Colorado's distinctive lodgepole pine trees are under attack from a beetle infestation described by scientists as a "perfect storm" which could destroy 90% of the western American state's pine forests.

The bark beetle outbreak was responsible for the death of 4.8m lodgepole pines in Colorado last year, up from 1m in 2005. The infestation has spread across 1,000 square miles of forest - nearly half the total in the state. Forty three per cent of the state's lodgepole pines have died as a result of the infestation. But it is not limited to Colorado: the beetles have munched their way through the western US and Canada, affecting 36,000 square miles of forest.

"I knew we would have an infestation," says Jan Burke, a silviculturist for Colorado's White River national forest, "but I never remotely imagined this. Nobody predicted this." She looks up at the mountains behind the ski resort of Vail, sweeping hillsides of pine pockmarked with the orange stain of dead trees and the delicate feathery grey of aspens. "I guess we're the lucky ones because in our lifetime we got to see these forests. Our children won't. For many that's a bitter pill to swallow."

The results of the infestation in Colorado appear catastrophic for the pine forests that are familiar to thousands of visitors each year, many of them Britons heading for ski resorts including Aspen, Breckenridge and Beaver Creek, as well as Vail.

If the image of Colorado in the holidaymaker's mind is of snowcapped peaks and pine-covered slopes, those notions will have to change. As the lodgepole pines die, the dominant tree species will be aspen, its grey bark and light foliage replacing the dark green of the pine.

For now, the hillsides look familiar, with snow covering the dramatic slopes west of the Rockies. But even a cursory inspection reveals the devastation caused by the beetle. Among the green of the pines, the orange patches indicate beetle infestation and dead trees, what Burke calls "microsites of mortality". Forestry officials calculate that for every orange-brown pine that can be seen by the naked eye, another five to 10 are infected.

Once the beetle gets to the tree, nothing can be done.

Scientists, environmentalists, ski resort managers and forestry officials agree that by the time the beetle has finished, it will have killed 80-90% of the mature lodgepoles in Colorado. Mature trees account for 90% of the lodgepoles. While beetle infestations are part of the natural order of the forest, the current infestation defies all expectations.

"We're two to three years from seeing virtually the death of mature lodgepole pines," says Burke. "To the casual observer it will look like all of them. It's wholesale mortality. It's difficult to watch these really beautiful stands die. It just makes you want to go home."

The beetle behind the infestation, the half-centimetre long Dendroctonus ponderosae, is indigenous to the area, and has been responsible for epidemics in the past, most recently in 1983. The beetle only attacks mature pines, boring through the bark to lay eggs inside the tree. When the larvae reach maturity they in turn bore a hole out and leave the tree en masse for the next host, where they will lay their eggs.

But unlike previous infestations, which experts view as part of the forest's system of self-management, this time a combination of factors has conspired to propel the beetle population to epidemic proportions. Almost the entire stock of lodgepole pines in Colorado is mature, a result of the settlement and logging of much of the area 100 years ago. Consequently, the beetles have an abundant source of food. A long-term drought has put the trees under stress and deprived them of one of their defences against the bugs, the ability to exude sap and expel the insects.

Warmer weather - a recent study showed that over the last 50 years Colorado's average high temperature had risen by nearly half a degree Celsius and the average low temperature by nearly one degree - has accelerated the growth of the insects and enabled them to survive the winters. Normally Colorado would expect to experience a sustained period of cold weather during the winter months. But the week or more of temperatures lower than -10C has not materialised.

"The current outbreak is so far along that there's nothing much that can be done," says Bill Romme, professor of fire ecology at Colorado state university and lead author of a report into the infestation published in November. "If we get a period of bitter cold temperatures, that could cause the outbreak to collapse. But we're not getting the temperatures. Usually an insect outbreak will go on for one to three decades. We're about a decade into this one but at the rate they're moving they could run out of food before then."

Romme is reluctant to attribute the outbreak to global warming. "We can't say that the current outbreak is the product of global warming," he says, "but we can say that this is a harbinger of what could be produced by global warming." The devastation has caused much soul-searching among those who manage the forest, as well as those who have pursued a dream of moving to the pine-covered slopes of Colorado.

"It's not so much an ecological challenge as a social and economic challenge," says Don Carroll, acting supervisor of the White River forest, who heads the Colorado Bark Beetle Cooperative. "If it wasn't for all the people that lived here it wouldn't be much of an issue." He reels off a list of the safety issues raised by the death of the pines, from the fire hazards caused by dead trees to the possibility of timber falling on homes or power lines and the threat posed to the water table.

He is also concerned about extracting the fallen timber from the forest, from trees that have died and fallen and from those that have been logged to prevent them falling.

"We don't have the industry to handle this stuff," he says. "We're not going to get 2x4s out of these trees in the future, so we're looking at new technologies to utilise biomass fuels."

But the forest service has met opposition from those who are against the large-scale felling of trees and it has been difficult to persuade communities - some of them among the US's richest - of the need for mass logging. Vail has committed $1.5m (£770,000) to logging trees in the hope that it will be able to retain some of its green mountainside.

"I fail to see how a massive outbreak of logging will do any good," says Rocky Smith, forest watch coordinator for the environmental group Colorado Wild. While not opposed to logging to protect populated areas, he thinks the forest service is going too far.

"Logging in the back country isn't going to achieve much," he says. "They really ought to prioritise a little more than they are."

Jan Burke gestures beyond the skiers enjoying the spring snow among the pines in Vail. "It doesn't look so infested," she says, "but you're looking at millions of dollars of work. They want to hang on to the green forest as long as they can but it's too little, too late."

Burke retains a sense of determined optimism. "You can drive around and it does look like a thermonuclear device has hit," she says, "but in two to three years it will start to regenerate. It's not going to look like a moonscape. Some areas will, but only in the very short term. There will be a flush of flowers and then we'll see a lot more aspen."