YELLOWSTONE NATIONAL PARK - Scientists are increasingly alarmed that global warming has removed any control that bitterly cold winters once had over the mountain pine beetle, which co-evolved with pine trees over millennia in the West.

That sets the stage for explosive growth for the pest - an outbreak that some say may sweep through North America's coniferous forests like smallpox swept through American Indian populations.

The mountain pine beetle is a natural part of pine tree ecosystems in western North America, from Mexico to British Columbia. The beetle lives in pines such as lodgepole, ponderosa, scots and limber. The beetle has a symbiotic relationship with two fungi species, and together they attack and kill pine trees as part of their life cycle and reproduction strategy. Trees die as the phloem layer is damaged enough to cut off the flow of water and nutrients between needles and roots, turning the needles a distinctive rust-red.

Most Western pine species have evolved defenses that can hold off limited numbers of beetles, but can be overwhelmed by larger numbers. Periodic outbreaks - in which thousands to millions of trees might die - run their course until a bitter cold snap of minus 40 degrees for several days can slow or stop the outbreak by killing overwintering adults and larvae.

In the past, the mountain pine beetle was always present in Western forests. If there were only a few beetles, trees were generally successful in defeating them, experts say.

"Usually, the beetle would pick off a few trees, make a few spotty kills and poop out for decades," said Diana Six, an entomologist at the University of Montana. "Pine beetles were beneficial at thinning stands, returning nutrients to soil, creating snags for birds and in general, more biodiversity."

But mountain pine beetles also played ecological roles when they had large outbreaks in lodgepole forests, she said. These periodic outbreaks have happened for millennia, setting lodgepole forests up for massive wildfires and the ultimate regeneration of those lodgepole forests.

In recent years, milder winters have triggered a massive pine beetle outbreak in British Columbia, sweeping through lodgepole pine forests for hundreds of miles.

Yet much worse may be on the horizon.

A clean sweep

Jesse Logan is a recently retired research entomologist and systems ecologist at the Utah Forestry Science Lab in Logan, Utah. Pointing to a map of coniferous forests in North America, Logan notes that it is only a thousand miles from Logan to Nacogdoches, Texas, at the western edge of the Southeast's vast loblolly pine forest.

Yet for the mountain pine beetle, which is ubiquitous in Western forests, the distance is more like 5,000 miles, as the forest pest cannot cross the Great Plains.

What could happen, Logan said, is that the mountain pine beetle could break out of the confines of Western pine forests to enter the vast boreal forests of jack pine that sweep across northern Canada.

Already, much to the dismay of foresters and entomologists, the mountain pine beetle has unexpectedly found its way over the summit of Pine Pass, near Chetwynd, British Columbia, and is less than 10 kilometers from the western edge of the jack pine forest, she said.

"Jack pine has never encountered a pine beetle," Logan said. "It has no defenses."

With no biological defenses and a dwindling prospect for bitterly cold winters, the pine beetle could sweep across Canada, then south through the Eastern and Southeastern conifer forests of the lower 48 states - a distance of 5,000 miles, Logan said.

None of those pine species at risk of mountain pine beetle invasion has evolved defenses to deal with such an attack.

"In effect, the mountain pine beetle would be acting as an exotic invader," Logan said.

High-energy defenses

Most pine species in the continent's western forests have evolved defensive strategies to deal with attacking pine beetles. According to Six, a healthy, vigorous tree can produce massive quantities of pitch to "flood out" or drown the invading beetle.

"You know the wonderful smell of pine?" Six asked. That's actually a complex chemical compound of pesticides and fungicides which can be intensified in strength when under attack, she said.

Yet such defenses are costly in terms of energy reserves, Six said. A tree stressed by drought, too much or too little sunlight, physical injury from a lightning strike, or some other disease or infestation cannot muster a sustained defense and can be overwhelmed.

Research indicates that mountain pine beetles like jack pines and other pine species just fine, Six said. Indeed, of all North American pines, only the Jeffrey pine seems to be immune to the unwanted attentions of the mountain pine beetle.