Common Tree Pests and Diseases Affecting Urban Canada

Updated: June 12, 2026

Several invasive insects and fungal diseases now pose serious risks to tree populations across Canada’s urban centres. Recognizing the signs early, understanding the reporting pathways, and knowing when a tree is beyond treatment are the practical skills that matter most for property owners and community stewards.

Emerald Ash Borer (Agrilus planipennis)

First confirmed in Canada near Windsor, Ontario in 2002, the emerald ash borer (EAB) is a wood-boring beetle native to northeastern Asia that targets all species of ash (Fraxinus) in North America. In the absence of natural predators, EAB populations can kill untreated ash trees within three to five years of initial infestation.

The adult beetles are metallic green and approximately one to one-and-a-half centimetres in length. Their larvae feed in S-shaped galleries beneath the bark, disrupting the tree’s ability to transport water and nutrients. External signs include:

• D-shaped exit holes approximately 3–4 mm wide in the bark
• Increased woodpecker activity on the trunk (as birds forage for larvae)
• Vertical bark splitting or serpentine galleries visible beneath loose bark
• Dieback beginning in the upper canopy and progressing downward
• Epicormic sprouting from the trunk and major branches

EAB has been confirmed in Ontario, Quebec, Manitoba, New Brunswick, Nova Scotia, and Prince Edward Island, with ongoing monitoring in adjacent provinces. Regulated areas restrict the movement of ash wood, nursery stock, and firewood. The Canadian Food Inspection Agency (CFIA) maintains current regulated zone maps and movement restrictions.

Preventive insecticide treatments—typically systemic applications of imidacloprid or emamectin benzoate applied by a licensed applicator—are available and can extend the life of high-value ash trees in early-infestation zones. Biological control agents, including several parasitic wasp species, have been released in affected areas as part of a long-term management program coordinated through Natural Resources Canada.

Dutch Elm Disease

Dutch elm disease (DED), caused by the fungal pathogen Ophiostoma ulmi and the related species O. novo-ulmi, spread across North America through the twentieth century, killing large portions of the elm population in eastern Canadian cities. The disease is spread primarily by elm bark beetles that carry fungal spores from diseased to healthy trees.

Symptoms appear as yellowing and wilting of leaves on individual branches—a progression known as flagging—followed by rapid dieback as the fungus spreads through the tree’s vascular system. Affected wood, when cross-sectioned, shows a characteristic ring or arc of brown discolouration just beneath the bark.

Alberta has maintained one of the largest protected elm populations in North America through a rigorous municipal management program that includes pruning bans during high beetle flight periods, rapid removal of infected trees, and coordinated public education. Edmonton and Calgary have maintained significant street elm populations through this approach.

In provinces where DED is established, managing the spread involves prompt removal of infected material, sanitizing tools between trees, and avoiding pruning during periods of high beetle activity, which generally spans late spring through early summer.

Bronze Birch Borer (Agrilus anxius)

The bronze birch borer is a native beetle in Canada, but it preferentially attacks white and paper birch trees already under stress from drought, soil compaction, or root damage. Urban birch trees are particularly vulnerable because of the difficult conditions they face in paved and compacted environments.

Signs of infestation include swelling and ridging under the bark from larval feeding galleries, D-shaped exit holes (similar to but smaller than those of EAB), and dieback that progresses from the top of the canopy downward. Once the upper third of the canopy has died, trees rarely recover regardless of treatment.

Maintaining birch health through adequate irrigation during dry periods and avoiding root zone disturbance are the primary preventive measures. Birch is generally not recommended for planting on hot, south-facing street sites where water stress is predictable.

Recognizing Stress Versus Active Infection

Many symptoms that property owners attribute to pest or disease problems are expressions of root zone stress—compacted soil, insufficient water, physical damage to the root system, or grade changes that buried the root flare. Distinguishing between stress and active infection matters because the management response differs substantially.

A tree dropping leaves early due to drought stress does not require pesticide application. A tree showing wilt caused by a vascular pathogen may require prompt removal to prevent spread to adjacent plants. A certified arborist can conduct a site assessment that considers soil conditions, root zone health, crown condition, and visible symptoms together to reach an accurate diagnosis.

Misidentification of the problem leads to treatments that are ineffective, expensive, and sometimes harmful to the tree or surrounding soil biology.

Reporting and Resources in Canada

The Canadian Food Inspection Agency manages the federal response to invasive forest pests. Canadians who suspect they have found a new infestation of a regulated pest can report it through the CFIA’s online reporting portal or by contacting their provincial natural resources ministry.

Natural Resources Canada’s Canadian Forest Service maintains accessible research on forest pest monitoring and biological control programs. Provincial ministries—including the Ontario Ministry of Natural Resources and Forestry and BC’s Ministry of Forests—publish current pest advisory bulletins that are updated seasonally.

Early detection and prompt reporting remain the most effective tools available to property owners and community members. Regulated pest populations detected early are significantly more manageable than those that establish before monitoring systems flag them.