Part of Forest health

Forest pests and damage agents

Read about forest pests and use the interactive tool to diagnose forest and tree pests or damaging agents.

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Forest pests
Spruce Budworm Larva
Spruce budworm larva


The term ‘pest’ is value laden. We must recognize that forest insects and diseases are often essential components of forested ecosystems and play a dynamic role in promoting forest health and resiliency. The same is true for other types of natural disturbance (for example, fire, wind events). However, when forest damaging organisms or other forest disturbance occurs in in ways that place constraints on the management, enjoyment, utilization or ecosystem values of trees and forests, they are considered pests.

Forest pests can threaten the health and vigour of trees and forests. Therefore, they may threaten biodiversity and the many social, cultural, and economic values Albertans enjoy from forested lands. See the list of noteworthy forest pests in Alberta.

In this section, ‘pest’ is an umbrella term for all kinds of forest damaging agents. Many agents can have negative effects on the health of trees and forests. Some are living (biotic), while others are non-living (abiotic).

Table 1. Examples of biotic and abiotic agents

Examples of biotic agentsExamples of abiotic agents
Disease-causing organismsChanging climatic conditions
Parasitic plantsFlooding
MammalsOther severe weather events (hail, ice storms)
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Online pest diagnostic tool

The Alberta Forest Pest Diagnostic System is an online database of host trees, symptoms, biology, impact, terms and images related to tree pests in Alberta. Search by symptom, pest common name or tree, and find pest management information.

Forest Pest Diagnostic System

Noteworthy forest pests

Learn how forest pests and damage agents negatively impact Alberta trees and forests.

  • Mountain pine beetle

    Latin name: Dendroctonus ponderosae

    Mountain pine beetle (MPB) is the most serious insect pest of mature pines in western Canada. Millions of hectares of pine forests have been affected by this pest in British Columbia and Alberta (and to a lesser extent in SW Saskatchewan), over the past 2 decades. Relying on mass attack and introducing a symbiotic fungi, the beetles overwhelm pine tree defences. Successfully attacked trees die within one year. MPB may kill vast tracks of mature pines.

    There are major social, economic and ecological affects associated with MPB outbreaks. Fire frequency and severity can be increased, biodiversity and wildlife habitat can be degraded. The Forest industry may also be negatively impacted, affecting jobs, wood supply and long-term timber resources.

    For more information:

    Mountain pine beetle in Alberta

    Alberta Forest Pest Diagnostic System

    Figure 1: Mature mountain pine beetle

    Mountain Pine Beetle on Bark
  • Spruce budworm

    Latin name: Choristoneura fumiferana

    Until the recent mountain pine beetle (MPB) epidemic, spruce budworm (SBW) was the most damaging conifer pest in North America. White spruce, the primary host tree for SBW in Alberta, are quite resilient to defoliation. Therefore, unlike MPB, SBW do not kill trees in a single year. However, reduced tree growth results if severe defoliation extends into a second summer. By the fourth consecutive summer of severe defoliation, young trees may begin to die. Larger, mature trees may begin to die after five consecutive years of severe defoliation.

    Over-mature spruce and spruce-fir stands are the most susceptible to defoliation by this insect. They can support large numbers of SBW when populations periodically increase in size.

    For more information:

    Spruce Budworm in Alberta

    Alberta Forest Pest Diagnostic System

    Figure 2: Spruce budworm larva on spruce

    Spruce Budworm Larva on Spruce
  • Forest tent caterpillar

    Latin name: Malacosoma disstria

    Forest tent caterpillar (FTC) is a major defoliator of broadleaf trees in the boreal forest. Not all of FTCs impacts are bad (for example, important role in recycling nutrients). However, reduced radial stem growth and branch mortality can result from successive years of defoliation. Tree mortality may occur if severe defoliation repeats for several years.

    An FTC outbreak can be awe-inspiring, entirely devouring tree and shrub leaves over vast areas. Masses of caterpillars blanketing everything in sight and producing copious webbing provide an ‘ick’ factor seldom rivalled by forest insects. Severe or repeated defoliation also makes the host trees more prone to other pests. Drought and defoliation are the major drivers of growth reductions and mortality, interacting such that their combined effects are worse than the impact each would cause individually. Some areas in Alberta are already experiencing widespread aspen mortality due to combined impacts of FTC and drought.

    For more information: Alberta Forest Pest Diagnostic System

    Figure 3: Forest tent caterpillar larva on aspen leaves

    Forest Tent Caterpillar Larva on Aspen Leaves
  • Armillaria root disease

    Latin name: Armillaria ostoyae

    Armillaria root disease occurs in all regions of Canada, with seven species found in the Western Provinces. Among them is Armillaria ostoyae which, in Alberta, is common south of 60oN. Armillaria is a natural component of forest ecosystems. Above-ground symptoms and signs are present in many forest stands, but above-ground indications do not reflect the extent to which the fungus is present.

    An enormous A. ostoyae mushroom in the Malheur National Forest of eastern Oregon, dubbed the ‘Humongous Fungus’, is possibly the largest organism in the world. Samples taken were genetically identical over an area of almost 10 square kilometres – indicating it is one organism.

    Host trees often have mechanisms used to fight off armillaria infections. Trees may be infected but keep fungal development in a state of equilibrium. This host/fungus equilibrium is upset in favour of the fungus by any activity that creates stumps. Once host trees are cut (or fall over), the fungus rapidly colonizes the stumps. Fungal mycelium and rhizomorphs (string-like structures produced by the fungus) spread through the tree’s roots and the surrounding soil, infecting new hosts. Infections occur when tree roots contact infected roots or rhizomorphs.

    These diseases are capable of causing significant losses. In some areas of western Canada, armillaria has been suggested as a potential threat to sustainable forestry management.

    For more information: Alberta Forest Pest Diagnostic System

    Figure 4: Armillaria spp. fungal mat under bark

    Armillaria Spp. Fungal Mat Under Bark
  • Lodgepole pine dwarf mistletoe

    Latin name: Arceuthobium americanum

    Dwarf mistletoes are common, perennial, flowering, parasitic plants with many different, largely host-specific species. In the prairie provinces, lodgepole pine dwarf mistletoe (LPDM) causes greater annual losses of merchantable pine volume than any other disease agent.

    Losses come not only from mortality, but also from reductions in growth and defects related to the ‘witches’ brooms’ caused by this disease. Mistletoes spread by firing sticky eggs from aerial shoots (the only parts of these plants that grows outside the host tree’s bark). The seeds land on nearby trees, germinate and specialized roots penetrate the new host’s bark.

    Reductions in growth by one third of heavily infected mature pine can be expected, while heavily infected young trees will not produce merchantable volume. LPDM can also influence fire behaviour, promoting crown fires. Despite its negative effects it should be noted that LPDM is important ecologically, often providing important habitat for birds and small mammals.

    For more information see Alberta Forest Pest Diagnostic System.

    Figures 5: Dwarf mistletoe aerial shoots on lodgepole pine

    Dwarf Mistletoe Aerial Shoots on Lodgepole Pine
  • Large aspen tortrix

    Latin name: Choristoneura conflictana

    Large aspen tortrix (LAT) is a broadleaf defoliator, sometimes a severe one. Major outbreaks can adversely affect trees, causing delayed leaf flush, reduced radial growth and (in some instances) cause mortality. Infestations last typically 2 to 3 years and occur approximately every 10 years. Successive years of defoliation stresses trees, increasing their susceptibility to other insects and disease. LAT prefer aspen, but if those hosts are completely defoliated during outbreaks, other deciduous species begin to look yummy to them. In addition to defoliation, the presence of large numbers of LAT caterpillars may become a public nuisance.

    For more information see Alberta Forest Pest Diagnostic System.

    Figures 6: Large aspen tortrix larva on a stump

    Large Aspen Tortrix Larva on a stump
  • Yellow-headed spruce sawfly

    Latin name: Pikonema alaskensis

    The yellow-headed spruce sawfly (YHSS) is usually a pest of young trees and stands. Spruce greater than eight metres in height are seldom damaged by these pests. The larvae look like caterpillars; however, they are more closely related to wasps than moths or butterflies. They prefer open grown spruce (not surrounded by other vegetation), like those in found in plantations, shelterbelts, nurseries or lawns. They are also occasionally found in large numbers in naturally regenerating stands. A few years of consecutive severe defoliation can cause branch mortality, top-kill and tree mortality.

    For more information: Alberta Forest Pest Diagnostic System.

    Figure 7: Yellow-headed spruce sawfly larvae

    Yellow-headed Spruce Sawfly Larvae on Spruce
  • Pine needle casts

    Several different types of fungi cause pine needle cast diseases, causing discoloration of needles and needle loss for infected pine. Damage appears to work up from the bottom and out from the interior of the crown. Current year's growth is usually green.

    Some of these diseases can be serious and cause mortality. However, most pines die only if they are already stressed by some other factor, or heavy repeated infections occur over several years. Severe infections negatively affect host tree vigour and growth, also predisposing infected trees to other forest health damaging agents. Needle casts are not usually serious diseases. Still, repeated extensive defoliation from these diseases can lead to growth loss or even death. Mortality is uncommon for mature trees. Extensive defoliation may lead to a change in growth and shape of pines, particularly in younger trees.

    For more information: Alberta Forest Pest Diagnostic System.

    Figure 8: Pine needle cast disease

    Pine Needle Cast Disease
  • Willow leafminer

    Latin name: Micruapteryx salcifoliella

    The willow leafminer (WLM), also known as the willow leaf blotch miner, is a small gray moth native to North America. The caterpillars of this moth feed on willow leaves, periodically causing extensive damage to willow stands in the province. In 2011, an inter-provincial outbreak of this insect damaged vast tracks of willow stands across western Canada. WLM feed on most species of willow. Although, some willow species (such as feltleaf willow) having leaf undersides covered ‘felt-like’ with dense hair are not attacked.

    Willows are not typically killed by WLM. However, if consecutive years of severe leafminer damage occur, willows under stress may be killed. Severe infestations may result in branch kill. Significant die-back of willows in some parts of Alberta has occurred in the last decade. Affected leaves may drop prematurely and damaged parts of leaves affect plant nutrition and growth. This may affect forage for wildlife, such as moose, dependant on willow as a food source.

    For more information:

    Willow Leafminer factsheet

    Willow Blotch Miner factsheet

    When Moths Turn Away Moose

    Alberta Forest Pest Diagnostic System

    Figure 9: Willow leafminer larva on willow leaf

    Leafminer Larva on Willow Leaf
  • Invasive species

    Invasive (non-native) species are organisms that are intentionally or unintentionally introduced into Alberta from elsewhere, of which some have the potential to threaten our province’s ecosystems and biodiversity. The term ‘invasive species’ encompasses a broad range of insects, diseases, plants, and other organisms. In the absence of the natural enemies (organisms that would normally regulate pest populations), invasive species can gain advantage over native species.

    Spongy moth (Lymantria dispar)

    One invasive insect species of concern is the spongy moth, originally introduced to eastern North America by an amateur entomologist in 1869. Since escaping confinement, the spongy moth has expanded its range, inflicting severe damage on eastern forests. In order to slow its westward spread, the spongy moth is regulated under the Canada Plant Protection Act. Alberta’s government augments federal population survey efforts by setting out monitoring traps in some high-risk locations such as campgrounds, railyards, and industrial sites. While spongy moths have been detected in several locations across Alberta, no established populations have been found.

    Human activity is often a major cause of invasive species introduction and spread; spongy moth is commonly spread through the transportation of goods infested with eggs, for instance. Serious potential economic and ecological consequences can result from the introduction of invasive species into new areas. People should be vigilant when entering Alberta’s natural areas.  Ensuring that recreational vehicles (particularly off-road vehicles) are clean and firewood is locally sourced are steps we can take to limit the spread of invasive species. To learn more about spongy moth and other invasive insect species, and what you can do to help limit their spread, visit the Invasive Species Centre website.

    For more information:

    Invasive species (Canadian Food Inspection Agency)

    Hazards of moving firewood (Canadian Food Inspection Agency)

    Alberta Forest Pest Diagnostic System

Integrated pest management

Integrated pest management (IPM) focuses on the prevention of pest outbreaks and, if justified, control of pests using multiple tools. The concept of IPM was created to address the problems associated with the heavy reliance on chemical control in agriculture and to provide a framework for making economically sound management decisions. Over-use of pesticides leads to soil and groundwater contamination and encourages the development of pesticide resistance. Resistance develops when a small portion of the pest population survives exposure to a pesticide because of their genetic makeup. The survivors pass along this resistance to their offspring and, over time, the population develops resistance to that pesticide.

Another issue can occur when using broad-spectrum pesticides. This type of pesticide is not species-specific; it harms not only the target pest but also the non-target insects like pollinators or natural enemies that help keep the pest population under control. Finally, pesticide application can cause or promote outbreaks of secondary pests. This happens when the suppression of the target pest releases a non-target insect from interspecific competition, which allows it to outbreak.

Accurate pest identification is essential to IPM. This is important because prevention, monitoring and control activities are specific to a particular pest. Once the pest has been identified, the forest manager can learn about the pest’s life cycle and population dynamics, and the host response to the pest. This information enables the tailoring of prevention, monitoring and control activities to target the correct insect at the most vulnerable life stage using the effective monitoring and treatment methods.



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