Tree-Killing Beetles Spread into Northern U.S. Forests as Temperatures Rise

A study finds strong links between climate change and the spread of southern pine beetles, whose damage increases the risk of ecosystem harm and forest fires.

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Trees damaged by an infestation of Southern Pine Beetle. Credit: Erich Vallery/USDA
Southern pine beetles lay their eggs under the bark of trees, eventually killing them as they cut off water flow and nutrients. Credit: Erich Vallery/USDA

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Southern pine beetles are among the most destructive insects invading North America’s pine forests today, and they’re spreading farther north as global temperatures rise, putting entire ecosystems at risk and creating fuel for wildfires as they kill the trees they infest. A new study shows the insects’ range could reach Nova Scotia by 2020 and cover more than 270,000 square miles of forest from the upper Midwest to Maine and into Canada by 2080.

Winter cold snaps that once killed the beetles in their larval stage are becoming less frequent at the northern edge of the beetles’ current range, which will allow them to multiply and spread into new territory quickly, the study’s authors say.

Even if greenhouse gas emissions are cut drastically to the level envisioned by the Paris climate agreement, the level of heat-trapping pollution will still raise temperatures enough to continue to drive the spread of the beetles at least through 2050, according to the study, published this week in the journal Nature Climate Change.

Southern pine beetle. Credit: Erich G. Vallery/USDA
Southern pine beetle. Credit: Erich Vallery/USDA

Co-author Radley Horton, a Columbia University climate scientist, said the study shows how some ecosystems can be very sensitive to changes in temperature extremes, and how small shifts in average temperatures can lead to big changes in those extremes.

“One of our messages is to look at these outcomes and embrace the uncertainty,” Horton said. “You can’t only plan for what you see as the most likely outcome.”  

By 2050, conditions will be suitable for the southern pine beetle from Southern Maine to Ohio under any emissions scenario. Beyond that, the insects could even spread into parts of sub-Arctic Canada after 2080 under the highest emissions scenario commonly used in global climate policy talks.

“We’re looking at some pretty devastating effects on localized forests, like the coastal pitch pine forests, which are especially susceptible,” said lead author Corey Lesk, a graduate student working with Horton.

Southern pine beetles have been particularly successful in damaging pitch pines, which form the coastal forests of Maine. Those forests support rare and threatened ecosystems, including birds that depend on pitch pine seeds for survival, Lesk said. The New Jersey pinelands, forests on Long Islands and the Adirondack pine forests are particularly susceptible.

Pine forests are important to regional industries, from tourism to logging, as well as ecosystems. Timber producers lost an estimated $43 million per year to southern pine beetle infestations from 1977 to 2004, according to the U.S. Forest Service.

The 14-Degree Threshold

The beetles are native to Central America and the southeastern U.S., but they’ve increasingly been spotted in the Northeast over the past decade, with significant outbreaks in New Jersey starting 2001, and sightings in New York and Connecticut in the past three years. 

In summer, the small, rice-sized black bugs bore into the bark of pitch pines, red pines and jack pines to lay their eggs. The insects burrow into the living wood just below the bark and cut off the flow of water and nutrients.

Southern pine beetle damage under the bark of a tree. Credit: Erich Vallery/USDA
Southern pine beetles burrow tunnels under the bark of a tree. Credit: Erich Vallery/USDA

In the study, the scientists found that the southern pine beetle’s northernmost appearance correlates with latitudes at which tree bark temperatures dropped to 14 degrees Fahrenheit. When they focused on matching beetle sightings with temperatures in New Jersey, they found that the 14-degree threshold had moved northward by about 40 miles per decade. The northernmost sightings of the beetles have drifted north by about 53 miles per decade since 2002, the study found.

Seasonal Warming

The findings show that global warming nuances matter. The world’s average temperature has gone up by about 1 degree Fahrenheit in the last 50 years, but the coldest night of winter has warmed by about 6 to 7 degrees Fahrenheit at many weather stations in the region. The study projects that the winter’s coldest temperatures will warm another 6.3 to 13.5 degrees Fahrenheit in the region by 2050 to 2070.

By 2060, the scientists expect the beetle will have established itself from southern New England through Wisconsin, and by 2080, climates suitable for the beetle should reach 71 percent of red pines and 48 percent of jack pines, growing across more than 270,000 square miles of the Northeast, Upper Midwest and Canada.

The study’s authors say there is some debate as to whether the southern pine beetles will spread into the pine forests of the Great Lakes region, which are somewhat buffered from the spread by swaths of deciduous forests.

“The climate will be totally suitable for southern pine beetle. The question is whether they can jump across those barriers. But by late in the century, they’ll have a continuous path from New England to the Upper Midwest through Canada,” Lesk said.

The scientists tried to account for other influences that could affect how fast the beetles move north, including whether the bugs can travel through hardwood forests where there are few pines, or how warming temperatures may affect southern pine beetle predators. When those uncertainties are factored in, it could affect the arrival date of the insects by about 20 years in either direction, Horton said.

“When you start looking at some of the more dire projections for late in the century, if we can manage to avoid the high-emissions scenario, it might delay the emergence of the pine beetles,” he added.

Prevention Options?

The study is a warning to logging and resort communities that depend on forests economically. Preventive options include trying to cut and remove infected trees, or thinning forests before the beetles arrive to slow their spread, Horton said.

The spread in the Northeast won’t be as visually dramatic as in the Rocky Mountains, where mountain pine beetles killed trees across millions of nearly contiguous acres, because eastern forests are smaller and patchier. And there is some hope that southern pine beetles won’t spread to white pines, valuable as commercial timber and as wildlife habitat.

The spread of forest pathogens in the Northeast fits a widespread pattern linked with global warming. Bark-boring insects related to the southern pine beetle have also hit forests across Europe, from the Alps to the forests around Siberia’s Lake Baikal.

In parts of Europe, forest managers may have more of a fighting chance against the spread of tree-killing pests because forests tend to be more fragmented and actively managed than in the U.S., said Alistair Jump, head of biological and environmental sciences at the University of Stirling, Scotland.

“On the longer time scale, letting ‘nature’ take its course might be advocated – hoping that the natural enemies of the pest catch up with it, and acknowledging that some species will be lost, and balanced by the gain of others,” Jump said.

He said a middle way could be to see how forests change naturally and to try to facilitate that change elsewhere. “That includes assisting the spread of key species and preparing in advance for more severe impacts,” he said. “But it takes a very coordinated multi-agency, multi-community approach with a lot of foresight and political will to do so.”

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