Outbreaks of eastern spruce budworm occur regularly in the boreal, Great Lakes and Acadian forest regions of Canada. As natural disturbances, they are an integral part of these forest ecosystems.
Nevertheless, the spruce budworm is one of the most damaging native insects affecting spruces and true fir in Canada. During a major outbreak, tens of millions of hectares of trees can be severely defoliated by the insect. An outbreak may last several years, and cumulative defoliation can cause significant levels of mortality and growth loss in mature softwood forests. This in turn can result in significant losses of important timber and non-timber resources, negatively affecting the forest industry and forestry-dependent communities.
The dead and damaged trees that remain after a spruce budworm infestation create further problems if they become a major source of fuel for wildfires and brood material for other insects.
The last extensive outbreak of spruce budworm in Canada reached its peak in the 1970s, damaging more than 50 million hectares. By the late 1990s, the outbreak had declined to fewer than 1 million hectares. Since that time, there has been a resurgence and then decline of populations, especially in the western boreal portions of the spruce budworm’s wide range in Canada.
- Between 1999 and 2001, an infestation in northeastern British Columbia damaged nearly 1.6 million hectares.
- Between 2002 and 2003, an infestation in the Northwest Territories damaged 2.4 million hectares.
- Populations continue to decline in Saskatchewan, but recent increases have been reported in Alberta. Manitoba has reported a relatively stable population of around 100,000 hectares of spruce budworm defoliation since 1997, and a sharp drop occurred in 2011.
- In Ontario, a relatively small infestation—covering 850,000 hectares—peaked between 2006 and 2007. It has now declined to around 300,000 hectares.
- Populations of spruce budworm in Quebec have increased steadily since 2006, reaching about 2.6 million hectares in 2013. This outbreak started unusually far north, on the north shore of the St. Lawrence River and north of Lac St-Jean. It reached the Lower St. Lawrence River near Rimouski in 2010.
- The Atlantic provinces have remained free of budworm-caused damage since 1995. However, the development of the outbreak in the Lower St. Lawrence in Quebec suggests that outbreaks may soon occur in nearby northern New Brunswick.
- A steady increase in the area of forest damaged by spruce budworm has been observed in eastern Canada since 2006, perhaps heralding a new, extensive outbreak in that part of the country.
Outbreaks of spruce budworm recur at intervals of approximately 35 to 40 years in eastern Canada. Damage is most severe where there are uninterrupted forest stands dominated by mature balsam fir and white spruce.
Although weather is often cited as an important factor, there is no consensus as to how weather may influence the beginning or end of outbreaks. It is more likely that weather interacts with forest conditions to determine where, rather than when, outbreaks occur.
The availability of extensive forests of susceptible host trees is a primary contributor to the development of widespread outbreaks, mostly by supporting the survival of small larvae and maturation of moths that reproduce and migrate to new areas.
New evidence suggests that spruce budworm populations increase first in areas where natural enemies are unable to curtail increases in the local density of budworms. Reproductive success of budworms then increases and migrating moths may subsidize local populations, leading to a rise in spruce budworm populations over a very wide area.
Outbreaks end when a combination of reduced resources, resulting from damaged trees, and increases in spruce budworm mortality, caused by many natural enemies, such as birds, parasites and diseases, reduces local budworm survival.
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