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VANCOUVER, British Columbia (Reuters) - This past winter did not provide the bitter cold needed to kill the infestation of mountain pine beetles in the forests of the western Canadian province of Alberta, so human efforts must continue, officials said on Tuesday.
The risk of the beetle's spread remains highest in northwest Alberta, making it likely it will eventually reach Canada's huge boreal forests, although its tree-killing march east becomes more difficult because there are fewer pine trees to infest.
The Rocky Mountains of southwestern Alberta are also infested, and the area remains under threat from bugs flying east across the border from infested forests in British Columbia, officials said.
Nature has historically controlled the beetle population with early winter cold snaps that kill them before they are ready for a deep freeze. Up to 97 percent of the insects have to be killed to stop their spread.
"Clearly that level of mortality was not achieved in some of the areas of Alberta this year," said Allan Carroll of the Canadian Forest Service.
Some experts have attributed the lack of early cold snaps in recent years to global climate change.
Alberta officials, who released their latest survey of the beetle population on Tuesday, said the province will have to continue its control efforts by removing infested trees and burning forests before the beetles can attack them.
"Where we have typically targeted beetles in the past we will continue to do so in the future," said Erica Lee, senior health officer at Alberta's Sustainable Resource Development department.
Western Canada's pine beetle outbreak began in the 1990s in British Columbia, where the government estimated last year it has now killed or infested pine trees in an area of 140,000 square km (56,000 square miles).
The tiny beetles, Dendroctonus ponderosae, lay eggs under the bark of mature lodge-pole pines, eventually killing them. Once beetles infest a tree, it cannot be saved.
In addition to killing the trees, the insects spread a fungus that stains the wood blue and lowers its value as lumber.
Although the insects pose a direct financial threat to the timber industry, they can also cause environmental problems such as increased flooding, by killing trees that control the rate of the spring snow melt.
"We look at the mountain pine beetle issue as a landscape issue that can effect a wide variety of resources," Lee said.
Although officials say it is likely the beetles eating their way through the pines of northern Alberta will eventually reach the boreal forest, that does not mean they can continue their spread to Eastern Canada.
"The pine moving eastward toward the boreal forest through eastern Alberta and into Saskatchewan tends to get so spotty that it probably can't support a massive outbreak the way we've seen it in British Columbia," Carroll said.
Reporting Allan Dowd, editing by Peter Galloway