Bark Beetle Infestation Offers Warning on Delicate Workings of Climate Disruption

Earlier this month, a small group of interested citizens gathered at the Evergreen Library in Evergreen, Colo., to attend a Tuesday night program called “Beyond the Headlines: The Pine Beetle Infestation.” For some, it may seem a strange time to hold such a program, as the weather has been pretty wet and there hasn’t been much pine beetle activity in their neck of Colorado’s Front Range.

But a potential disaster looms just over the next ridge, figuratively and literally.

“The community does know that it is a problem that is going to be upon us itself in two or three years,” said Janice Tang, a librarian who attended the event. “They’re coming over the continental divide at this point. They’re up in Vail. They’re wreaking havoc in the pine forests in Vail.”

Several species of bark beetles – such as mountain pine beetle (Dendroctonus ponderosae), piñon ips beetle (Ips confusus), and spruce beetle (Dendroctonus rufipennis) – are attacking and devastating the predominantly conifer forests of western North America from British Columbia to New Mexico. Tens of millions of acres of western forests have been affected by die-offs of infected trees the past few years, causing more than $1 billion in damage annually in the United States alone.

The problem will likely worsen, unless steps are taken to the reduce greenhouse gas emissions that lead to a warmer climate, according to a recently released government report, Global Climate Change Impacts in the United States.

How global warming is affecting the delicate biological cycles of a single family of insects offers a stark warning on unforeseen ecosystem disruptions that climate change will usher in if left unchecked.

Bark beetles are a natural part of the forest ecosystem. They help increase the diversity of forest stands, killing older or weaker trees and creating patches of forest that differ in species composition, stem density, age, and successional stage. The beetles, by helping break down dead wood, also contributing to the recycling of minerals and nutrients. But, when outbreaks are widespread, they can cause massive die-offs and increase the likelihood of massive, devastating fires that can undo their normally beneficial effects.

Climate change influences the frequency, intensity, and distribution of bark beetle outbreaks by affecting both the beetles as well as the trees themselves. Kenneth F. Raffa, a professor in the departments of entomology and of forest and wildlife ecology at the University of Wisconsin and lead author of 2008 paper on the dynamics of bark beetle outbreaks in the journal BioScience, said that two climatic factors are important: temperature and drought.

“Elevated temperature has beneficial effects on bark beetles in two ways,” Raffa said. “One, it reduces the amount of mortality they experience in the wintertime. That’s obviously particularly important at the higher latitudes and the higher elevations. The other thing that elevated temperature does is that it can reduce the time needed for them to complete a generation. … You put those two elements together, their shorter life cycle and their higher survival, and you have more beetles, and more beetles allow them to attack healthier trees.”

All life stages of bark beetles can be killed by low temperatures, but the degree of coldness is not as important as its timing. Barbara J. Bentz, a research entomologist at the USDA Forest Service’s Rocky Mountain Research Station, explained why.

“They’re not like us. When we get cold our tissues freeze and, boom, we’re dead. They metabolize these – basically we call them anti-freeze compounds. Glycerol is the one mountain pine beetle uses. What glycerol does is allow their tissues to not freeze” said Bentz. “We’ve recorded them down to minus 40 C when they have the maximum amount of glycerol in their bodies.”

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