Here's How Bark Beetles and Other Insects Harm Trees
Sydney Jones
Rainmaker & Tree-Planter.
When you think of deforestation, you might conjure up images of loud and heavy machinery bulldozing 200-year-old trees, or rampant forest fires turning trunks and branches black with ash. But there’s another killer out there, and it’s small enough to fit in the palm of your hand. Tree-killing insects are responsible for large-scale deforestation across North America.
While some insects work by defoliating and weakening healthy trees, others, like bark beetles, target trees that are already sick or under stress. According to the Food and Agriculture Organization of the United Nations, 35 million hectares of forest are damaged by tree-killing insects each year!
The Mountain Pine Beetle affects western Canada and the U.S.
The Emerald Ash Borer affects central and eastern U.S. & southern Canada.
The Spruce Beetle affects western North America.
The Mountain Pine Beetle
Native to North America’s west coast, the mountain pine beetle has devastated forests from British Columbia to Mexico. No larger than a grain of rice, the invasive insects fly from tree to tree, weakening and killing species like lodgepole, western white, sugar, whitebark, and limber pines by boring into the bark. These boring holes, known as “pitch tubes” are tell-tale signs of a mountain pine beetle infestation. Piles of dark dust, known as boring dust, may also be found around the base of the tree. Large populations of mountain pine beetles can be determined by the presence of woodpeckers, which feed on developing larvae under the bark, punching holes into thin-barked trees like the whitebark pine. Within eight to 10 months of a successful attack, the tree will fade and eventually die. Here's a look at a pine beetle infested forest from above.
According to the U.S. Forest Service, more than 1.5 million acres of forest in northern Colorado and southern Wyoming are affected by the mountain pine beetle epidemic, which was triggered by an extended drought in the late 1990s and early 2000s.
"By about 2012, beetles will have killed nearly all of the mature lodgepole trees in northern Colorado and southern Wyoming. Besides affecting watersheds, future timber production, wildlife habitat, recreation sites, transmission lines, and scenic views, beetle-killed trees also present a fuels build-up situation that could result in catastrophic wildland fires."
The Emerald Ash Borer
The emerald ash borer is an Asian insect species native to Taiwan, Mongolia, Korea, Japan, China, and the Russian Far East, targeting species like white, green, blue, black, and pumpkin ash. Although the insect was first discovered in North America in 2002 in the Detroit, Michigan area, tree ring evidence suggests it landed on the continent sometime in the early 1990s. Females lay eggs under flaps of bark or in crevices found in the tree trunk. Once hatched, the larvae carve out an S-shaped tunnel, eventually chewing out a chamber to pupate. Although fully grown emerald ash borers feed primarily on foliage, the death of the tree is ultimately caused by the destruction of the bark by the larvae. Mature emerald ash borers are perhaps most recognizable by their metallic green exoskeleton. It has killed at least tens of millions of ash trees so far and threatens to kill most of the 8.7 billion ash trees throughout North America. Below is what the emerald ash borer larvae tunneling looks like beneath the bark of the infested tree.
The Spruce Beetle
The spruce beetle is one of the most damaging insect species native to North America. Preying primarily on Engelmann and blue spruce, mature females carve “galleries” out of fallen or living trees to lay their eggs. Once the eggs hatch, they spend the winter season burrowed under the bark in the phloem layer, which transports valuable nutrients to the tree gained from photosynthesis. Eventually, the fully grown beetles carve their way out of the tree between late May and July to find a new host. Trees dying as a result of spruce beetle infestations will display small piles of boring dust around their trunk, and pitch tubes closer to the base of the tree. Strip attacks, which consist of an infestation on only one section of the tree, are common, and less fatal than full-blown spruce beetle outbreaks.
The Effects of Climate Change
Insect outbreaks are a normal part of a forest’s biological cycle. In fact, by eliminating dead or sick trees, insect infestations can encourage a more productive ecosystem and healthier forest. Rampant and recurring outbreaks, however, are more than just natural disturbances; they can have long-lasting or even irreversible effects on forests and ecosystems. The reason for these outbreaks is in large part caused by climate change.
Warmer, drier seasons contribute to environments ideal for travel, mating, and survival among tree-killing insects. As temperatures increase, tree-killing insects are moving into forests previously untouched by invasive species. Leaves, needles, and bark shed as a result of damage caused by insects also increase the risk of forest fires. These changes not only affect trees, but entire ecosystems that surround and rely on them.
How Can I Help Stop the Spread of Invasive Species?
There are many things you can do as an individual or community to reduce the spread of tree-killing insects. Here are just a few:
- Refrain from moving firewood from one location to another.
- Report sightings of invasive or unfamiliar species.
- Plant trees and plants native to your area.
- Refrain from moving organic matter like wood, flowers, or plants across borders.
- Stay on designated pathways and trails.
- Wash your boots of seeds and plant matter after hiking.
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