IN HOMER, Alaska, jazz singer Mary Jane Shows once belted out songs to her spruce trees every night. Now all the tall trees are dead and Shows has moved away. “It will never go back to what it was,” she says.
Just south of Alaska in the Canadian province of British Columbia, artist Annerose Georgeson often gets lost in her home town. The pines that once served as local landmarks in the city of Prince George are dead and gone. “People all over the place are disoriented. Everything looks so different,” says Georgeson.
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And in the high alpine forests of Montana outside Yellowstone National Park, Jesse Logan, a skier and biologist, has witnessed similar changes. There the ancient whitebark pines, the region’s mountaintop guardians, have fallen one by one. These long-living pines once provided highly nutritious seeds for grizzly bears. Logan says that he sometimes feels like he is watching the collapse of a great and remote ecosystem.
The killer is a tiny beetle no bigger than a grain of rice – the bark beetle. 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, including the now infamous mountain pine beetle, have taken down nearly 30 billion conifers from Alaska to Mexico. The beetle has changed entire watersheds, ecosystems and logging communities. No other creature on the planet can change a landscape as fast.
The epidemics are still raging in places. They end only when the beetles eat themselves out of house and home. What’s more, the beetles have been behaving in ways never seen before. Researchers have had to revise their thinking about this improbable creature. So what is responsible for these phenomenal outbreaks? And is there any way to stop them?
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 (7500) than mammals. Most colonise a tree’s inner bark, or phloem, and breed in broken, stressed or diseased timber. In a treatise published in 1982, the late Utah entomologist Stephen Wood estimated that half of all tree deaths in North America were due to bark beetles.
Their highly social nature makes them formidable predators. Members of what foresters call “the beetle logging company” hunt in packs. After populations build up in windblown or lightning-struck conifers, they form swarms that can bring down living trees. During the Alaska beetle tsunami in the 1990s, which lasted a decade, the biomass of beetles in the forest may have been 20,000 tonnes, equivalent to a wolf pack of half a million animals, according to Ed Berg, an ecologist with the US Fish and Wildlife Service before he retired.
The beetles hijack the trees’ defences to produce powerful pheromones that control the pace and scale of their attack. “The more the tree tries to fight, the more the beetles pour into the tree. It’s an early version of ju-jitsu,” says Ken Raffa, a bark-beetle specialist at the University of Wisconsin-Madison.
All-out assault
Among mountain pine beetles (Dendroctonus ponderosae), 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 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 Raffa. 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 living 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. Every one appears to be a veritable biological bus carrying a load of strange and often hostile passengers. The spruce bark beetle alone, for example, can carry up to 10 species of fungus, six different kinds of mite and nine species of bacteria. Under its wings it also ferries nematodes that in turn pack in more fungi.
Some of these fungi assist the beetles. The mountain pine beetle usually infects trees with the blue stain fungus, for instance, which is the preferred food of its larvae – wood being tough to digest. “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, Missoula.
Sometimes the fungi can be even deadlier than the beetles. Mites hitching a ride on bark beetles spread the fungus causing Dutch elm disease, which has devastated elm tree populations in Europe and elsewhere. And researchers are finding additional passengers all the time. “The number of new species being discovered is unbelievable,” Six says. Like many folk in Montana, she lost all the ponderosa pines on her property to bark beetles.
While occasional bark beetle outbreaks are a natural occurrence in many forests, than ever before. Trees with garish red needles – the sign of an attack – and greying ghost forests of dead trees are common sights in the North American west. In Alaska, spruce beetles ate their way through around 10,000 square kilometres of forest. In British Columbia, pine beetles have , an area the size of England. In New Mexico, the pinyon ips beetle, normally a marginal tree killer, erupted over an area of 8000 km2. All in all, over the past couple of decades, the beetles rolled across an area 10 times larger than previous outbreaks. Every day, 100,000 beetle-killed lodgepole pines crack and fall in the forests of Wyoming and Colorado.
During these outbreaks the beetles have behaved in unexpected ways. In addition to taking out large mature trees, they also attacked young trees. Pine beetles stunned researchers by successfully attacking and reproducing in spruce trees. Historically, only about 600 mountain pine beetles would attack a lodgepole pine. But during the “hyperepidemic” as many as 6000 insects overwhelmed individual trees. “In some cases it was a suicide mission; they had no room to breed,” says Staffan Lindgren, a beetle expert at the University of Northern British Columbia in Prince George, Canada.
Two factors helped fuel these exceptional outbreaks. The mountain pine beetle in particular is primed to attack and bring down ageing lodgepole pines. In a natural forest, wildfires keep the percentage of prime beetle fodder at around 25 per cent, says Allan Carroll, one of Canada’s foremost insect ecologists at the University of British Columbia in Vancouver. But thanks to dedicated fire-fighting and other factors, ageing 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. “It’s a perfectly tuned, complex system. We gave it a bit of a jab and buggered it up,” says Carroll.
“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 just a 2 to 3 °C increase 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 °C. Average temperatures shot up in December and January by 3 °C and 4 °C 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.
Poor defences
Warmer temperatures not only boost beetle growth, they can stress trees, too. After a string of hot summers in Alaska, both white spruce and Sitka spruce had become extremely drought-stressed. That, in turn, reduced their ability to produce resin and defend themselves. The beetles also attacked in May while the trees’ roots were still frozen in the ground and unable to move water for resin production.
In other places, the beetles have moved up mountains and decimated high alpine whitebark 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 defences. 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. Berg, who has lost all the spruce on his property, says that the massive tree die-off in southern Alaska and the Yukon is a warning of things to come throughout North America.
The rest of the world will not escape either. Northern Europe, for example, can expect an expansion of beetle country . Norwegian researchers recently calculated that warming temperatures would allow a species confined to the southern part of the country to by 640 kilometres by 2070. In 2008, an Austrian study concluded that .
Little can be done to stop this. People have been trying to find a way to halt outbreaks for centuries. After Europe’s hardwood forests were replaced by fast-growing monocultures of spruce and pine from the 14th century onwards, bark beetles became a major scourge, sometimes causing wood famines and fuel shortages. 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 US Forest Service tried electrocuting ponderosa pines infested with western pine beetles. But only broods near the contact point got fried. Next, researchers injected trees with deadly chemical cocktails, including nicotine sulphate, but the chemicals killed the tree too. Others applied “goop”, a putty-like material composed of magnesium and bitumen, to infested trees with limited success. After the second world war, foresters mixed DDT with diesel, chlordane and toxaphene, again with little success.
One enterprising 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.
“Once they are in full boom mode, there isn’t much you can do,” says Mary Reid, a beetle biologist at the University of Calgary in Alberta. Jesse Logan, a retired US Forest Service entomologist, agrees. He has a standard reply when asked about controlling massive beetle outbreaks: “Could you build a fan big enough to blow a hurricane back out to the ocean?”
This has not stopped North American authorities from trying. Many have resorted to making massive clear-cuts ahead of the beetles. The clear-cuts have done a lot of damage to watersheds and biodiversity, by increasing erosion among other things, but they have not stopped the beetles. concluded that there is no evidence logging can control bark beetles after an outbreak has started.
In Europe, there is growing opposition to clear-cutting. After the spruce beetle invaded a national park in the Czech Republic last year, foresters and politicians wanted to pull out the chainsaws. Entomologists and greens, however, cried foul, and . Across the border in Germany, the Bavarian Forest National Park decided in the 1980s to let beetle outbreaks run their course, believing in the long term.
In North America, entire landscapes have been transformed in the wake of the beetle. In Alaska, a fast-growing grass has smothered the ground and prevented young spruce growth. Many former lodgepole sites in British Columbia will likely become grasslands. Forests of pinyon and juniper trees have been reduced to just juniper. Climate change may make restoration of some forests impossible. “You can’t force something back to existing conditions when they no longer exist,” says Six.
Some see this destruction as a warning. They think the significance of bark beetle epidemics goes far beyond forests. Buzz Holling, a Canadian ecologist famous for his work on the resilience of ecosystems, argues that the collapse and renewal of forests by bark beetles shows how small things can rapidly undo big complex systems. “The future is not just uncertain. It is inherently unpredictable,” says Holling. “The forest makes a comprehensive metaphor, and it should be used to understand other phenomena on the planet.”
“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 US 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.”
Due to an editing error, the original version of this article stated that pine beetles in British Columbia had swept through 13,000 km2. This has been corrected to 130,000 km2
