Bark beetles, aided by climate change, are devastating U.S. pine forests

December 5, 2011

In the high alpine forests of Montana outside Yellowstone National Park, Jesse Logan, a skier and biologist, has watched ancient whitebark pines, the region’s mountaintop guardians, die off one by one. These long-living pines once provided highly nutritious seeds for grizzly bears. Logan says that he sometimes feels as if he is watching the collapse of a great and remote ecosystem.

The killer, active across North American pine forests, is the bark beetle, an insect no bigger than a grain of rice. Foresters around the world have been battling bark beetles for centuries, but the scale of recent outbreaks in North America has been unprecedented. Since the 1990s, vast swarms of nearly a dozen species have taken down nearly 30 billion conifers from Alaska to Mexico.

Bark beetles are probably the world’s oldest forest engineers and tree surgeons. They evolved along with conifers 300 million years ago. There are more species of bark beetles (7,500) than mammals. Most colonize a tree’s inner bark, or phloem, and breed in broken, stressed or diseased timber. According to a 1982 estimate, half of all tree deaths in North America were due to bark beetles.

Their highly social nature makes them formidable predators. After populations build up in windblown or lightning-struck conifers, they form swarms that begin their destructive work. During the Alaska beetle tsunami in the 1990s, which lasted a decade, the biomass of beetles in the forest may have been 20,000 tons, equivalent to a wolf pack of half a million animals, according to Ed Berg, who was an ecologist with the U.S. Fish and Wildlife Service before he retired.

All-out assault

Among mountain pine beetles, a female selects the prey, usually a drought-stressed tree. She then launches a coordinated assault by releasing a pheromone that attracts hundreds of other beetles, which hit the tree like a hail of bullets. For several hours, all-out biological and chemical warfare ensues. A lodgepole pine will try to deter attackers with copious amounts of its resin, which oozes out to form characteristic “pitch tubes” where beetles tunnel into the tree. If it fails to “engoo” the besiegers, the tree will try to gas any beetles that breach its bark with poisonous hydrocarbons.

In the end, it’s a numbers game. “If you were attacked by 10 guys in a bar, the fact that you could manufacture some resistance almost becomes irrelevant,” says Ken Raffa, a bark beetle specialist at the University of Wisconsin Madison. After a successful attack, the beetles secrete another pheromone, verbenone, which signals that the castle has been stormed and is now full. It takes anything from several hundred to several thousand beetles to overcome a conifer. The beetles lay their eggs in tunnels under the bark, and the larvae finish off the tree by destroying the tissues that carry water and nutrients.

Nor do the beetles work alone. The spruce bark beetle, for example, can carry up to 10 species of fungus, six kinds of mite and nine species of bacteria. Under its wings it also ferries nematodes, which in turn pack in more fungi.

Some of these fungi assist the beetles. Wood being tough to digest, the mountain pine beetle usually infects trees with the blue stain fungus, the preferred food of its larvae. “Without the fungi, the beetles would be nothing. It’s the most amazing thing,” says Diana Six, an entomologist and fungus specialist at the University of Montana at Missoula.

Worse than ever

While occasional bark beetle outbreaks are natural events in many forests, recent occurrences have been bigger and more severe than ever. Trees with garish red needles — the sign of an attack — and graying ghost forests of dead trees are common sights in the West.

During these recent outbreaks, the beetles have behaved in unexpected ways. In addition to taking out large, mature trees, they also have been attacking young trees. Pine beetles have also attacked spruce trees. Historically, only about 600 mountain pine beetles would attack a lodgepole pine. But during one “hyperepidemic,” as many as 6,000 insects overwhelmed individual trees. “It was a suicide mission; they had no room to breed,” says Staffan Lindgren, a beetle expert at the University of Northern British Columbia.

Two factors helped fuel these exceptional outbreaks.

In a natural forest, wildfires keep the percentage of prime beetle fodder at around 25 percent, says Allan Carroll, one of Canada’s foremost insect ecologists at the University of British Columbia in Vancouver. But thanks to dedicated firefighting and other factors, aging pines made up more than half of some forests by 1990. In other words, human management of the forests has turned them into an incredible smorgasbord for beetles.

The other factor is climate change. “As things warm up, everything for the insects speeds up,” says Six. With an increase of just two or three degrees Celsius (about four or five degrees Fahrenheit) in average temperatures, some species have doubled their reproduction rates as well as their tree consumption.

When the first of the continent’s great beetle epidemics erupted in southern Alaska in the 1990s, normally a cold and wet place, mean annual temperatures had already increased by an average of 1.5 degrees Celsius (about three degrees Fahrenheit). Average temperatures shot up in December and January by three and four degrees Celsius, respectively. In response, the beetles switched from a two-year life cycle to a one-year cycle, a change that poured more beetles on the landscape.

In other places, the beetles have moved higher up mountains and decimated high alpine white-bark forests. They have also invaded northern pine and spruce forests where there appear to have been no previous beetle attacks and the trees therefore have fewer defenses. The mountain pine beetle has spread into Canada’s boreal forest, where it is now dining on jack pine, the nation’s best-known tree. Cold Arctic blasts and the tree’s scattered distribution could slow the beetles’ march to the Atlantic Ocean to an insignificant crawl, but nobody knows for sure.

“The long and short of all this is that now is a bad time in history to be a conifer tree of any kind,” says Berg, who has lost all the spruce on his property.

A long history

People have been trying to find a way to halt outbreaks for centuries. Johann Gmelin, a German botanist and polymath, devoted 500 pages to the problem in 1787. He prescribed the felling and burning of attacked trees.

Two hundred years later, after Gmelin’s fell-and-burn technique failed to stop big outbreaks, the U.S. Forest Service tried electrocuting infested ponderosa pines, but only broods near the contact point got fried. Next, researchers injected trees with deadly chemical cocktails, but the chemicals killed the tree, too. Others applied a puttylike material composed of magnesium and bitumen to infested trees, with limited success. Similarly, a mix of DDT, diesel, chlordane and toxaphene produced little change.

One group of scientists even tried wrapping lodgepole pines with plastic explosives to blow beetles under the bark to smithereens. To their chagrin, the beetles recovered from their concussion hours later.

Clear-cutting ahead of the beetles has been tried, but this has not stopped them. One recent review concluded that there is no evidence that logging can control bark beetles after an outbreak has started.

Some think the significance of bark beetle epidemics goes far beyond forests.

“Bark beetles are one of the agents that require us to revise our sense of ourselves in this world and understand our place on this Earth,” says wildlife biologist David Mattson of the U.S. Geological Survey. “That’s why I kind of like them, for the same reason I like grizzly bears. They both challenge our sense of who we are.”

Nikiforuk, author of “Empire of the Beetle: How Human Folly and a Tiny Bug Are Killing North America’s Great Forests,” wrote this article for New Scientist magazine.

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