forest ecology insects population ecology wildfires

How are forest insect outbreaks like wildfires?

Wildfire and insect outbreak both affect our forest in complex and interconnected ways. Learn more in this post.

How are forest insect outbreaks similar to wildfires? There has been a lot in the news recently about wildfires. Similarly, another influence—forest insects—is sweeping through North American forests. Wildfire and insect outbreak are both large-scale forest disturbances and affect our forest in complex and interconnected ways.

We’ve been hearing a lot in the news about wildfires

In 2015, 61,922 wildfires burned over 10 million acres of the United States, setting a record for the largest area burned in North America in a single year [1]. To put this in context, the combined size of Massachusetts and Connecticut is approximately 10 million acres. However, most large-scale fires occur in the western states, not in the east. In Washington State, a series of fires—the Okanogan Fires—burned over 304,782 acres (123,341 ha). This wildfire, the largest in Washington state’s history, threatened numerous towns, and took over 1,200 firefighters (including crews brought in from Australia and New Zealand) to control [2].

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Fig. 1 Fire damage following the 2012 Whitewater-Baldy Fire in Gila National Forest, New Mexico. Photo credit: Kari Greer, US Forest Service. Source

Unfortunately, 2016 looks no better; the season got off to a fiery start. In the first three months of this year, the U.S. already suffered three times more damage than the 2000 - 2010 average for the same time period [1]. Luckily, fire acreage since has dropped some in the ranks, but this still means that from January through July of this year, we’ve experienced a total of almost 3.5 million acres burned in the U.S. alone. The Fort McMurray fire in Alberta Canada was in the news a lot this spring. Even in the early stages of its fiery life, it was deemed ‘extreme’ by Alberta Agriculture and Forestry [3]. These fires have important implications for human and environmental wellbeing.

Insect outbreaks are also sweeping through our forests

Meanwhile, another source of forest disruption is simultaneously sweeping through North American forests (Fig. 2); however, this forest ‘fire’ is biological in nature. In 2010, 9.2 million acres of U.S. forest had tree mortality from insects and plant disease [4]. Remember how many acres were burned by forest fires last year? About 10 million acres. Insects and plant pathogens affect about the same acreage of land in the United States as wildfires.

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Fig. 2 A view from Mount Fraser, British Columbia. Looks like wildfire damage, but all the red trees are dead or dying from mountain pine beetle. Photo Source: Wikipedia Commons

North American forests experience disturbances from both native and non-native insects. Bark beetles (e.g. mountain pine beetle, southern pine beetle, and spruce beetle) are all native, but are responsible for eruptive outbreaks that kill or weaken millions of acres of trees. Mountain pine beetle alone has affected about 3.4 million acres in Colorado since 1996 [5] (Fig. 2). Similarly, invasive insects such as the hemlock wooly adelgid and emerald ash borer are sweeping through our forests; their overwhelming numbers and voracious appetites leave a wake of dead stalks similar to the wooden remains of a fire (e.g. Fig. 3). Since its introduction to Richmond, Virginia from Asia in 1951, the hemlock wooly adelgid has spread through the Appalachians Mountains from Georgia to Maine. This insect can kill a tree in 4 to 5 years by depleting it of its vital nutrients and stimulating a hypersensitive response—a sort of allergic reaction by the tree[6]. Clearly slower than the ravages of a wildfire, a pest outbreak covers about the same area and leaves in its wake a similar impact. Insect and fire damage both dramatically alter the forested landscape, reduce active growth, and affect ecological process such as nutrient cycling and water systems.

Ecological comparisons aside, it should be acknowledged that wildfires, more than insect outbreaks, can cause extensive damages to human property and lives. And wildfires, when roaring through a landscape, are terrifying.

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Fig. 3 The aftermath of a hemlock wooly adelgid infestation. Photo credit: U.S. Forest Service

Wildfires and insect outbreaks are connected in more ways than one

Forest fires and insect outbreaks are two different large-scale forest disturbances, but they don’t act independently of one another; they fluctuate together. The incidence and impact of fires and insect outbreaks have both increased in recent decades, likely due to climate change [7; 8]. Additionally, research has found that forest kill-off by beetles may, in turn, be contributing to global warming due to the loss of moisture that trees circulate when alive. The effect is estimated to be about the same as that from wildfires [9]. At the same time, however, insect outbreaks can act to reduce wildfire severity. A 2016 study by researchers at the University of Vermont found that, contrary to popular belief, outbreaks of insects can reduce forest burn severity [10]. The researchers [11] think that the insects ‘thin’ the forests, killing some while leaving others in place. This decreases the density of the forest, decreasing the amount of fuel (wood) that a fire has to burn. So the increasing fires and more severe insect outbreaks of our warmer future might dampen rather than fuel each other’s effects.

Are wildfires and insect outbreaks all bad news?

No, forests are dynamic and always in flux. Defoliation and mortality by insects and fires are an integral part of a forest’s life cycle. These processes thin out weakened and old trees and other plants to make room for new growth. They also help to recycle dead biomass (organic material left behind from recently living organisms) and replenish the soil [12]. Some ecosystems only exist because of fire or disease. For example, lodgepole pines depend on wildfires to release their seeds from pine cones for the next generation. Trees such as aspens and ponderosa pines are able take advantage of space cleared by fires or insect damage to regenerate [13]. While the destructive aspects of fire and insect outbreaks are obvious, regeneration and new life often follow.


  1. “U.S. Wildfires.” National Centers for Environmental Information National Oceanic and Atmospheric Administration. Accessed 22 May 2016. https://www.ncdc.noaa.gov/societal-impacts/wildfires/ytd/12?params%5B%5D=fires&params%5B%5D=acres&end_date=2015

  2. Toppo, Greg. “Washington wildfires now largest in state’s history.” USA Today. August 25, 2015. http://www.usatoday.com/story/news/2015/08/24/washington-wildfires-largest/32302927/

  3. Jenner, Lynn. “Fort McMurray Wildfire in Alberta Canada Deemed Extreme.” Fire and Smoke. NASA.** May 4, 2016. http://www.nasa.gov/image-feature/goddard/2016/fort-mcmurray-wildfire-in-alberta-canada-deemed-extreme

  4. Man, Gary. “Major Forest Insect and Disease Conditions in the United States: 2011.” United States Department of Agriculture Forest Service. June 2012. http://www.fs.fed.us/foresthealth/publications/ConditionsReport_2011.pdf

  5. “Aerial Detection Survey: Highlights for 2015.” United States Department of Agriculture Forest Service. Accessed 22 May 2016. http://www.fs.usda.gov/detail/r2/forest-grasslandhealth/?cid=fseprd489834

  6. “Hemlock Woolly Adelgid.” Pest Alert. United States Department of Agriculture. Forest Service. NA-PR-09-05. August 2005. http://na.fs.fed.us/spfo/pubs/pest_al/hemlock/hwa05.htm

  7. Weed, Aaron S. Matthew P. Ayres, and Jeffrey A. Hicke. Consequences of climate change for biotic disturbances in North American forests. Ecological Monographs, 2013, 83 (4).

  8. “Is Global Warming Fueling Increased Wildfire Risks?” Union of Concerned Scientists. Accessed 22 May 2016. http://www.ucsusa.org/global_warming/science_and_impacts/impacts/global-warming-and-wildfire.html#.V0Oq-Ga1A4A

  9. Richards, Sabrina. “Beetles Warm BC Forests” The Scientist. 27 November 2012. http://www.the-scientist.com/?articles.view/articleNo/33447/title/Beetles-Warm-BC-Forests/

  10. “Insect outbreaks reduce wildfire severity” Science Daily. 28 April 2016. https://www.sciencedaily.com/releases/2016/04/160428122500.htm

  11. Meigs, Garrett W., Harold S J Zald, John L Campbell, William S Keeton, Robert E Kennedy. Do insect outbreaks reduce the severity of subsequent forest fires? Environmental Research Letters, 2016; 11 (4): 045008 DOI: 10.1088/1748-9326/11/4/045008

  12. “Why forests need fires, insects, and diseases.” Natural Resources Canada. Last modified 20 May 2016. http://www.nrcan.gc.ca/forests/fire-insects-disturbances/forest-need/13081

  13. Boyle, Alan. “Will forest flourish after fires? In a warming world, not always” NBC News. 5 July 2013. http://www.nbcnews.com/science/will-forests-flourish-after-fires-warming-world-not-always-6C10534178

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