Natural Disturbances

Photo: beetle damage © Epp
Mountain pine beetle damage


Large-scale insect outbreaks are an important natural disturbance regime in Canada. Changes in patterns of outbreaks of some insect species are evident but they are not uniform, with some increasing in severity, some decreasing, some showing no sign of change, and many without long-term data. Insect outbreaks and fire each affect the other and both are influenced by climate. For example, the suppression of wildfire has caused changes in forest structure in some areas, increasing their susceptibility to outbreaks of some insects. At the same time, insect outbreaks can influence fire dynamics, for example, increasing wildfire intensity in post-outbreak stands.


Spruce budworm

Map: spruce budworm. Click for graphic description (new window).
Source: adapted from Canadian Forest Service, 200738
Photo: spruce budworm © Thérése Arcand, NRCan, CFS

The spruce budworm, native to Canada’s boreal and mixedwood forests, is one of Canada’s most prevalent and influential insect defoliators. Of the four species that occur in Canada, the most widespread is the eastern spruce budworm. Its preferred hosts are balsam fir and white and red spruce, but it can also defoliate black spruce.39 It is most damaging to older, denser forest stands although during severe outbreaks all host stands are vulnerable.40 Together with fire, the eastern spruce budworm is the dominant natural disturbance in the boreal forest.41 Cycles of spruce-budworm outbreaks, recurring approximately every 30 to 55 years,42 influence species composition, age-class distribution, successional dynamics, and forest condition, thereby playing an important role in shaping forest ecosystems.43, 44 Outbreaks occur somewhat synchronously over extensive areas, but outbreak duration varies regionally.45 The last peak outbreak was in 1975, when over 510,000 km2 were defoliated nationally.46

Western spruce budworm affects a much smaller area. The last peak defoliation was in 2007, when about 8,600 km2 were defoliated nationally.46 Severity of attack is low, for example, 95% of affected area in B.C. was classified as light in 2008.47 One study mapped historical attack in the Kamloops Forest Region and found an increase in attack over the four outbreaks between 1916 and 2003, particularly after 1980.48

Area defoliated by eastern spruce budworm east of the Manitoba border and in Maine, U.S.

Thousands km2 of moderate to severe defoliation, 1909 to 2007
Graph: area defoliated by eastern spruce budworm. Click for graphic description (new window).
Source: pre-1909 to 1980 (blue line) adapted from Kettela, 1983;49 1974 to 2008 (red line) adapted from National Forestry Database, 201046 and Strubble, 200850

There is no consensus on whether there has been a change in frequency of eastern spruce budworm outbreaks.44, 45, 51, 52 An overall increase in the area it has defoliated is apparent for Ontario and Quebec, however, which represented 98% of the area affected during the last peak outbreak.46, 49 There is no consensus on whether this constitutes a trend. At the same time, the severity of outbreaks in New Brunswick decreased between 1949 and 2007.53 Studies that conclude there have been changes in the pattern of attack have attributed them to fire suppression, forest harvesting practices, temperature increases in the spring, insecticide spraying, and less reliable reconstructions of historical outbreaks.44, 54, 55


Mountain pine beetle

Photo: mountain pine beetle © Leslie Manning, CFS

The mountain pine beetle is native to western North America and at least four large-scale outbreaks have occurred in B.C. in the last 120 years.25 The disturbance has changed in the last decade, however, with an infestation of unprecedented intensity in B.C.58, 59 In 2005, it spread to Alberta,60 where it has spread rapidly, including to jack pine/lodgepole pine hybrids.61, 62 Attack results not only in changes to the forest, but can result in changes in water temperature and flow patterns, and increased soil and stream bank erosion.63 Beetle-killed stands are also more vulnerable to fire,64-67 and the combination of increased insect attack and past fire suppression can lead to an increase in intense, stand-replacing wildfires.68 The infestation appears to have peaked in B.C., likely because most host trees in the central plateau have already been attacked, and because variable terrain and greater tree diversity have slowed the spread in other areas.58

Cumulative area affected
1999 and 2009
Two maps of cumulative area affected by pine beetles. Click for graphic description (new window).
Source: adapted from B.C. Ministry of Forests and Range, 2010;56 Alberta Sustainable Resource Development, 201057
Area affected annually by mountain pine beetle in B.C.
Thousand km2, 1928 to 2009
Graph: area affected annually by mountain pine beetle in B.C. Click for graphic description (new window).
Source: adapted from B.C. Ministry of Forests and Range, 2010;56 Alberta Sustainable Resource Development, 201057

Host availability, climate, and forest management practices all influence mountain pine beetle dynamics.25 Changes that have contributed to the current infestation include:

  • The proportion of older age classes of lodgepole pine stands, which are more susceptible to attack, increased from 17% in the early 1900s to 55% in 2002,64 largely as a result of fire suppression,25, 64, 67, 70 and harvest practices that change forest structure.64, 67, 71
  • Climate has changed since 1920 to become more suitable for the beetle.72 Warmer winters73 have led to increased beetle survival. Temperatures in spring and late fall also affect mortality.71 For example, earlier onset of spring has increased spring survival.58, 72, 74
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