Forest Pests and Climate Change

Forest Pests

Many of Canada’s most notorious forest pests and diseases have become household names in recent years:

  • The mountain pine beetle killed off a large portion of British Columbia’s Lodgepole Pine trees from the late 1990s through the 2010s and has also spread east, threatening forests in Alberta [1];
  • The emerald ash borer has aggressively attacked eastern Canada’s Green Ash trees, killing 99% of Toronto’s 850,000+ Ash trees [2], and is now spreading west to the prairies [3]; and
  • Dutch elm disease is slowly but surely stripping cities and towns across eastern and central Canada of their majestic American Elms. [4]

Under normal conditions, forest pests and tree diseases can be natural agents of disturbance that promote forest health and diversity. Unfortunately, our warming climate is tipping the ecological balance and turning them into a worsening threat.

"We’ve seen a vast area being infested faster and faster. The reason for that is climate change."

Terry Teegee knows the forests of the west coast intimately, and has seen the astonishing results of insect damage first hand. He is Regional Chief of the British Columbia Assembly of First Nations, Tribal Chief of the Carrier Sekani Tribal Council, and a registered professional forester. Teegee and his community have witnessed sporadic pine beetle outbreaks going back many generations: “our elders talked about it: we’d hear stories about the forest being blood red.” Recently however, pine beetle infestations have become massive in scale and in consequences.

The numbers are astounding. During the early 1990s, the beetle destroyed an average of about 45,400 hectares of forest per year; between 2004-2014 the beetle was a hundred times more destructive, killing over 6.4 million hectares per year. [5] Teegee watched this devastation unfold. “We’ve seen a vast area being infested faster and faster,” he says. “The reason for that is climate change.”

Teegee says that climate change is leading to warmer winters and summers, and that both of these seasonal changes are contributing to the beetle’s massive impact. In the past, he says, “we’d get an early freeze of the land and of the trees in October, and that kept the mountain pine beetle in check. That hasn’t occurred often enough since the 80s.” Teegee also observes that “with climate change we’ve seen a lot longer summers, meaning that there are two flights of mountain pine beetle. And that’s unprecedented, but has happened more and more in the past twenty years.”

The latest pine beetle infestations in BC have largely run their course, primarily because they’ve killed off most of their preferred tree species. But that doesn’t mean the threat is over. Teegee says that “2008 was basically where mountain pine beetle exhausted its use of the pine tree because there were none left, and now it’s carrying on into the boreal forest.” The beetles have begun to attack jack pine [6], and forestry researchers have identified climate change as a major risk factor in the likelihood of this destructive species spreading to the vast pine forests of eastern Canada. [7]

Deep winter cold spells are needed to limit outbreaks of many pests, including ash borers and tent caterpillars.

Teegee’s experience of warming winters and summers leading to a sudden explosion of insect damage is a pattern that has also been seen with other pests across the country. In Toronto, for example, hotter summers allowed ash borer populations to undergo two reproductive cycles rather than just one, doubling their normal rate of infestation. [8] Research has also shown that deep winter cold spells are needed to limit outbreaks of many pests, including ash borers [3] and tent caterpillars. [9] The warming climate thus weakens natural controls on insect pest populations at the same time as it accelerates their rates of growth and reproduction. This combination allows pests to spread much farther and faster than before.

Pests attacking new species and making their way into new ecosystems are especially concerning consequences of climate change. Insects can now be found in unexpected places, such as near the tops of mountains or far north, near the tree line. [10] These shifts in habitat and species can happen rapidly and can have devastating consequences when infestations reach forests that haven’t evolved to resist these invasive threats.

Trees, of course, have natural defences that allow them to repel and recover from many kinds of pests and diseases. Unfortunately the same changes in climate that promote the aggressive spread of insects also impact trees’ capacities to defend against them. During warmer and drier conditions associated with periods of drought, trees are less resilient to the effect of insects and disease. [11] And when faced with multiple sources of stress – such as an onslaught of insects during a drought – trees are much more likely to die. [12][13]

Read more: Defoliators

Insects such as the Ash Borer and the Mountain Pine Beetle get a lot of public attention, but tree health problems caused by defoliators (insects that consume the leaves of trees) are also a serious concern. Two common defoliators are the forest tent caterpillar [14] – which feeds on the leaves of a variety of trees including trembling aspen, oak, ash, maple and white birch – and the spruce budworm, a very destructive and widely distributed defoliator in North America, targeting fir and spruce. [15]

Climate change is likely to cause increased tent caterpillar activity because warmer winters cause less insect mortality. [9] Similarly, the severity of Spruce Budworm defoliation is correlated with milder winters and warmer, drier summers [16], both of which are projected to become more likely in many parts of Canada. And, as Natural Resources Canada points out, budworm infestations increase vulnerability to forest fire [9], which also becomes more likely in a warming climate. (See our Forest Fires and Climate change article for more information.)

Mitigation and Adaptation

Urban and wild forest management strategies can play an important role in reducing the impact of forest pests in the face of climate change.

The city of Winnipeg and the province of Manitoba, for instance, have implemented strategies to reduce the spread of Dutch elm disease, including practices around firewood storage, tree pruning, early detection, and the rapid removal of infected trees. [17] Alberta and Saskatchewan have implemented a variety of strategies targeting the mountain pine beetle, in the hopes of slowing its spread, though they understand it may be difficult to stop it altogether. [18]

Climate projections such as those presented in the Climate Atlas are essential to inform management strategies in both urban and wild settings. Projections show shifts in temperature, which could have implications about where pests might be found in the future, as well as what conditions trees will face as climate change alters the seasonal distribution of warmth, cold, and precipitation.

A key message arising from forestry research is that climate change will likely bring on sudden and unpredictable disturbances. [19] Forest managers will have to cultivate diverse and resilient tree populations and management practices, because climate change means having to be ready for the unexpected.

Ultimately, the most direct way to preserve our forests in the face of these threats is to take prompt and effective action to prevent climate change from accelerating. The less warming that comes to pass, the less stress will be placed on the natural world, and the less our practices will have to adapt to more serious risks.

Teegee says “I think we’re in dire straits with the reality of climate change,” but notes that “the good thing about human beings is that we’re resilient. We’ll make change.” For Teegee, responding to climate change means recognizing that “we’ve lost that real connection with the land” and that fundamentally “we’ve really got to think about what’s important in our lives” in order to live in balance with nature.


  1. Natural Resources Canada. “Mountain Pine Beetle.”
  2. CBC News. “Toronto's ash trees face extinction.” (2011)
  3. City of Winnipeg. “Emerald Ash Borer.”
  4. National Resources Canada. “Dutch elm disease.”
  5. Canadian Council of Forest Ministers. National Forestry Database. “Forest Insects.”
  6. Natural Resources Canada. “Slowing the march of the mountain pine beetle.”
  7. Canadian Forest Service. “Predicting the risk of mountain pine beetle spread to eastern pine forests: considering uncertainty in uncertain times.”
  8. CTV News. “Hot, dry weather accelerated Toronto's emerald ash borer tree crisis.”
  9. Haynes, Kyle & Allstadt, Andrew & Tardif, J. (2014). “Effects of climate change on forest tent caterpillar outbreak dynamics based on a century of tree-ring data.”
  10. Weed, Aaron S.; Ayres, Matthew P.; Hicke, Jeffrey A. 2013. “Consequences of climate change for biotic disturbances in North American Forests.” Ecological Monographs 83(4): 441-470.
  11. Allen, Craig D. et al. “A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests.” Forest Ecology and Management 259.3: 660-684.
  12. Anderegg, William R. L et al. (2015) “Tree mortality from drought, insects, and their interactions in a changing climate.” New Phytologist. doi: 10.1111/nph.13477.
  13. Globe and Mail. “Drought-stricken forests in B.C., Alberta face new threat from insects.”
  14. Natural Resources Canada. “Forest tent caterpillar.”
  15. Natural Resources Canada. “Spruce budworm.”
  16. Natural Resources Canada. “Effects of climate change on the impacts of spruce budworm infestations.”
  17. City of Winnipeg. “Dutch Elm Disease Research.”
  18. Province of Alberta. “Alberta's Strategy - Mountain Pine Beetle in Alberta.”
  19. Williamson, T.B et al. Climate Change and Canada’s Forests: From Impacts to adaptation.
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