IN THE PALM OF YOUR HAND, they don't look like much: a black bug the size of a chocolate sprinkle. Put one under a microscope and the mountain pine beetle, Dendroctonus ponderosae, is hairy and lethal-looking, a shovel-faced, tanklike chewing machine with legs. Since the first outbreaks began, in the early 1990s, the beetles have infested more than 81,700 square miles of North American forests, an area larger than Nebraska. Last year, in my home state of Montana alone, the plague more than doubled—from 1.2 million acres in 2008 to 2.7 million in 2009. Depending on whom you talk to, this makes Dendroctonus either the most destructive insect in the recorded history of North American forests or, as some ecologists see it, a massive influx of "ecosystem engineers" working tirelessly to improve woodland biodiversity. Call me anthropocentric, but the idea of a bug in the driver's seat doesn't inspire much confidence.

Take this lodgepole I'm standing next to, one of the beetle's favorite pines. It is green-needled and vigorous-looking, but it's actually dead as a phone pole. The bark is riddled with hundreds of bulging, popcorn-size hits. Made of sawdust, pitch, and bug scat, these spots mark where last year's hatch of adult beetles chewed their way in to lay their eggs, which turn larval by the fall. Infused with natural antifreeze that withstands temperatures down to 30 below, Dendroctonus larvae hibernate till summer, when they pupate, abandon their brood tree, and take flight to attack fresh stands. The short of it is, this green tree's a goner. By next spring the needles will fade; by summer, they'll turn red. Not some feel-good autumn-in-Vermont red, either. More like the rocker-panel rust on a junkyard Dodge.

There's something about a red evergreen that says we're deeply screwed. Take a drive over Montana's 6,320-foot MacDonald Pass and you'll see these trees by the millions, spilling down the Continental Divide like leftover marinara. Some ecologists say there's little to worry about from Dendroctonus. They're quick to point out that these beetles are endemic—that they've been around as long as the forests themselves, that they cull older, weaker trees, and that these outbreaks come and go naturally over the centuries.

But this time, things have changed. Stressed by less rainfall and a warmer climate, our high-altitude lodgepole stands can't muster enough pitch to flush out the invaders in the numbers the bug now presents. Low temperatures used to control the beetle population, killing most of them off each winter, but the last time my hometown of Helena saw sustained temperatures of 30 below was 1996. As populations continue to multiply, as they continue to thrive at higher elevations and latitudes and develop tastes for new species (the whitebark pine is now functionally extinct in some places), human intervention has so far proved maddeningly ineffective.

There's never been a better time to be a beetle. With the forests spread before them like a cruise-ship buffet and no serious predators to worry about, the beetles are writing a whole new script, one in which they're no longer mindless, ravening insects but a particularly exuberant tribe of eco-nihilists. Or perhaps just another few billion insatiable consumers and, in that way, not so different from us.

Nobody knows how all this will play out. But hard-hit Colorado already stands to lose virtually all of its mature lodgepoles. High-altitude trees help retain winter snowpack; if they go into decline, spring runoffs could radically change, affecting local water management. In places like Yellowstone, the whitebark's demise could eliminate one of grizzly bears' main food sources, the tree's cone seeds. And what happens when all these carbon-absorbing trees decay and become carbon producers? The temperature goes up another notch.

At this point, the situation is much like what happens when a Third World country unexpectedly goes nuclear: There's widespread alarm followed by a rush to learn about a long-ignored, now ominous threat. Who are these bugs anyway, and what do they have in mind for the Rockies?

IT'S ONLY NATURAL THAT a panorama of several million dead pines would have a disquieting effect on the psyche.