Tree Maintenance

Threats to Trees

The City of Toronto's Urban Forestry branch is responsible for the identification and management of forest health threats using integrated pest management practices. Toronto's Forest Health Threats describes methods to control the introduction, establishment, spread and impact of current and potential forest pest invasions.

Learn more about Toronto's forest insects, diseases and threats by reviewing the factsheets below.

Common insect pests that are a threat to Toronto's trees and urban forest include:

Aphid colony on a twigAphids

Aphids are small soft-bodied insects that feed on plants by sucking sap from leaves and stems.

Adult Asian longhorned beetle. Asian Longhorned Beetle

The Asian longhorned beetle (Anoplophora glabripennis) is a very serious threat to the hardwood trees of North America.

Birch leafminer - adult sawflyBirch Leafminer

The birch leafminer (Fenusa pusilla) is included in a group of insects known as sawflies.

Adult Bronze birch borer beetle Bronze Birch Borer

The bronze birch borer (Agrilus anxius) is a native, North American beetle that can cause serious damage to a host of birch trees.

Bronze poplar borer larval tunnelsBronze Poplar Borer

The bronze poplar borer (Agrilus liragus) is a flat-headed borer that feeds on poplars and is native to North America.

Adult carpenter antCarpenter Ants

The carpenter ant (Camponotus pennsylvanicus) is a social insect and lives in colonies. They nest in wet wood in the cavities of trees.

Eastern tent caterpillars on the tent

Eastern Tent Caterpillar

The Eastern tent caterpillar (Malacosoma americanum) is a native insect that rarely occurs in large enough numbers to cause tree death in Toronto.

Native Bark BeetleElm Bark Beetles

There are two species of bark beetles that feed and breed on elm trees, the smaller European elm bark beetle (Scolytus multistratus), and the native elm bark beetle (Hylurgopinus rufipes).

Adult Elm Leaf beetleElm Leaf Beetle

The elm leaf beetle (Xenthogaleruca luteola) is originally from Europe and was first discovered in Ontario in 1945.

Mining damage from the Elm LeafminerElm Leafminer

The elm leafminer (Fenusi ulmi) is a European insect that has been known in North America since the late 19th century.

Emerald Ash Borer resting on a human finger.Emerald Ash Borer

The Emerald Ash Borer (EAB) is an introduced insect pest from Asia that attacks and kills all species of ash (genus: Fraxinus) trees.

Male and female European Gypsy MothsEuropean Gypsy Moth

European gypsy moth (Lymantria dispar) is a defoliating insect that can severely weaken or kill trees. It is a major introduced pest to North America.

Fall cankerworm male and wingless female mothsFall Cankerworm

Fall cankerworm (Alsophila pometaria) is a native insect, and can be one of the most damaging defoliators of Toronto’s urban forest.

Honey locust leafhopperHoney Locust Plant Bug and Leafhoppers

There are several species of plant bugs and leafhoppers that feed on honey locust (Gleditsia triacanthos).

Adult Japanese beetleJapanese Beetle

Japanese beetles were introduced to North America in 1916. They are a voracious eater of the leaves and fine roots of over 300 different species of plants, grasses and trees.

Adult lace bugLace Bug

Lace bugs are a native North American insect that feeds on a wide range of trees.

Termites - soldiers and workersTermites

The eastern subterranean termite was introduced to Toronto in 1938 and has since become established in homes throughout the city.

Common diseases that are a threat to Toronto's trees and urban forest.

Apple Scab symptoms on leavesApple Scab

Apple Scab is one of the most serious diseases of apple and ornamental crabapple trees.

Ash Anthracnose on older leavesAsh Anthracnose

Ash anthracnose is a common disease of ash trees, caused by the fungus Apiognomonia errabunda.

Beech bark disease - beech scale on barkBeech Bark Disease

Beech bark disease is a devastating disease of beech tree and is caused by a complex of a beech bark scale (Cryptococcus fagisuga) and species of Nectria fungi.

Black knot on a branchBlack Knot

Black knot is a disease of Prunus species that causes twig and branch swelling and discolouration, resulting in girdling and dieback of branches and sometimes the main stem.

Canker - fruiting bodies on barkCankers

There are many different types of cankers that affect a broad range of trees.

Cytospora - resin on branchCytospora Canker of Spruce

Cytospora canker is one of the most common and damaging diseases of spruce.

Dutch Elm Disease - brown streaking in woodDutch Elm Disease

Dutch elm disease (DED) is the most devastating disease of elm trees in North America.

Eastern filbert blight - cankers in rowsEastern Filbert Blight

Eastern filbert blight is caused by the fungus (Anisogramma anomola) and is indigenous to northeast North America.

Fire blight canker on apple branchFire Blight

Fire blight can be a serious disease that affects many trees and plants in the Rosaeceae family.

Horse-chestnut leaf blotchLeaf Blotch of Horsechestnut

Leaf blotch of horse-chestnut is a leaf disease caused by the fungus Guignardia aesculi.

Oak anthracnose - deformed leavesOak Anthracnose

Oak Anthracnose is a disease affecting oak trees caused by the fungus Apiognomonia quercina.

Oak wilt - leavesOak Wilt

Oak wilt, caused by fungus Ceratocystis fagacaerum, is a serious disease of oak trees.

Pear trellis rust orange spotsPear Trellis Rust

Pear trellis rust is cause by the fungus Gymnosporangium fuscum. The pear trellis rust fungus has been introduced to southern Ontario in recent years.

Powdery mildew - norway maplePowdery Mildew

Powdery mildew is a common foliar disease of many tree species. 

Scales - wooly alder aphidScales

A large number of scales feed on trees by sucking sap from leaves, twigs or stems.

Sudden oak death oozeSudden Oak Death

Sudden oak death (Phytophthora ramorum), is a disease affecting species of oak trees caused by a soil borne, fungal-like organism. 

Sycamore anthracnose - leaf blightSycamore Anthracnose

Sycamore anthracnose is the most serious disease of sycamore trees.

Tar spot - fallTar Spot

Tar spot is a fungal leaf disease that may occur on several plants, but it is most common on maple.

Verticillium wilt branch diebackVerticillium Wilt

Verticillium wilt is a vascular disease caused by the soil-borne fungus Verticillium dahlia.

Hazards and other threats to Toronto's trees and urban forest include:

Beaver lodge in a Toronto wetlandBeaver Damage

The beaver (Castor canadensis) is the largest rodent in North America. In Ontario, they have been known to eat almost every tree and shrub species available.

Bald-faced hornet nestBees, Wasps, Hornets

Bees are important pollinators and honey producers.  Wasps and hornets are considered beneficial insects because they feed on a large number of insect pests.

Hackberry Nipple GallsGalls

Galls are unusually-shaped formations that grow on plants including trees.

Girdling roots around the base of a tree Girdling Roots

Girdling roots are roots that grow around other roots or around the main stem of a tree.

A tree displaying damage cause by a lawn mower Mechanical Injuries to Trees

Mechanical injuries are one of the most common causes of dieback of trees in an urban environment.

Witch's Broom Close UpSalt injury

De-icing salt used in winter to provide safe road and walking conditions can cause significant damage to trees and other vegetation.

Stand of three trees submerged in waterWater stress

Water is vital to trees. Without water a tree cannot maintain its physiological functions and will die.

Wood decay showing fruiting body on a beech tree Wood decay

Wood decay is a process of wood disintegration that is caused by fungi or other micro-organisms.