January 13, 2010

FeatureBark Beetles Wreak Havoc in Western U.S. and Canadian Forests

If you live, travel, or ski in the American West, you may have noticed a large number of standing dead trees among the region’s pine forests. What is causing so many trees to die? The answer lies in a tiny insect called a bark beetle.

Bark Beetles

Bark beetles, tiny insects the size of a grain of rice, are behind the demise of western pine forests. Bark beetles have hard, cylindrical bodies that are typically red, brown, or black in color. There are 600 different species of bark beetles in the United States. Not all species of bark beetles attack trees; some just live within the bark of already-dead or dying trees. Two species of special concern that do attack trees are the mountain pine beetle (Dendroctonus ponderosae), which attack lodgepole pine and sugar pine trees, and the western pine beetle (Dendroctonus brevicomis), which attack coulter pine and ponderosa pine trees. (The term “dendroctonus” means “tree killer” in Latin.)

Like most insects, the life cycle of a bark beetle goes from egg to larva to pupa to adult; these changes typically occur over a period of a year. At higher elevations, where temperatures are normally cooler, bark beetles tend to have a two-year life cycle. The bark beetle’s life cycle begins when a female burrows into the bark of a tree, where she builds an “egg gallery.” During the summer or early autumn, the female lays her eggs along the sides of the gallery. After a period of 10 to 14 days, the eggs hatch. The newly-hatched beetles remain larvae for about 10 months. Larvae spend this stage of their life within the tree bark, where they feed on the tree’s phloem. Following their development from pupa to adult, the beetles bore a hole through the tree’s bark and leave the tree. After emerging from the tree, the adult bark beetles move to other trees, and the cycle begins again.

Though their feeding on a tree’s phloem is harmful, the bark beetles themselves are not the ultimate cause of a tree’s death. A major agent of tree death is blue-stain fungus. The bark beetles carry this fungus on their bodies. Once introduced to the tree, the blue-stain fungi spread throughout the tree and interrupt the flow of phloem to the tree’s crown. The fungi also decrease the flow of pitch (thick, sticky pine sap) through the tree, which reduces the tree’s defenses against bark beetle attack. (Most healthy trees are able to survive an attack by bark beetles by throwing out large amounts of pitch, which drowns the beetles.) In addition to killing trees, blue-stain fungus, as its name implies, stains the sapwood (outermost wood) of a tree blue.

Impact on Forests

Conifer forests in the western United States have been particularly hard-hit by bark beetle outbreaks. A total of over seven million acres have been affected in the United States. Of this acreage, one million acres have been affected in Montana and 1.5 million acres have been affected in Colorado and Wyoming. In 2002, the U.S. Forest Service estimated that over the next 15 years, bark beetles could affect nearly 22 million acres of western forestland. Canadian forests (particularly those located in British Columbia) have had it even worse—over 34 million acres have been lost because of bark beetle attacks. Initial signs of tree death include red or orange needles at the crown (or top) of a tree; by the time such signs are present it is too late to save the affected tree. Though it may be distressing to see large swathes of dead or dying forestland, a larger concern is what happens to the forest after the trees die. Many forests are being clearcut due to the large amount of dead trees. This management practice is done to prevent devastating crown-based wildfires. Fire is also a concern for trees that have fallen to the ground. Wildfires at ground level can bake the ground, which severely damages soils, prevents regrowth, and may lead to devastating erosion and mudslides. Standing dead trees are also a cause for concern, as they may fall at any time. This is particularly worrisome in heavily trafficked sites, such as campgrounds and other recreational areas.

Bark Beetle Outbreak Control

Once bark beetles have attacked a tree, there is no effective method to save the tree. The best time to prevent the spread of bark beetles is either before an attack has occurred or after a tree has succumbed to an attack. Trees that are particularly vulnerable to bark beetles are old trees and unhealthy or stressed trees. Scientists think that the current bark beetle outbreak is particularly severe because of an ongoing drought in the western United States. Watering trees in order to prevent drought-induced stress can help keep trees healthy and make them less vulnerable to a bark beetle attack. Researchers have also found that certain insecticides, if applied to trees prior to an infestation, can also help prevent trees from being attacked by bark beetles.

For trees that are already infested or dead, forest managers recommend that the affected limbs be removed or the entire tree felled. Removed branches or trees should then be destroyed, either through burning or chipping. Another option is to pile cut wood in an area that receives direct sunlight and tightly cover the woodpile with a plastic sheet. The trapped heat kills any bark beetles still in the wood as well as dries out the wood, making it unsuitable as a bark beetle food source. Trees that are cut down due to bark beetle infestation do not go to waste. Cut lumber can be used as firewood, in biomass boilers, as a source of pellets for woodstoves, as newsprint, and as material for furniture or home construction.

Natural Process Gone Awry

One thing to remember is that bark beetle outbreaks are not abnormal events in and of themselves. In fact, even the loss of lodgepole pine forests is not shocking—lodgepole pines have evolved to go out with a stand-replacing event, though the cause is typically fire. In this case, bark beetles are taking the place of fire—when they attack and kill older trees, they allow younger tree species to take over. There are other benefits to the current bark beetle attack as well. As dead trees decompose, they add nutrients, including nitrogen, to the soil. Openings in the forest also let other trees grow at a faster pace as they are able to receive more sunlight to fuel photosynthetic activities. Fallen and standing dead trees also provide habitat for animals and other organisms.

Though bark beetles themselves are a normal part of western forest habitats, the severity of the current outbreak, however, is unusual. Scientists studying the current bark beetle infestation point to several factors that may explain why the infestation has lasted for so long. One major factor is the policy of suppressing forest fires. Fire is a natural part of a forest’s life cycle. A long-standing policy of fire suppression has led to the development of stands of trees that are all relatively the same age and same size. Forests that contained a mosaic of tree ages and sizes were historically able to quell bark beetle attacks as the beetles tended to be unable to move through the diverse forest stand. Also, as discussed above, continuing drought conditions have weakened trees to the point where they are unable to stave off an attack by bark beetles.

Climate change is also identified as another culprit behind the strength of the current bark beetle attack. A warmer climate has led to warmer winters, which helps bark beetle populations flourish. A warmer climate is also helping bark beetles extend their ranges into new territories. In general, climate change is causing bark beetles to act like an invasive, non-native species. The beetles are now able to attack trees at higher elevations and are now even going after younger and healthier trees than they would typically attack.

Future of Western Forests

Given the severity of the current bark beetle outbreak, foresters are working on the development of a management plan to deal with the massive amount of affected trees in western forests. The Western Forestry Leadership Coalition (WFLC), a partnership of state and federal foresters, was created in part to answer management issues such as these that involve many agencies and cross multiple boundaries. The WFLC recommends a four-pronged approach to the management of forest impacted by bark beetles. First, they recommend prevention through the use of forest-thinning techniques. Second, they recommend the suppression of bark beetle activities through the removal of affected trees, the use of pheromone-baited traps to capture bark beetles, and the use of pesticides. Third, they recommend active restoration of particularly hard-hit forests or high-value forests (such as those that provide habitat for endangered species). Fourth, and perhaps most importantly, the WFLC recommends continued research and public outreach to better understand the current outbreak and how it might be curtailed and inform the public as to what is happening to the region’s iconic forest regions.

Clearly, over the next several years, western forests will change. The species composition and density will also change over time, as normally occurs during the natural process of succession. In the end, the bark beetle-pine tree relationship is a natural one that has occurred for thousands of years. Though its results may be devastating to see, it is a gentle reminder that nature is not always pretty.

More to Explore

• Western Bark Beetle Assessment: 2009 Update
• Pests in Gardens and Landscapes: Bark Beetles
• Pine Bark Beetles Affecting More than Forests
• Biology, Ecology and Management of Western Bark Beetles
• Rocky Mountain Region Bark Beetle Information
• Mountain Pine Beetle Leaflet
• Our Future Forests
• Bark Beetles Kill Millions of Acres of Trees in West
• Some See Beetle Attacks on Western Forests as a Natural Event

Last updated January 13, 2010 | Comments (0) | Permalink | Feature