
Spruce Beetle in Non-Spruce:
When beetle populations are high, spruce beetles and other related bark beetles are known to occasionally attack non-spruce conifers. In 2017, there were a few reports of non-spruce conifers (Scots pines (Pinus sylvestris) and Siberian larch (Larix sibirica)) being attacked by bark beetles in Susitna River Valley. Taxonomist Jim LaBonte (Oregon Dept. of Agriculture) confirmed that spruce beetle was the bark beetle attacking these non-native conifer hosts.
Many of the initial attacks on both non-native tree species appeared to have been unsuccessful, though gallery initiation was observed at several attack sites in Scots pine and in at least one attack site in larch. Lodgepole pine (Pinus contorta) were also present but unattacked in many of the locations where Scots pines have been attacked. It is unclear whether these beetle attacks in non-spruce hosts will be successful, both in terms of beetle reproductive success and tree mortality.
Spruce Beetle and Temperature:
Many people are interested in whether periods of winter cold temperatures will kill the spruce beetles. Unfortunately, the answer is not as simple as one might think; there are several factors that must line up to result in mortality of overwintering beetles. While bouts of cold temperatures are helpful in checking the population to some extent, we can’t predict the impact it may have on local beetle populations or on the overall outbreak.
Warm temperatures also impact spruce beetle populations. When developing spruce larvae are exposed to temperatures beneath the bark that are greater than ~62°F, some beetles are able to complete their life cycle in one year instead of two. This is more likely to occur on the south/east side of a tree along the margin of the forest where the trunk receives direct sunlight. The occurrence of beetles able to complete development in one year rather than two may be a contributing factor in the rapid population buildup of spruce beetles that lead to outbreaks and increased spruce mortality. For more information, see Werner, R. A, and E. H. Holsten, 1985. Factors influencing generation times of spruce beetles in Alaska. Canadian Journal of Forest Research 15: 438-443.