Armies of tree-boring bugs are transforming some of Yellowstone National Park's green forests into seas of red, dead trees.
The bugs - four species of bark beetles and one budworm - have infested more than 50,000 acres in perhaps the largest insect outbreak recorded in Yellowstone.
In the past, the trees of Yellowstone have faced one species of bug or another at any given time but, at least in the past 100 years or so, haven't had to deal with so many types at once.
"They've never really been that synchronous to the degree they are now," said Roy Renkin, a Yellowstone biologist who's tracking the insect outbreak.
The bugs have come on strong in the past five years, taking advantage of trees weakened by drought, boring beneath the bark, breeding and feasting on the tree until it dies.
The insects are native to the Yellowstone ecosystem and are fulfilling their role as regulators of the region's forests. Like wildfire, the bugs operate in cycles and help spur life, death and a vital exchange of nutrients.
"It's something that has happened in the past and will continue to happen," Renkin said. "Disturbances are very integral to these forests."
Swaths of red-needled trees - those that were attacked and killed by insects last summer and will be ghosts of gray next year - are visible across Yellowstone. Visitors have taken notice.
"We've been getting a lot of questions about the trees," said Cheryl Matthews, a park spokeswoman.
No one knows how long the insect infestations will last and how many acres they will eventually cover.
"From the historic information, outbreaks can last three or four years or last 15 years," Renkin said. "But one common theme from them is they all end very abruptly."
Biologists have been tracking the bug outbreaks in Yellowstone from the air, documenting each year's growth and change.
Lately, the most voracious bug has been the mountain pine beetle, a shiny black-shelled insect about the size of a pencil eraser. The bug has taken over about 18,000 acres of whitebark pine in Yellowstone and 700 to 800 acres of lodgepole pine, according to park biologists
In some places, such as the northwestern corner of the park, the beetle has killed about five whitebark pine trees per acre. Farther south, about 80 trees per acres have died.
The beetle's taste for whitebark pine seems relatively new.
Typically, mountain pine beetles have gravitated toward lodgepole pines and only nibbled on whitebark pines, which grow at higher elevations.
But in 2000 and 2001, biologists saw a dramatic increase in beetle activity in whitebark pines. There's some evidence from a University of Montana study that indicates the drought is having a more severe effect on whitebark pines than lodgepoles because of the difference in elevation where they grow. But if the drought continues, lower-elevation lodgepoles soon may be more susceptible to bug attacks.
"But right now, whitebark pine at higher elevations is taking the brunt," Renkin said.
Whitebark pine trees play an important role in the Yellowstone ecosystem because they produce a fatty, high-energy nut that is a staple for some grizzly bears. Studies have shown that, when the trees produce a good crop of nuts, grizzlies spend more time in higher elevations and less time in valleys, where they run into trouble with people.
Elsewhere in Yellowstone, the Engelmann spruce beetle has attacked about 8,700 acres; the western balsam bark beetle has infested about 6,300 acres of subalpine fir; the Douglas fir beetle has hit 4,000 acres; and the western spruce budworm has affected about 14,000 acres of Douglas fir and Engelmann spruce.
Typically, a female beetle bores into a tree and lays eggs beneath the bark. Soon the young bugs and the adults form a girdle around the trunk, choking off important nutrient supplies for the tree.
In normal conditions, most trees have enough strength to squeeze out the beetles by releasing pitch. But in a drought, the trees' defenses are weaker.
Several insect outbreaks have occurred in the past at Yellowstone. Each time they have died out on their own, but park managers have also taken aggressive steps, including logging, burning and, in the 1950s, spraying infested Douglas fir with about 62 tons of DDT, an insecticide that was banned in the United States 20 years later.
"It's been proven over time that attempts to do something about it tend to be futile," Renkin said.
These days, there are no plans to try to kill or drive away the bark beetles in Yellowstone. Managers look at the bugs as just another part of Yellowstone's complex ecosystem.
If there's going to be any letup in the rash of beetles, it's likely to come from a sustained cold spell that will make it hard for them to survive the winter, biologists said.
"We're letting the natural process take its course," said Dan Reinhart, a resource-management specialist at Yellowstone.Billings Gazette
