Reproductive swell worsens beetle kill
Discovery of pine beetles’ producing two generations a year helps explain increasing damage, CU researchers say
By Clint Talbott
Just a few decades ago, Colorado mountain pine beetles generally emerged from trees in the second or third week in August. They flew to healthy trees nearby, then burrowed inside, often killing the trees.
Once inside the pines, they laid eggs, which grew into beetles that emerged the following year. Like tax day, a new generation of beetles came just once a year.
Jeffry B. Mitton knows this very well. He is a professor of ecology and evolutionary biology at the University of Colorado Boulder and has been studying mountain pine beetles for more than three decades.
But in 2008, Mitton and graduate student Scott Ferrenberg noticed something unusual at CU’s Mountain Research Station. It was June, but the beetles were already flying.
Mitton later mentioned this to a forester, who assured Mitton that, “Those aren’t mountain pine beetles.” Further, the forester added, early beetle flying “just doesn’t happen in Colorado.”
The skepticism fueled Mitton’s curiosity, and he and Ferrenberg studied the beetle life cycle for the next two years.
As they ultimately confirmed, beetles do fly early in Colorado now. Further, beetles are flying so early that they are often able to produce two generations a year, rather than only one. That finding, believed to be the first confirmation of this reproductive explosion, helps to explain the staggering scope of the current pine-beetle epidemic.
Because of the extra annual generation of beetles, there could be up to 60 times as many beetles attacking trees in any given year, their study found. And in response to warmer temperatures at high elevations, pine beetles also are better able to survive and attack trees that haven’t previously developed defenses.
These are among the key findings of Mitton and Ferrenberg, whose article on the rapid reproductive cycle of beetles was published in The American Naturalist this spring.
“It used to be the case that a single female would put out approximately 60 eggs,” Mitton says. “If all of those eggs survived and emerged as adults, then she would have 60 offspring in one year. Now, a female can have 60 offspring from June to August. And each one of those would have 60 offspring to give her, so in one year, 60 offspring plus 3,600 grandchildren.
“So what that means is there has been an exponential increase in the number of bark beetles flying in the forest, and very simply, the more beetles that fly, the more trees are attacked.”
This exponential increase in the beetle population might help to explain the scope of the current beetle epidemic, which is the largest in history and which extends from the Sangre de Cristo Mountains in New Mexico to the Yukon Territory near Alaska.
“This thing is immense,” Mitton said. The duo’s research, conducted in 2009 and 2010 at CU’s Mountain Research Station, located about 25 miles west of Boulder, helps explain why. The research was funded by the U.S. Department of Energy.
“We followed them through the summer, and we saw something that had never been seen before,” Mitton said. “Adults that were newly laid eggs two months before were going out and attacking trees” — in the same year.
These effects may be particularly pronounced at higher elevations, where warmer temperatures have facilitated beetle attacks. In the last two decades at the Mountain Research Station, mean annual temperatures were 2.7 degrees Fahrenheit warmer than they were in the previous two decades.
Warmer temperatures gave the beetle larvae more spring days to grow to adulthood. The number of spring days above freezing temperatures increased by 15.1 in the last two decades, Mitton and Ferrenberg report. Also, the number of days that were warm enough for the beetles to grow increased by 44 percent since 1970.
The Mountain Research Station site is about 10,000 feet in elevation, 1,000 feet higher than the beetles have historically thrived. In their study, Mitton and Ferrenberg emphasize this anomaly.
“While our study is limited in area, it was completed in a site that was characterized as climatically unsuitable for (mountain pine beetle) development by the U.S. Forest Service only three decades ago,” they write.
But in 25 years, the beetles have expanded their range 2,000 feet higher in elevation and 240 miles north in latitude in Canada, Mitton said.
Ferrenberg had the idea to monitor the beetles at higher elevations partly because trees at lower elevations have been attacked by beetles for centuries and have developed some defenses.
Lodgepole pines at higher elevations tended to have a lower density of resin ducts, which transport resin, the sole defense against beetles. The number of resin ducts in a tree can be a “marker” for whether a tree has a higher or lower resistance to a beetle attack, Ferrenberg said.
The trees at higher elevations had not faced the same intensity of beetle attacks as those at lower elevations until temperatures warmed, and they have not faced pressures of natural selection exerted by attacking beetles. “The trees in that area are somewhat naïve in their response,” Ferrenberg said.
These data help explain why westbound motorists emerging from the Eisenhower Tunnel on I-70 can look up, from 11,000 feet in elevation, and see beetle-killed trees. “We think we see some of the reason for the fact that this epidemic is so widespread,” Mitton said.
Nonetheless, some remain skeptical.
“Quite literally, I have been told that we didn’t see bark beetles in June,” Mitton says. “When I confirm that we have, the response is, ‘This is not possible.’”
“There’s a fair amount of inertia in the way some people are thinking about the world,” and Mitton understands why. For beetles to be thriving 2,000 feet higher than was possible only a few decades ago, “it’s such a transformation that it’s hard to accept.”
But as ecologists and evolutionary biologists, Mitton and his colleagues have been observing similar transformations in many species. Robins now get to the Colorado mountain town of Gothic three weeks earlier than they used to, and yellow-bellied marmots emerge 23 days earlier than they did a quarter-century ago.
“The fact that a bark beetle would do this is not surprising to us.”
At the same time, Mitton suggests that the epidemic should not spawn despair. “The ponderosa and lodgepoles are not going to disappear,” he notes. “The big trees will.”
Since the Europeans settled in North America, the number of aspens has declined. A likely Rocky Mountain future will include more aspen intermixed with conifers, he adds.
“If you want to look on the bright side, aspen are quite pretty in the fall.”
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