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Big Bad Bug Blog – Episode #3: The Mountain Pine Beetle

Updated: Feb 11, 2021

The Invasion

Not all enemies invade. Sometimes, they’re here all along, living right beside us. Flying under the radar, waiting until the timing and the conditions are right. Then they attack.

The mountain pine beetle, native to the forests of western North America, from Mexico to central British Columbia, is one of 200 different species of bark beetle. Normally, these insects lend a helping hand, by attacking old or weakened trees, and speeding development of a younger forest. However, unusually hot, dry summers and mild winters throughout the region during the last 20 years, along with forests filled with mature pines, have led to an unprecedented epidemic. We’re talking the zombie apocalypse of pine trees. Outbreaks develop irrespective of property lines, being equally evident in wilderness areas, native forests, mountain subdivisions, and back yards, with epidemic cycles occurring about every 10 to 30 years, depending on forest condition, weather and other factors poorly understood.

The current outbreak in the Rocky Mountain National Park began in 1996 and has caused the destruction of millions of acres of ponderosa and lodgepole pine trees in Colorado. According to an annual assessment by the state’s forest service, 264,000 acres of trees in Colorado were infested by the mountain pine beetle at the beginning of 2013. This was much smaller than the 1.15 million acres that were affected in 2008 because the beetle has already killed off most of the vulnerable trees. But the damage doesn’t stop here, with more than 16 million of the 55 million hectares of pine forest in British Columbia being destroyed. This may just be the largest forest insect blight ever seen in North America.

The Life Cycle

Mountain pine beetles have a one-year life cycle in Colorado. In summer, adults leave the dead, yellow to red-needled trees in which they developed. Females seek out living, green trees that they attack by tunneling under the bark. Coordinated mass attacks by many beetles are the norm. If successful, each beetle pair mates, forms a vertical tunnel (egg gallery) under the bark, and produces about 75 eggs. Following egg hatch, larvae (grubs) tunnel away from the egg gallery, producing a characteristic pattern.

MPB larvae spend the winter under the bark. They continue to feed in the spring, transforming to pupae in early summer. Emergence of new adults can begin in early July and continue through September. However, the great majority of beetles exit trees during late July (lodgepole pine) and mid-August (ponderosa pine).

The ability of the mountain pine beetle to survive and thrive is highly sensitive to temperature and precipitation. Outbreaks have been correlated with warmer winter temperatures, which allow more beetles to survive. In one study of higher elevations, biologists found that warmer temperatures allow the pine beetles to complete a full generation in just one year instead of two, allowing them to multiply faster. And recent periods of drought have stressed trees, making them more susceptible to attack

The Damage

Mountain pine beetles affect pine trees by laying eggs under the bark. The beetles introduce blue stain fungus into the sapwood that prevents the tree from repelling and killing the attacking beetles with tree pitch flow. The fungus also blocks water and nutrient transport within the tree. On the tree exterior, this results in popcorn-shaped masses of resin, called “pitch tubes”, where the beetles have entered. The joint action of larval feeding and fungal colonization kills the host tree within a few weeks of successful attack (the fungus and feeding by the larvae girdles the tree, cutting off the flow of water and nutrients). When the tree is first attacked, it remains green. Usually within a year of attack, the needles will have turned red. This means the tree is dying or dead, and the beetles have moved to another tree. In three to four years after the attack, very little foliage is left, so the trees appear grey.

The Defense

Management techniques include harvesting at the leading edges of what is known as “green attack”, as well as other techniques that can be used to manage infestations on a smaller scale, including:

  • Pheromone baiting – is luring beetles into trees ‘baited’ with a synthetic hormone that mimics the scent of a female beetle. Beetles can then be contained in a single area, where they can more easily be destroyed.

  • Sanitation harvesting – is removing single infested trees to control the spread of beetle populations to other areas.

  • Snip and skid – is removing groups of infested trees scattered over a large area.

  • Controlled, or mosaic, burning – is burning an area where infested trees are concentrated, to reduce high beetle infestations in the area or to help reduce the fire hazard in an area. Controlling wildfires has significantly increased since the 1980s and 90s due to firefighting technology.

  • Fall and burn – is cutting (felling) and burning beetle-infested trees to prevent the spread of beetle populations to other areas. This is usually done in winter, to reduce the risk of starting forest fires.

  • Pesticides – Biopesticides such as chitosan have been tested for protection against the mountain pine beetle, and pesticides such as carbaryl, permethrin, and bifenthrin are used for smaller area applications. But as all 3 have documented negative effects on the local mammals, birds and aquatic systems, other alternatives are recommended.

The concept of natural plant defense holds hope for eliminating pine beetle infestation. Beneficial microbial solutions are being researched and developed that work with the plant to activate and enhance its resistance mechanisms against insects and disease.

The Aftermath

Wood from beetle-affected trees retains its commercial usefulness for 8 to 12 years after the tree has died. The timber can be used for any wood product from standard framing lumber to engineered wood products, such as glue-laminated products and cross-laminated panels. Though there are many small wood working shops that are making furniture and crafts out of this beautiful and exotic appearing blue-stained wood, and despite the massive supply and the increasingly apparent need to utilize this dead timber, there are very few companies that use this otherwise wasted and hazardous natural resource by creating product lines that require large volumes of the dead trees. This is due to the significant difficulties and increased expense inherent to processing beetle kill timber, and the correspondingly lowered profitability. But blue stained pine is now available at some big box stores like Lowes and makes beautiful woodwork.

There has been concern that the huge number of beetle-killed trees may pose a risk of devastating forest fires. Forest thinning to mitigate fire danger is expensive and resource-intensive. Attention is turning to ways to turn this liability into a source of cellulosic ethanol. Leaders in western U.S. states and Canadian provinces have promoted legislation to provide incentives for companies using beetle-killed trees for biofuel or biopower applications. Sellable commodities resulting from MPB damage can help subsidize the cost of forest thinning projects and support new job markets. Colorado’s Department of Energy recently provided $30 million toward construction of the state’s first cellulosic ethanol plant, to convert beetle kill into ethanol. Lignin, a byproduct of the process, can be sold for applications in lubricants and other goods.

The Future

A healthy forest normally stores significant amounts of carbon. By killing off millions of acres of trees, which then decay or provide fuel for wildfires, the mountain pine beetle has instead turned lodgepole pine forests into a source of carbon emitted into the already overloaded atmosphere.

The pine beetle epidemic shows how warming temperatures can lead to a problem that, in turn, causes temperatures to rise even further. Such a feedback loop serves to amplify global warming. The added carbon in the atmosphere warms mountain temperatures even more, allowing the beetles to continue to multiply and destroy more trees, which give off more carbon. The feedback loop continues until the preferred host trees are gone.

The choices we make today can help determine what our climate will be like tomorrow. Significantly reducing our heat–trapping emissions—by developing cleaner energy technologies such as solar and wind power—would help minimize the rise in temperatures and the resulting severe impact on our forests and parks.

If you see signs of the mountain pine beetle, contact Plant Escape immediately, so that our trained horticulturalists can begin evaluation and treatment of your plants, shrubs and trees, ASAP.


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