Pine on the Decline: The Mountain Pine Beetle Epidemic

Though they are only one-eighth to one-third inch long—the length of a grain of rice—mountain pine beetles have made a devastating impact on our nation’s forests.

A multiple-year outbreak of the small but mighty insects (Dendroctonus ponderosae) and other bark dwelling beetles has compromised thousands of acres of trees, and there’s no sign of them stopping any time soon.

Bark beetles have populated North American forests from Canada to Mexico for thousands of years, but within the last two decades, different species of beetles, including the destructive mountain pine variety, have killed pine and spruce trees throughout the Rocky Mountains and elsewhere. The current outbreak is the largest ever recorded in U.S. and Canadian history.

Pine forests have suffered in the past, and come back even stronger after previous infestations. And an ebb and flow of insect outbreaks is natural. However, there are a number of elements contributing to this current, more severe infestation.

An extended drought in the late 1990s and early 2000s is considered the spark that set off the explosive assault. Beyond that initial drought, three other factors have facilitated the epidemic: warmer winters, old forests, and continued dry conditions. With global temperatures on the rise, beetle eggs and larvae have a better chance of surviving the winter; it is only with a continued deep freeze of five days or more that the beetle population can be controlled. Secondly, bark-dwelling beetles prefer mature trees to carry out their lifecycle. The insects prefer older trees because the bark is thicker and more protective; larger trunks provide more space to colonize; and the phloem, which provides beetle food, is more abundant. As large portions of pine forests reach a mature age of 80 years old and up, due to in part effective wildfire suppression programs, those trees become more susceptible to bark beetle infestation.

Too many trees can be problematic as well. Overstocked forest stands mean trees have to compete for water, nutrients, and sunlight. “This causes stress that the beetles can sense,” says Gene Phillips, Forest Health Specialist at the Nevada Division of Forestry. Though small populations of beetles prefer older, weak trees, growing populations can destroy young, healthy trees. And to add to the perfect storm: Prolonged periods of low precipitation have weakened trees. All of these have contributed to an outbreak of epidemic status.

Image from Firewise Landscapes Inc.
Image from Firewise Landscapes Inc.

The Lifecycle of a Mountain Pine Beetle

A single beetle is all it takes to initiate an attack. That beetle will select a tree and begin feeding. Chemical reactions in the insect’s intestines create an aggregation pheromone, an airborne molecule that attract members of the same species. This pheromone, combined with volatile tree chemicals, draws in more beetles. The crew of beetles then tunnels through the bark to create vertical tunnels, called egg galleries. Inside the vertical egg galleries, females mate with males, then lay eggs along the sides of the gallery; each female beetle lays approximately 60 to 75 eggs. Roughly two weeks later, larvae hatch; those larvae mine galleries horizontally under the bark, where they stay protected from cold temperatures and predators until they begin feeding again in the spring. From late June to September, new adult beetles dig exit holes through the bark of the dying trees, at which point they fly off to select a new, live tree as a host.

And it’s not just the beetles’ living in and off of the trees that leads to a forest’s demise. “All of the beetles’ boring weakens the tree by damaging the cambium and exhausting defense reserves, but they also carry a symbiotic fungi which they seed into the tree from specialized chambers in their mouths,” says Scott Ferrenberg, a PhD candidate at the University of Colorado.

When the beetles first attack trees, they introduce blue stain fungi from spores near their mouths. It not only gives infested wood a distinct blue-gray color, but also aids beetles in killing the trees. “The fungi grows and clogs the tree’s cells, ensuring that weak trees don’t recover and eventually die,” says Ferrenberg; he has studied mountain pine beetle and other insect attacks. In other words, with the dozens to thousands of attacks on a single tree, that tree has little to no chance of successfully fighting back; all of the boring significantly cuts off nutrient and water supply.

Though mountain pine beetles usually live for only one year, some beetles may live two years in higher elevation areas. And they’re now breeding twice in some years instead of once.

Signs and Symptoms of a MPB Attack

While not all attacks are successful, there are characteristics of infested trees. Some signs are visible on the outside of the tree, while others will surface when a tree is cut down.

Under the tree
Look for a sawdust-like material at the base of a tree. The fine, dry mixture of fecal matter and chewed up wood, called frass, can be found on the ground around it as well as in crevasses in the bark.

The bark
Popcorn-shaped masses of resin, called pitch tubes, coat tree trunk bark and denote the spots where beetles started to tunnel. They may be brown, pink, or white. Some patches of bark may also be missing, removed by woodpeckers searching for larvae.

Below the bark
Where bark is missing, there may be areas of exposed tunneling. It’s possible some of the surface-layer wood will feature a blue hue from the blue stain fungus carried in by the beetles. The color will be more apparent when cuts are made in the tree.

It’s possible some of the surface-layer wood will feature a blue hue from the blue stain fungus carried in by the beetles. The color will be more apparent when cuts are made in the tree.

The needles
Tree crowns of beetle-attacked trees can fade from a lush green color to a pale yellow, then to a reddish brown hue as the tree dies. This process usually takes 1 to 3 years.

Image from US National Forest Service.
Image from US National Forest Service.

The Greater Impact

In addition to the death of young and old trees, there’s another side effect of the infestation: soil erosion. That in turn puts water quality and wildlife habitats at risk and increases the chance for landslides, as well as forces the closing of hiking and biking trails due to the risk of falling trees. “It’s important to know that large regions of the west are supported by tourism related income,” Ferrenberg says, “and many of the possible effects are hard to measure because of the complexity of the landscape.”

Towns that run through national parks or towns thriving on hunting, fishing, and skiing tourism are all affected by the mountain pine beetle epidemic. Effected trees are hazardous since the weakened wood puts those trees at a higher fall risk, forcing the closure of many trails and campgrounds.

Beetle attacks also directly affect the timber industry by killing off forest stands that could have been harvested. “Once a tree dies there is a finite amount of time where the potential lumber becomes too dry and brittle if left standing too long,” Phillips says.

But one of the most significant concerns is the effect on snow. Because so many trees have been lost to beetle attacks, there is less protective cover from trees. Winds can move more snow than in the past, and the sun’s rays also have more access to the snow, impacting how pace of snow melts. “This is a very serious issue for ski resorts and for agriculture since snow pack is really utilized as a natural reservoir that maintains stream flow late into the fall each year,” Ferrenberg says. The loss of just three or four trees can change snow pack over an acre or more depending on how the winds change, and that has water managers very concerned, he says.

Wildfires are often considered an ill effect of pine beetle attacks, but the jury’s still out on the connection. “The majority of scientific evidence says fire risk is not increased by bark beetle epidemics, but the U.S. Forest Service and other groups push the risk to gain extra funding from Congress,” Ferrenberg says. “This topic is an example of where politics and emotion are currently trumping science.” Though dried-out, hollowed, dead trees are in fact at risk for catching fire, compounds in pine trees’ needles and resins, called monoterpenes, are highly flammable whether the tree is alive or not. Ferrenberg says fire risk is generally high, so it’s hard to pinpoint beetle attacks as a major contributing factor. Though research does suggest flammability is higher in infested trees, those flammable chemicals begin to degrade a year or two after a tree has died. Length and severity of an outbreak as well can influence the chance for wildfire.

Controlling an Outbreak

Bark beetle infestations can’t be stopped, but there are methods of controlling the spread and destruction. Preventive sprays, considered moderately effective, can be used on living trees early in the summer to kill or deter beetles. But because of the difficulty in large-scale application and concerns over the effects on drinking water, chemical measures are usually limited to small campgrounds or private land. A more labor-intensive way to slow down beetle reproduction is to remove tree bark, exposing larvae to the elements, though Phillips says it’s not very effective. “If people catch the signs before the new adults emerge, they can cut the trees down and remove the wood with the beetles from the site,” Ferrenberg says. Disposing of already-attacked trees by cutting them down and burning them, or burying them is one of the most effective ways to keep beetles from a continued onslaught.

Nature also has its own ways of controlling beetle populations from growing, but again, it can’t put the brakes completely on an epidemic. Woodpeckers and some other insects feed on mountain pine beetles and their larvae, but they don’t really prevent future attack.

Periods of extreme cold are a more effective means of reducing the size of beetle populations, but it takes a prolonged deep freeze to do the job. Other options include pheromone lures (attracting beetles with a synthetic hormone to contain them in a single area) or diversifying tree species. Planting a better species mix and thinning out susceptible trees helps the trees that are left survive. “They now have room to grow and there is less competition for water, nutrients and light,” Phillips says.

Though mountain pine beetle attacks have substantially declined in the past three years due to the death of a large portion of mature pine trees, the beetle can continue to infest and kill other pine species. “There is still great potential for an epidemic, largely in Ponderosa pine, which has an enormous range across the west,” Ferrenberg says. He believes controlling attacks will never be easy. Because outbreaks cross borders of both countries and states, agreeing on control methods as well as objectives is a difficult task. “Many agencies have different mandates and management directions which often conflict,” Phillips says.

Management for beetles is also a multi-generational effort, Ferrenberg explains, “and that’s one reason why it fails. Few of the residents of the west can be convinced to care about long-term insect pest management when fire risk is a more constant and emotional issue.” Even so, insect damage across North America is 10 to 50 times greater in total acreage than fire in a typical year.

Though research continues on the impact of mountain pine beetles and potential methods for controlling current and future outbreaks, beetle attacks will continue, despite best efforts to prevent and treat them. Research and improving technology may prove more effective, but it’s clear there is no simple, immediate fix.

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