IDAHO FALLS, Idaho — U.S. Forest Service officials say the number of trees being killed by pine beetle infestation is on the decline, in part because the insects have eaten themselves out of house and home.
The beetles burrow under the bark of pine trees to lay eggs. Once the eggs hatch, the larvae eat the inner layer of bark, killing the tree. An aerial survey in 2010 showed beetle-killed trees on 9.2 million acres of public and private land in Western states, including nearly 2 million acres in Idaho.
The number of infested acres in Idaho has dropped 63 percent in the years since, to about 719,000 acres in 2012.
Experts feel confident the decline will continue.
"(It) is declining for multiple reasons, but the main factor is they've killed most of their suitable hosts," U.S. Forest Service entomologist Carl Jorgensen told the Post Register (http://bit.ly/W2gmHo ).
Once a tree is infested, it can't be saved.
"There's nothing we can do for it at that point," said Tom Eckberg, Idaho Department of Lands Forest Health Specialist.
The beetles prefer lodgepole pine, whitebark pine and ponderosa pine. They seek out trees with the thickest layer of phloem — the inner layer that larvae feed on — and that means they typically attack trees that are at least 8 inches or larger in diameter, that are 80 years old or older, and that are growing at an altitude of 6,500 feet or lower.
When adult beetles find a suitable host, they send out a pheromone to signal other beetles to the tree, Eckberg said.
After the tree is full of beetles, another pheromone is sent out telling new insects to find another home, said Sandy Kegley, a Forest Service entomologist.
"They will continue to kill trees until they run out of suitable hosts or very cold weather kills a large portion of beetles," Eckberg said.
Last year's massive wildfires, which burned about 1.7 million acres in Idaho, also may have contributed to a decrease in the acres lost to beetles this year.
"The fires could have killed the trees the beetles would have eaten," Eckberg said. "But that's uncertain."