Bark Beetles and Fire
Authors: Logan Jackson and Andrew Hettick
This case study is part of a collection of pages developed by students in the 2012 course, "Megafires: rare events or the new norm," in the Department of Earth Sciences, Montana State University. Learn more about this project.
Mountain pine beetles (Dendroctonus ponderosae) are native insects that have shaped North America's pine forests for thousands of years. Ranging across western forests, from Canada in the north to Mexico in the south, pine beetles can be found from sea level to as high as 11,000 feet. Measuring only about the size of a grain rice, pine beetles inhabit a variety of pine species, but are most commonly found in ponderosa or lodgepole pine forests. Periodic outbreaks of the insect kill millions of trees across wide swaths of forest. The infestations of bark beetles can have dramatic effects in landscape flammability, discussed in more detail below. Although bark beetle infestations are a regular force of natural change in forested ecosystems, several of the current outbreaks, occurring simultaneously across western North America, are the largest and most severe in recorded history. The causes behind these outbreaks has been the topic of research by numerous ecologists. Possible causes have focused primarily on climate change induced stress on trees, rendering them more susceptible to attack, as well as lower winter mortality in the larval stages of the beetle.
Bark beetles often attack trees that are already weakened by age, disease, drought or physical damage. Healthy trees are able to put up a defense by producing resin, which may contain a number of insecticidal and fungicidal compounds that can kill or injure attacking insects, or simply trap and suffocate them with the sticky fluid. On the exterior of the tree, this results in popcorn shaped masses of resin, called "pitch tubes", where the beetles have entered. Under outbreak conditions, however, a single beetle can signal other beetles with pheromones, at which point the sheer number of beetles can overwhelm the tree's defenses and ultimately kill the tree.
Bark beetles bore through the bark of mature pine trees and dig a gallery in the tree where they lay their eggs. By first injecting a blue stain fungi they carry with them into the sapwood, they prevent the tree from repelling and killing the attacking beetles with tree resin. In concert with the fungus, the larvae hatch and feed underneath the bark, girdling the tree and cutting off the flow of nutrients and water. This joint attack kills the host tree within a few weeks of a successful attack. Within a year, the needles will have turned red and in three to four years, with few needles remaining, the trees appear grey.
Anatomy of an Outbreak
Bark beetle outbreak dynamics are complex, and a variety of circumstances must coincide and thresholds must be surpassed for an outbreak to occur on a large scale. Moreover, large areas of suitable hosts are an essential requirement for a widespread outbreak. Although outbreak dynamics differ from species to species and from forest to forest, climate change is one factor that appears to be driving at least some of the current bark beetle outbreaks. Temperature influences everything in a bark beetle's life, from the number of eggs laid by a single female beetle, to the beetles' ability to disperse to new host trees, to individuals' over-winter survival and development timing. Elevated temperatures associated with climate change, particularly when there are consecutive warm years, can speed up reproductive cycles and reduce cold-induced mortality. Shifts in precipitation patterns and associated drought can also influence bark beetle outbreak dynamics by weakening trees and making them more susceptible to bark beetle attacks.
Except for a few days during the summer when adults emerge from brood trees and fly to attack new host trees, all life stages of the bark beetle are spent beneath the bark of pine trees. Adult bark beetles emerge from trees and fly to new host trees between June and August, although weather and location my affect exact timing. Females are the first to attack new trees, releasing the pheromones that attract other beetles for a mass attack. The beetles bore into the tree and excavate areas to lay their eggs. Once hatched, the larvae feed on the tree until cold winter temperatures initiate dormancy. After spending the winter under the bark, bark beetles emerge in the summer to start the process over again. The adaptive problem for the mountain pine beetle is for adult emergence to occur early enough to allow for the maximum time for oviposition but late enough to avoid lethal spring, or early summer, temperatures. In many areas, the maturation rate for bark beetles is one year. In some colder areas, like Alaska or Canada, that rate may be as long as two years. However, new studies are showing that, in response to warmer temperatures, bark beetles are emerging more than a month earlier than historically recorded. This early emergence is leading to beetles laying their eggs earlier, which, driven by warm temperatures, develop more quickly through the summer to produce adults that emerge by August. This in turn leads to greater beetle numbers, attacking greater numbers of trees, spreading farther, and producing more eggs.
The bark beetle can play an important regulatory role in forest ecosystems. Normally, these insects attack old or weakened trees, speeding development of a younger forest. But their role in increasing fire activity is largely unfounded. It could be assumed that all the trees beetles are killing would provide more than ample fuel for a fire. This is only true immediately after the tree dies, while it still has its needles, or after the tree has fallen down. "Outbreaks of mountain pine beetle...do not appear to substantially increase the risk of subsequent fire under most conditions. Instead, fire risk is strongly tied to warm and dry conditions, such as those of recent decades. As long as the severe droughts we have been seeing in recent years persist, we can expect a high risk of fire - regardless of beetle outbreaks. (Black and Noon 2013)"
Bark Beetle Management
Because resource objectives will differ and the factors influencing a bark beetle outbreak differ depending on the species of beetle, host tree species, local ecosystem, and geographical region, there is no single management action that is appropriate across all affected forests.
There are natural controls of bark beetles, including woodpeckers and clerid beetles that feed on adults and larvae under the bark. However, under outbreak conditions these natural controls often fail to stem attacks. Extreme cold can also reduce bark beetle populations. For winter mortality to be a significant factor, sufficiently cold temperatures (around -30 F) must persist for up to a week while the beetles are in their most vulnerable stages. But due to global climate change, winter temperatures are not falling as low or lasting as long as is necessary to be an effective check on the bark beetle population.
Once a tree becomes infested with bark beetles it is beyond saving. Therefore, there are two options for human control and management. One is to save non-infected trees and the other is to dispose of those that are. A healthy forest is the best defense against a bark beetle outbreak. It is important that tree stands are thinned to limit competition for light, water, and nutrients. Ensure the resilience of the trees by removing stress factors for the trees. Diversifying tree age and species will also help to minimize stand-level tree mortality. If a tree does become infected, it is important to dispose of the tree before the beetles can reemerge and attack another tree. Disposal should be complete removal or burning of the tree, as immature beetles can continue to develop in cut trees. Other options are spraying non-infected trees with preventative chemicals once a year or placing anti-aggregation chemicals on trees. For more information regarding control and management, please click on the following links.
Facts About Bark Beetles
Bark beetle fact sheet comprised by British Columbia: Mountain Pine Beetle Fact Sheet
Website about bark beetles comprised by the state of Montana: Beetles in Montana
Trees At Risk
Guide to 'at-risk' trees comprised by Colorado State University: Are Your Trees at Risk?
USDA comprehensive guide for people interested in learning more about management practices: Management Guide for Bark Beetles
Colorado State Forest Service's guide to chemical preventative measures: Guide to Preventative Chemicals
Colorado State Forest Service's guide to solar treatment options: Solar Treatment for Bark Beetles
Frequently Asked Questions
Frequently asked questions about the Mountain Pine Beetle in Colorado: Pine Beetle FAQ
Here is a resource for younger audiences, grade 5 and older : A Year in the Life of a Mountain Pine Beetle: By Buford the Mountain Pine Beetle
Here is a resource for all ages, which also includes a Spanish-language section: Interactive information and games on invasive insects in our forests
Black, S. and Noon, B. (2013). New Study: Bark beetle outbreak not the culprit in recent rash of western fires. The Xerces Society. April 24, 2013.
Hicke, J. and Ojima, D. (2006). Changing Temperatures Influence Suitability for Modeled Mountain Pine Beetle (Dendroctonus ponderosae) Outbreaks in the Western United States. Journal of Geophysical Research, G2(111). April 27, 2013. https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JG000101
Leatherman, D., Aguayo, I., and Mehall, T. (2013). Mountain Pine Beetle. Colorado State University. April 28, 2013. http://web.archive.org/web/20070605063836/http://www.ext.colostate.edu/pubs/insect/05528.html
Logan, J. and Powell, J. (2001). Ghost Forests, Global Warming, and the Mountain Pine Beetle. American Entomologist, 47(3): 160-173. April 26, 2013.
Mountain Pine Beetle, Montana. DNRC. April 20, 2013.
Mountain Pine Beetle, Wikipedia. April 24, 2013.
Mountain Pine Beetle Attacks. Mountain Research Station, University of Colorado Boulder. April 28, 2013.
Mountain Pine Beetle and Wildfire Risk. Wildland Fire Lessons Learned Center. April 23, 2013.