Mountain Pine Beetles Show Resiliency to Warming Climates

In a recent Ecological Monographs article, David Soderberg, a doctoral candidate from Utah State University’s Ecology Center, and his colleagues, asked how resilient mountain pine beetles (Dendroctonus ponderosae) are to changing temperatures, and their results have important management implications for forests threatened with climate change.

Larval mountain pine beetles live under the bark of pine trees and feed on the tissues that provide nutrients to the tree. When the nutrient supply is cut off, the tree dies. Adult beetles then emerge from their host trees synchronously, attack new trees en masse and lay new eggs.

Mountain pine beetles are native to western North America and are an important component of forest ecosystems on the continent. However, in recent years climate change induced temperature increases have had grim consequences for forests infested by mountain pine beetles.

“During the past several decades, mountain pine beetle caused tree mortality has resulted in more carbon loss than wildfires,” explained Soderberg.

Should scientists expect more of the same as temperatures continue to rise? How adapted are mountain pine beetles to temperature changes? To address these questions, Soderberg and the research team reciprocally relocated two mountain pine beetle populations by rearing beetles from a relatively cold climate in Utah and a relatively warm climate in Arizona. They also took beetles from both populations and placed them in an area that was hotter than either source population. All beetles and trees used in the experiment were enclosed in netting to ensure that the researchers were not propagating beetle infestations or causing additional tree death.

“We anticipated that if local adaptation was very strong, then a rapid shift to a new climate might be detrimental,” Soderberg explained.

As it turned out, this was not the case. The results showed that mountain pine beetles not only persisted in warmer temperatures but were more successful regardless of whether they had originally come from a relatively warm or cold climate.

One explanation for this result could be that mountain pine beetles don’t have to expend as much energy producing antifreeze in warmer climates. If that sounds a bit like a superpower from a comic book, it kind of is. It takes one full year, two in cold climates, for mountain pine beetles to reach adulthood which means they need a way of surviving harsh winters. They do so by producing antifreeze compounds in their own bodies, but it costs a lot of energy to do it.

“The climate is trending toward temperatures that are warmer, suggesting increased overwintering survival,” said Soderberg. “Unnecessary investment into antifreeze could waste vital metabolic resources and ultimately be detrimental to population success.”

In other words, when it is warm, mountain pine beetles are more likely to survive the winter and don’t need to invest as much energy in antifreeze. Instead, they can spend energy on things like reproduction and maturing more quickly.

So, how warm can it get before pine beetles start feeling the heat?

“Mountain pine beetles had the greatest reproduction at the warmest side in our study, but too much warmth at the wrong time could be a bad thing,” explained Barbara Bentz, study coauthor. “They do have plasticity that allows them to handle some variability, but the warming has to be within the bounds of that plasticity. This was the case for the ‘warm’ site in our study, the warmth occurred at the right times. If we were to place them in a really warm site, they might not do so good.”

Plasticity is essentially how much change an organism can withstand and still survive. For example, you might not like going outside when the temperature is 35°C (96°F) or 0°C (32°F), but you will survive it. Conversely, if you went outside when it was 100°C (212°F) or -100°C (-148°F) it would kill you because those temperatures exceed the ranges that your body can handle.

“The next step of our research is to figure out how much warming, and the time of that warming, mountain pine beetles can handle,” said Bentz.

“We hope that our work helps managers comprehend how pine beetles respond to warming temperatures,” added Soderberg. “Understanding how mountain pine beetle population success will be influenced by changing temperatures will inform management strategies for forests that provide important human and ecosystem services.”

Jim Vandygriff and Matt Hansen provided assistance with collection of D. ponderosae-infested and non–infested trees, laboratory rearing, and field assembly. We thank Dr. Monica Gaylord, Danny DePinte, and John Anhold for their help with field assembly and winter collections in Arizona. We also thank Zach Gompert, Justin Runyon, and Justin deRose for providing comments on an earlier version of the manuscript, and to William Pearse for statistical help. The experiment was designed by B. Bentz, K. Mock, and R. Hofstetter. D. Soderberg, B. Bentz, and R. Hofstetter implemented the field experiment and collected data. D. Soderberg conducted the statistical analysis and led the writing of the manuscript with assistance from B. Bentz and K. Mock. We thank USDA Forest Service Rocky Mountain Research Station and the S.J. & Jessie E. Quinney College of Natural Resources at Utah State University for financial support.