Canadian forests have always been subject to damage by insects. Reports of insect damage go back to early European settlers in eastern Canada who observed eastern spruce budworm (Choristoneura fumiferana) activity. Today, native insects that significantly damage forests include spruce budworm, tent caterpillars (Malacosoma spp.), and bark beetles (Scolytidae). Other non-native, or alien, insects, introduced through post-settlement activities also cause extensive damage. Examples are balsam woolly adelgid (Adelges piceae), which feeds mainly on balsam fir (Abies balsamea), and gypsy moth, European race (Lymantria dispar), which attacks many hardwood species. Alien species often do more damage than native ones because they have no natural predators or diseases here to limit the expansion of their populations. Forest insect pests acting in combination with diseases are even more damaging to forests.
Damage from insects may limit the economic use of certain tree species and/or render areas of forests unsuitable for recreation, wildlife habitat, or other uses. The biodiversity of forests may also be altered by the effects of insect damage.
Terminology clarifications
The NFD Web site reports insect damage in areas within which there is mortality, moderate to severe defoliation, or trees killed by bark beetles. Moderate to severe defoliation means that 30% or more of the current foliage has been removed. Different levels of defoliation, over various time periods, result in some growth losses; if the defoliation is prolonged, the trees will die. Significant growth losses are generally deemed to begin when crown defoliation reaches 40%.
In the case of budworm defoliation damage in spruce (Picea) and fir (Abies) stands, the reported areas may be large and contiguous. In the case of bark beetle infestations of lodgepole pine (Pinus contorta var. latifolia), the reported areas may be scattered pockets of dead trees, usually of larger diameter. There are seldom discrete areas of complete mortality from bark beetles. Factors such as site quality, tree age and health, and weather conditions may also affect the extent of area and distribution of trees that are killed.
Forest management activities include spraying with insecticides to protect foliage and keep trees healthy by reducing insect populations. Because insects such as the forest tent caterpillar (Malacosoma disstria) tend to cause growth loss rather than extensive tree mortality, growth losses are reported.
Data sources
The defoliated areas are mapped by provincial and industrial cooperators. The agency that collects and processes the data differs among jurisdictions.
Areas of forest lost due to insect pests and diseases are based on studies by CFS scientists and other researchers. Measurements for the defoliated areas are combined with inventory data, using GIS techniques, to determine tree mortality and growth reductions. Research reports on insects and diseases can be found in Forest Conditions, Monitoring and Reporting and in Forest Landscape Management on the CFS Web site.
Precautions in using the data
Researchers and managers try to use the most up-to-date methods of data collection and analysis and every effort is made to ensure that the data represent the actual forest conditions. However, the data should be treated with caution as the basic information varies greatly from place to place. The sketch mapping techniques are often imprecise and forest inventory data in some parts of the country are more recent than others. As well, standards of data collection and processing differ across the country.
A forested area may be defoliated by more than one insect, and this often results in overlap of the reported figures. The area within which there is moderate to severe defoliation can also include relatively small non-forested areas, roads, cultivated areas, small lakes, or burned areas. Areas reported as defoliated may include patches that differ in the degree of defoliation severity. Areas of tree mortality caused by an insect attacking one or more species may include other tree species which subsequently die from exposure because the stand is opened. Finally, some areas of defoliation may be missed in the surveys.
Data on mountain pine beetle (Dendroctonus ponderosae) damage are included in the NFDP Web site. Damage caused by these insects is not defoliation. Eggs laid by female beetles develop into larvae that feed on the inner bark; infested trees usually die from the combined effects of feeding and the pathogens (for example, blue stain fungi) that infect the trees when they are attacked by the beetles. The mountain pine beetle data describe areas within which trees have been killed by the beetle - death usually occurs within one year of infestation.
When interpreting data on insect damage, consider the following limitations: reporting areas of dead forest versus reporting areas where dead trees occur; the limitations of inventory data; and the ability (or lack thereof) to match inventory data with areas experiencing one or more years of insect defoliation. The extent of growth loss or mortality varies with damage intensity, site condition, tree age, tree health, weather conditions, etc.
A summary of areas affected by major defoliating insects or bark beetles is presented to illustrate the magnitude of the problem. Strict comparison is not possible; usually a succession of defoliation years is required to kill a tree, and this varies with the pest, the tree species being attacked, and other factors. Differences in survey methodologies and intensities for each pest further complicate comparisons. More detailed information can be obtained from CFS's Forest Health Web site. 
