Bark beetles in double-time
Bark beetles have always been part of Western forests, cycling from massive outbreaks into periods of low activity. But the current beetle outbreak is unprecedented – it has killed 30 million acres of lodgepole, ponderosa, jack pine and whitebark so far, in a swath from New Mexico up into Canada and even Alaska.
Now, scientists are finding that the immense destruction is being helped along by climate change. Warming temperatures mean that bark beetles mature from egg to tree-trashing adult in one year instead of two, and even produce eggs for a second generation.
The findings bear out predictions described in our 2004 story "Global Warming's Unlikely Harbingers", by High Country News contributing editor Michelle Nijhuis. That feature explained how years earlier, Jesse Logan, research entomologist for the Forest Service’s Rocky Mountain Research Station, and fellow scientists had begun wondering how global warming might affect beetle outbreaks, and had plugged various temperature increases into a computer model:
When temperatures hit two degrees Celsius higher than the average conditions at one of their whitebark pine study sites, prospects for the beetles improved dramatically. Beetles raced through a one-year life cycle at higher elevations. They also synchronized their emergence, allowing them to join forces and overwhelm tree defenses. High-mountain mass attack — and mass tree death — suddenly became possible.
"If Logan’s model is correct, even a few uninterrupted years of these widespread, unusually high temperatures will unleash the bark beetle as never before," wrote Nijhuis. Real-world observations seemed to bear out those predictions. Researchers in locations from southeast Idaho to Montana noticed that as temperatures warmed, the beetles' lifecycle accelerated from two years to just one.
And now, other scientists have confirmed those anecdotal observations, further validating Logan's model. Jeffry Mitton and Scott Ferrenberg of the department of ecology and evolutionary biology at the University of Colorado-Boulder have proved that pine beetles are breeding not once but twice per year, which could result in "up to 60 times as many beetles attacking trees in any given year," according to a press release describing their work. Their paper will appear in May in The American Naturalist.
Science Magazine describes how the researchers first became aware of the beetle's changed lifecycle:
Four years ago, Mitton and his graduate student Scott Ferrenberg discovered a possible explanation for this epidemic—almost by accident. While hiking in mid-June to survey pines along Niwot Ridge, due east of Boulder, they saw something strange: adult beetles out and flying. Many even landed on the researchers' clothing. The insects, Mitton says, were swarming close to 2 months too early that year. It seemed so implausible that when he told colleagues about the encounter, some didn't believe him. "This would really upset the apple cart," Milton remembers thinking.
So he and Ferrenberg spent the summers of 2009 and 2010 tracking the growth of pine beetles. They even cut observation windows into the bark of dead pine trees so they could look at larvae hiding in their nests. At first, the insects seemed to be developing as normal. But then, the beetles did the unexpected—they morphed into adults and, beginning in mid-June or even earlier, escaped from their trees. The cue for this early flight seemed to be unseasonably hot weather.
But the beetles weren't just bursting out early, Mitton adds. June-emerging bugs attacked nearby pines almost immediately, laying their own eggs. Those offspring developed speedily, becoming adults, by August or September, just in time to infest another round of pine trees—the second that season. Many Colorado beetles, then, have been able to fit a whole new generation—and an untold number of extra young—into their summers, the team found.
Logan, now retired, describes Mitton's and Ferrenberg's paper in an e-mail as "the first verifiable data for within-season completion of a full lifecycle." Logan notes that further study will be needed to figure out if that second generation of larvae survives through the winter, although the warming climate is making such seasonal mortality less likely. But, he says, "this (work) is the real deal and a significant contribution."
The author is High Country News's managing editor.
Image of mountain pine beetle courtesy U.S. Forest Service.