DENVER, Sept. 5 (UPI) -- Tiny bark beetles the size of a black bean have killed entire mountainsides of pine trees in the western U.S, but ecologists and forest managers disagree about whether they are a pest or a boon to wildlife ecosystems. Whether trees killed by the beetles spread wildfires quickly has been called into question by new studies.
Millions of gray, denuded trunks, called snags, cover the mountainsides of Arizona, Colorado, Wyoming, New Mexico and Utah. The beetles have killed off more than 40 million acres of Canadian pines in Alberta and British Columbia, Canada.
Other hillsides are dotted with browning and reddish pines that are slowly dying because their circulation systems are clogged with beetle larvae. Their bark is covered with tumors of red dripping resin, leeching life out of the trees.
Two waves of pine beetles have spread through the Rocky Mountain West in the last 20 years, encouraged by warmer temperatures and drought conditions that weakened the resistance of the largest, densely growing pines.
"It was a one-two punch," said Dan West, the Colorado State Forest Service's state entomologist.
Between 1996 and 2014, mountain pine beetles swept through ponderosa and lodgepole pine stands, especially after forest fires. Beetles affected pines in 3.4 million acres of National Forest land in Colorado, but the trend appears to have slowed considerably, West said.
Then in 2012, a wave of spruce beetles began to kill off high-altitude Engelmann and blue spruce trees.
Drivers heading west along Interstate 70 across the continental divide see entire mountainsides spotted with ghostly gray trunks throughout the White River National Forest and near the ski towns of Breckenridge and Vail. The beetles also have decimated forests in Rocky Mountain National Park, the San Juan Mountains, West Elk Mountains and Sawatch Range.
The bugs are native to the West and have co-evolved over millennia to battle with the pine trees, West said. The insects move slowly, but have spread in a wave across the western United States.
"They don't fly very well," West said.
Drawn by the terpenes scents of the trees, female beetles drill under the bark. If the carbohydrate-rich woody underlayer pulp meets her "gustatory acceptance" the beetle will release a plume of pheromones to draw males and other beetles to the tree, he said.
Beetle eggs stew over the winter in a sort of antifreeze and larvae hatch the next spring and begin to consume the underlayer of the bark, creating a girdle that interrupts the tree's ability to move water and nutrients. The beetles only emerge from the bark when they mature to fly off to find another tree.
Healthy trees fight back with a beetle-toxic resin that flushes the bugs out, West said.
Densely packed trees
The problem is that decades of growth and a forest-management policy of mandatory fire suppression have created a monoculture of trees of the same species and the same age. Larger trees with a 14-inch diameter, densely packed together, are preferred beetle targets.
"They like the bigger trees. They want to save something for their grandchildren," West said.
Several years of severe drought weakened the mature trees' resistance and gave them less liquid to produce the cleansing resin.
Also, temperatures in the mountains no longer drop to minus-35 degrees Fahrenheit or lower, which in the past froze out about 20 percent of the beetle eggs.
In a light airplane every year, West and members of the U.S. Forest Service create an aerial survey of new forest areas that show the first signs of beetle infestation. Both waves appear to be abating, but they have permanently changed the mountains in Colorado, and new-growth forest will take decades, West said.
In Colorado, custom carpenters now use blue-streaked beetle-killed pine to make unique furniture.
In Yellowstone Park, the impact of whitebark pine beetles was cited in 2011 by a federal appellate panel as one of the reasons that grizzly bears should remain protected by the Endangered Species Act.
Yellowstone Grizzlies gorge on high-protein white bark pine nuts at high altitudes in the fall before hibernation, the panel's decision said. A beetle infestation of the whitebark pines meant that grizzlies would find it harder to forage for food.
Conservationists also worry that the astounding number of dead trees is reducing the planet's ability to absorb carbon dioxide from the air.
But some ecologists say the dead trees killed by beetles are an important part of a forest ecocycle and help encourage wildlife.
"Snag habitats are unique and ecologically rich," said Chad Hanson staff ecologist and director of the California-based John Muir Project. He calls the beetles a "keystone species."
Hanson argues that beetle-killed trees provide habitat for woodpeckers, who eat the beetles and peck nest cavities that are used by other creatures like bluebirds, nuthatches and chickadees as well as chipmunks and flying squirrels.
Raptors such as hawks and owls feast on those animals, he said. The new scrub brush that grows in the sunny clearings where snag trees have died is also a habitat for forest mammals, he said.
Even though they're shocking to see, beetle infestations are not new, and multi-acre dead pine patches were described across the West during the 19th century in reports by the General Land Office, a precursor to the Bureau of Land Management, Hanson said.
Reports of beetle-killed mass pine die-offs were also described as early as 1803 in Southeastern states Tennessee and Georgia, according to a 1992 report from the Georgia Forestry Commission.
Hanson worries that beetle-kill forest patches give the U.S. Forest Service an incentive to sell lumber to private logging operations, using the excuse that they might spread wildfires.
But snag trunks are not nearly as combustible as live trees and ground cover, Hanson said. He has studied forest health and wildfires and is the author of The Ecological Importance of Mixed-Severity Fires: Nature's Phoenix.
"Forests with bark beetle snags burn less, not more," he said. "If you want to start a campfire, you don't start with the big log; you start with kindling."
Colorado's West also noted that snag trees do not spread wildfires through the forest canopy, but might contribute to the debris on the forest floor that could catch fire.
A new study published in the Proceedings of the National Academy of Science showed that although beetle-killed forest and wildfires both increased in the Western United States between 2002-2013, "[wildfires] did not increase in direct response to bark beetle activity," the authors said.
Research by NASA, using satellite imagery, also suggests that forests with higher levels of snags from beetles burn at lower intensities, Hanson said.
Instead of a wildfire risk, pockets of dead snag trees on a mountain slope are a sign of the "ecological resilience of the forest," Hanson said.
He added: "It can be shocking to a lot of people that the trees are dead, but the forest is full of life."