Why Here, Why Now?

A cartoon illustrating a spruce beetle enjoying its winter home under the bark of a spruce tree. Bouts of extreme cold during winter have the potential to kill over-wintering beetles, but extreme cold has been less frequent in recent decades than in the past.

NPS Illustration/M. Perez

What causes a beetle outbreak?

The growth of spruce beetle populations is complex and dynamic. Favorable summer climates for reproduction, winter climates for survival, an abundance of potential host trees, and some sort of trigger is often required for populations to reach epidemic stage. Examples of possible triggers include a severe wind event causing stress-inducing damage to many trees in one area, or an extended drought that weakens an already aged population of spruce.

What conditions are needed for an outbreak to continue?

Once an outbreak occurs, and beetle populations have built to high levels, the maintenance of those populations then becomes source of the complexity. Will an extended cold snap threatened the survival of over-wintering beetle larvae? Survival is thought to be limited below -40° F (-40° C). Will cool temperatures in early summer delay flight season, altering reproductive success rate? Adult beetles generally leave their winter host trees to find new host once temperature reach 60° F (15° C). What happens if the beetle run out of suitable host trees? Forests are not uniform, so the bigger the outbreak, and more of the landscape affected, beetles may be forced to travel farther or accept less desirable host trees.

What does this mean for Denali’s forests?

Denali’s spruce forests—and those farther into Alaska’s interior—generally experience cooler, wetter summers and longer, bitterly cold winters as compared to forests in south-central Alaska. This seems to have constrained past beetle outbreaks from causing widespread spruce mortality as documented further south. However, with warmer winter temperatures and longer summer seasons, climate control on beetle population may loosen. For example, spruce beetle larvae are protected from harsh winters in the phloem of their host tree, but struggle to survive when winter temperatures dip below –40 F. Over the last decade, the Park has experienced far fewer periods of deep cold than before. This could allow for a more successful build up and upkeep of high spruce beetle populations in the area.

Over the next several years, time will tell how the current beetle populations fare. There are plenty of preferred host trees, but they occupy forests and landscapes quite different from those further south. White spruce forests in the Park north of the Alaska Range, as opposed to areas of the Park south of the Alaska Range, are perhaps less uniformly distributed (i.e., frequently mixed with open shrublands or wetlands), more diverse in composition (i.e., mixed with black spruce and aspen) and generally less productive (i.e., do not reach as large of size). In addition, regionally speaking, this outbreak seems to be in decline, with 2024 forest health mapping efforts revealing the lowest amount of spruce beetle activity since 2015 (USDA Forest Service et al. 2024).

Information here was summarized from:

• USDA Forest Service, Forest Health Protection and its partners. 2024. Alaska's Aerial Detection Survey. USDA Forest Service, Forest & Grassland Health.
• Bentz, B.J., Régnière, J., Fettig, C.J., Hansen, M., Hayes, J.L., Hicke, J.A., Kelsey, R.G., Negrón, J.F. and S.J. Seybold. 2010. Climate change and bark beetles of the western United States and Canada: Direct and indirect effects. BioScience 60(8): 602-613.

More Information

• Thoman, R. and H. R. McFarland, editors. 2024. Alaska’s Changing Environment 2.0. Alaska Center for Climate Assessment and Policy, International Arctic Research Center, University of Alaska Fairbanks.
• Spruce Beetle Outbreaks and Climate Change in Alaska | USDA Climate Hubs
• Fresco, N., Littell, J., Redilla, K., Moan, J., Mullen, S., Parr, C. and C. Stephenson. 2024 (preprint). Modeling the loss of climate protection from spruce beetles (Dendroctonus rufipennis) under changing conditions in Alaska's forests. Information about project available here: Modeling the Potential Effects of Climate Change on Spruce Bark Beetles — SNAP.

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