Double Trouble From Mountain Pine Beetles?Posted: April 18, 2012
Source: NY Times
The evolution of mountain pine beetles to produce two generations of beetle per year instead of one has probably been a factor in the unparalleled damage that insects have caused in pine forests in the western United States and Canada over the last decade, according to a newstudy.
The findings, published online in The American Naturalist, may explain in part how the scale of beetle-killed forest in the West became so vast. The number of acres killed in the last decade or so is 10 times that of any previous epidemic recorded, and it’s not over. The damage stretches from California across the prairies to the east, and from New Mexico to northern Canada.
Jeffry B. Mitton, a professor of ecology at the University of Colorado at Boulder and the lead author of the study, and a graduate student, Scott Ferrenberg, watched the bugs attack 15 pine trees in the southern Rockies in Colorado in the first week of June, about six weeks earlier than normal. Then the bugs laid eggs that hatched, and the new beetles flew to other trees in early August, the time of year when the species would normally attack a tree.
Two generations of beetles in one summer greatly increases the army that attacks trees. “It’s not twice as many beetles, it’s an exponential increase,” Dr. Mitton said. Each beetle lays 60 eggs and, since nearly all survive, each of those beetles goes on to lay 60 eggs in the same summer, which means 3,600 more beetles.
Diana Six, an entomologist at the University of Montana who is researching forest die-off, said she considered it “completely possible that there are two generations a year.”
“We’ve seen it before,” she said, although she suggested this is the first time it has been documented.
Dr. Six said she did not believe the two-generation phenomenon necessarily caused the huge die-off. “Around the West, it might be happening more and more often in micro sites, but I don’t think it’s widespread yet,” she said. “But I think that’s on the way — it’s one of the things we’re worried about.”
“It’s a scary warning of things to come,” she said.
Because of warming in the West, the voracious mountain pine beetles have moved to higher elevations over the last 25 years. In the 1970’s they did not live at altitudes above 9,000 feet, but they are now found at 11,000 feet and even higher because winter temperatures there are not cold enough to kill them. It is a serious problem because trees above 9,000 feet have not evolved to deal with the bugs and therefore have few defenses. “High-elevation trees are naïve to this,” said Mr. Ferrenberg, a co-author of the paper.
Trees at lower elevations fend off the beetle attacks by pumping resin into the hole where the bug is boring in, often killing it.
Meanwhile, many spruce beetles, which needed two to three years to complete a generation, now need just one year, a trend that is expected to cause an increase in spruce deaths.
Like much of the rest of the country, the West has seen one of its warmest winters ever. “Missoula never had below-zero weather,” said Steve Running, an ecologist at the University of Montana who works with Intergovernmental Panel on Climate Change. “That’s rarely happened before. The average is 10 to 12 nights below zero.”
It takes temperatures that remain in the 30 and 40 below range for weeks to kill the bugs, which hasn’t happened in many years.
“These warmer winters and hotter summers are driving this ecosystem response,” Dr. Running said. “And you can expect to see more ecosystem changes just as dramatic as it gets warmer.”