The tiny insect can turn entire pine forests into bleak landscapes.
Mountain pine beetles, which have devastated lodgepole pine forests in western North America, have been found in a new type of tree, the jack pine.
Jack pines cross the entire continent, which could ultimately wipe out pines on the U.S. East Coast.
Cold temperatures currently keep the spread at bay, but they may not last as the climate warms.
The mountain pine beetle is one step closer to crossing North America and turning eastern pine forests into the same bleak landscapes that have scarred the Mountain West in recent years.
Some scientists say it is inevitable that the tree-killing insect will spread all the way east via a corridor of jack pines across northern Canada's boreal forest. The question is: When? Others are more skeptical.
Either way, researchers have now shown that the beetle has invaded a new species of pine on the eastern side of the Rocky Mountains in Alberta, Canada, with potentially serious consequences.
In a report published this week in Molecular Ecology, researchers analyzed DNA from trees at the frontier of the beetle invasion in Alberta to demonstrate that the beetle is now infesting jack pines in addition to lodgepole pines, its typical target.
Jack pines can hybridize with lodgepole pines, so it is difficult to be certain that an infected tree truly is a jack pine, rather than a hybrid. The genetic tests provided certainty for researchers' suspicions.
"I was able to say: 'Yes these are jack pines that have been attacked by the mountain pine beetle,'" said Catherine Cullingham of the University of Alberta in Edmonton, who led the new study.
While lodgepole pines' range is limited to western North America, they give way to jack pines on the eastern slope of the Rockies. Jack pines cross the boreal forest of northern Canada, stretching all the way to the East Coast and down into the United States.
Long wind drafts carried beetles across the Rockies from British Columbia's record pine beetle outbreak to the jack pines in Alberta where they have now become established, Cullingham said.
Two factors could help slow the beetle's eastward progress. The first is that jack pines occur in patchier stands than the continuous swaths of lodgepole on the west coast. This may offer some protection against a continental sweep, depending on how jack pine fares as a host compared to a lodgepole pine.
A second key factor is that so far, research indicates that the northern route across Canada is too cold in winter for the beetles to survive, although that barrier may be temporary as climate warms.
"Currently the climatic conditions over much of the northern boreal forest aren't quite suitable for these populations," said Allan Carroll of the University of British Columbia, who was not a part of the study. "We fully predict that in short order it will become good for mountain pine beetles -- certainly within the next 30 to 50 years."
"I think this work is highly significant," said Jesse Logan, a retired USDA Forest Service entomologist. "We hypothesized this even, and in fact included a map showing the linkage of lodgepole pine to eastern U.S. jack pine through the boreal forest in our 2001 paper. Unfortunately, it appears that our predictions are becoming realized."
He agrees that if global warming continues unabated, the cold barrier will disappear. A second important point, he noted, is that "mountain pine beetle is now established in a geographic region (and host) where it has never occurred."
"On the whole, we can conclude that the mountain pine beetle is now a component of pine forest east of the Rocky Mountains and will spread eastward," Carroll said. "The rate of that expansion is currently unknown."
"We're going to start seeing dead pine trees where we haven't seen dead pine trees before," he added.
Even if it takes a while to make it to the East Coast, the beetle's spread through the boreal forest can cause its own problems. The forest provides lumber, habitat and freshwater and it sequesters vast amounts of carbon. Beetle infestations could affect any of these, in part by making the region prone to massive wildfires, Carroll said.
If the beetle makes its way across the continent -- whether quickly or over several decades -- it will encounter more and denser stands of pine in eastern Canada and the United States, and warmer climates to the south, all the more favorable for expansion.
"I think it is inevitable," Carroll said. "Whether it happens in my lifetime, I don't know, but certainly now that it has managed to break the barrier that the Rocky Mountains had provided, I think its movement eastward is inevitable."
Others are less certain. "I would exercise a little caution that the mountain pine beetle has gotten over its hill and is going to roll to the Atlantic Ocean," said Steven Seybold of the USDA Forest Service and the University of California, Davis.
He suggests patches of pine much across the prairies in the United States could already have bridged the continent for the beetle if such a scenario were likely.
At the same time, he said, "if it were to get established in the east, it would be a formidable pest," he said. "It could get into a lot of (types of) pines in the east. You could even think about it getting into the south."
The beetle attacks mature trees only; its larvae ultimately kill the tree by feeding under its bark. But this age-specific targeting means species would probably not be wiped out completely.
But current forest management practices, including fire suppression, often create forests full of trees of uniform age, which are particularly susceptible. Careful monitoring and changing forest management practices to create stands of varying ages could help forests weather the coming swarm, researchers said.