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.
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.