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Why Bark Beetles are Chewing Through U.S. Forests

The conifer forests of the North American west have been under a massive assault over the past decade by bark beetles: one species alone, the mountain pine beetle, has killed more than 70,000 square miles’ worth of trees, equivalent to the area of Washington State, and two recent studies have shed some light on how climate change is helping fuel the assault, and what’s likely to happen in a world that continues to warm.

The first, published in the journal Ecology, shows how intense drought can bring on a population explosion in the voracious insects — and how this creates a vicious cycle of tree-killing even when drought subsides. The second, published in Proceedings of the National Academy of Sciences (PNAS) reveals that warming lets beetles move to higher elevations, where they’re encountering trees that are unusually susceptible to infestations.

Two recent studies shed some light on how climate change is helping fuel the assault of bark beetles, and what’s likely to happen in a world that continues to warm.
Credit: Colorado State Forest Service

Given the enormous destruction wrought by bark beetles in recent years, it’s natural to think of them as unstoppable eating machines. “Because we notice the big outbreaks,” said Ken Raffa, a University of Wisconsin entomologist who co-authored the PNAS study, in an interview, “it’s easy to picture forests as one big salad bar for the insects. But most of the time, they only go to the most stressed trees.”

That’s because trees have evolved defenses against the beetles in the form of natural chemicals that repel or even kill the attackers. In times of severe drought, however, the number of trees under stress in a forest will skyrocket. When that happens, as it did during a 2001-2002 Colorado drought documented in the Ecology study, the dramatic increase in relatively defenseless trees allows beetle populations to skyrocket.

“Once the beetles reach epidemic levels,” Teresa Chapman of the University of Colorado, the study’s lead author, said in an interview “the epidemic has a life of its own. Now, even if favorable conditions return, there are just too many beetles and [the trees] can’t defend themselves.”  

Favorable conditions will probably return less easily in years to come, however, as with a warming world, scientists expect droughts to be more intense, come more often and last longer.

Meanwhile the beetles, whose numbers would normally be held in check by cold winters that kill their larvae, are surviving in greater numbers from one year to the next as winters in the U.S. continue to get warmer. Not only that: as spring comes earlier and temperatures stay warm for longer, the beetles can fly further than they once did, allowing them to extend their range.

This range extension includes movement to higher altitudes, where beetles are encountering tree species they would normally not run into. That includes the white bark pine, the subject of the PNAS study. “What we find is that unlike the [lower-elevation] lodgepole pine, the white bark pine hasn’t evolved any defenses against the beetles,” Raffa said.

The combination of increasing stress on trees and decreasing limits on the beetles’ range and winter survival, say scientists, makes it almost inevitable that the insects will continue to experience a population boom — at the expense of the great conifer forests of America’s mountain west. 

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By deon geldenhuys
on January 8th, 2013

Forests that are under stress with trees to close to one another , do not have a natural ability to withstand attacks from certain insects such as wasp larvae etc….

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By Jim Bowron (Edmonton Alberta Canada)
on January 8th, 2013

Apart from the mention of NORTH America in the first line, one might draw from this article that it is only American forests that are being attacked. The attack stretches north of the border as well, into British Columbia and recently into Alberta. Not only are the beetles going into higher altitudes, they are going into higher latitudes, and the boreal forest is now under threat.

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By Guenter Ambron (Cave Junction, OR)
on August 29th, 2014

The Boreal has laready experience a 30% die back.

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By Lcarey
on January 8th, 2013

Is it correct that the beetles have now breached the Continental Divide in Canada and are heading east toward the great northern Boreal forests of Canada?

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By Mike Lemonick (Princeton, NJ 08542)
on January 8th, 2013

Jim Bowron reminds us that this is definitely not a problem that respects international borders. I believe there have been bark beetle outbreaks in B.C. for quite a while now. I hope I get SOME credit for not being entirely USA-centric, like so many of us are in this country (we do have a Canadian scientist on staff, Alyson Kenward, who helps keep things in perspective). But a single mention of North America is clearly not enough. So thanks!

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By Warren Clarida (Iowa City)
on January 9th, 2013

It was a little while back but there has been this paper:  “Mountain pine beetle host-range expansion threatens the boreal forest” doi:10.1111/j.1365-294X.2011.05086.x which definitively showed that beetles have been attacking the jack pines that cross the entire continent.  This doesn’t mean that they’ve “breached the Continental Divide” per se; colder temperatures due to latitude and elevation affect the ability for the population to last, obviously this will change.  Also there is some good discussion in the article concerning the differences in susceptibility between the jack pine and the lodgepole pine.  Long story short more study is needed…

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By wg (Houston Tx 77036)
on January 14th, 2013

Droughts are not uncommon in the west. In the late seventies Denver had to draw down Dillion reservoir and global warming wasn’t on the radar. In my eighty years I have seen Mother Nature do whatever she wants, when she wants.I wonder how Yellowstone is doing with the beetle infestation since Mother Nature did her cleanup? By the way I believe that even Hansen has admitted that there has been no warming in the last15 or so years.

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By Tom (Austin, TX 78739)
on January 14th, 2013

A ranger in North Park explained to me that whenever there had been an out break before they used to log it and spray. This time the greenines got an injunction so it went out of control.

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By Dan Andrus (SPEARFISH, SD 57783)
on December 13th, 2013

In 2007 I was cutting trees in the Arrowhead lake area of California that had been killed by the beettles, we were told we had to stop because the greenies wanted a invirnmental impact study done. A few weeks later a fire swept through and burned i believe almost 200,000 acres. Now i am in the black hills of SD watching the same idiots make all the wrong decisions.

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By john galt (Columbia Falls, MT)
on January 14th, 2013

You can’t cut trees here in Montana on federal land. Every time there is a proposal to log on fed forests, there is a lawsuit by the eco-fascists backed up by Judge Molloy in Missoula every single time.

I once asked a forest service emplyee in charge of supervising controlled burns the following question. Why are you guys burning the trees instead of logging and replanting. By harvesting trees which end up as lumber, no carbon is released and newly planted forests soak up more carbon. He agree totally but then said,“Well, it costs $50 an acre to burn the trees and $500 to litigate with the enviro nutcases. That plain enough for you. So we burn the trees”.

Also, these beetles are not from outer space. They have been around a long time. There have been other droughts and warming periods where the beetles thrived and the trees died. We are going through another time now. Why not plant spruce, western larch and firs instead of lodgepoles. Oh whoops I forgot no logging and no planting.


Oh one more thing. Wait until the fires are purposely started as terrorism not as forest service policy. I am excluding lightning caused and man caused fires from campisites etc.  Trust me it’s coming to a state near you. Not from people like me who are in favor of responsible logging and conservation but from others who will make themselves known. This isn’t a threat, just what is happening in another country. 

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By Steve Bloom (Oakland, CA 94610)
on January 20th, 2013

I’ve been tracking the MPB situation for some years.

Saying “I believe there have been bark beetle outbreaks in B.C. for quite a while now” is like saying that Hiroshima was slightly damaged by bombing in WWII. The forests there have been devastated.

Re the beetles crossing the continental divide, that’s history. There are considerable infestations in north-central Alberta. Now the first outliers are in the edge of the boreal forest proper in northwest Saskatchewan.  See this article, although since it’s from last spring the scientists involved will have updated information.

The upshot is that the spread of the beetles across the boreal forest is inevitable, noting that the only thing that’s been delaying them is the relative sparsenss of the forest in the central Alberta region. Just past that barrier, in Saskatchewan, the forested area quickly expands far to the south into climate zones far more welcoming to the beetle.

A further wrinkle to this story is that there are non-pine conifer species making up much of the boreal forest, at least some of them have their own insect predators that are also responsive to climate change.

So a follow-up on this article would be good.

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By Ludvig (Creel, CH, Mexico)
on March 1st, 2013

It’s not the bark beetle that kill the tree, it’s a fungus carried by the female insect that kills it. I’ve witnessed the expansion of this insect in northern Ponderosae mexican forests, becoming a gradually increasing problem all over northern and central Mexico. Forests are being devastated here too. I’ve detected Ips and Dendroctonus kinds of bark beetles in this area.

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