Insects and Diseases

Trees in urban environments face significant abiotic and climate change-related stresses, which make them particularly vulnerable to insects and diseases (Climate Atlas of Canada, n.d.). As climate change leads to warmer temperatures and drier conditions, the susceptibility of urban forests to these threats is expected to increase [see chapter: Air Quality and Climate Change]. While many native insect species contribute positively to urban biodiversity and ecosystems, some native and non-native insects can cause severe damage to urban forests. They can defoliate trees, suck sap, bore into bark, carry diseases, weaken trees, and lead to tree death and costly management consequences. 

Forest insects and diseases in Canada can be classified into three broad categories: native, alien, and invasive species (NRCan, 2023). Invasive species, whether native or alien, are species that spread beyond their known usual range and that are capable of causing environmental and/or economic damage. The mountain pine beetle, which has extended its range from British Columbia to Alberta, exemplifies a native insect behaving invasively (NRCan, 2024a). Native species outbreaks occur periodically and can be severe (e.g., spruce budworm (Choristoneura fumiferana), mountain pine beetle (Dendroctonus ponderosae)). Alien insects have been introduced into Canada and often become pests, invading new hosts and ecosystems. Notable examples are the emerald ash borer (Agrilus planipennis), which has significantly impacted populations of Ash trees (Fraxinus spp). Beetle species primarily spread Dutch Elm Disease (DED). DED, which includes fungal pathogens (Ophiostoma novo-ulmi, a more aggressive strain, and Ophiostoma ulmi), has already devastated elm trees (Ulmus spp) in Eastern North America and poses a threat to elms across Canada (Government of Saskatchewan, n.d.). 

Pests and diseases cause economic and ecological damage and affect the social fabric of communities by altering landscapes and reducing the aesthetic value of urban areas. Effective management and mitigation strategies, including monitoring, cultural practices, biological and chemical controls, public education, and promoting diversity, are essential to protect these valuable urban forests and ensure their continued contribution to the health and well-being of Canadian cities.

Insects

Historically, Canadian cities have faced significant challenges from various insects and diseases that have had profound economic and environmental impacts. The Emerald Ash Borer (EAB), first detected in Canada in 2002, is one of the most destructive pests in North America. It has killed millions of ash trees across Ontario, Quebec, and other regions, causing substantial economic losses (NRCan, 2024b) [see chapter: Economic Value and Appraisal of Trees]. For example, Windsor, Ontario had to remove over 10,000 ash trees, drastically altering the urban landscape and increasing municipal management costs (Arnberger et al., 2017). The City of Montreal has also faced severe impacts, with large-scale tree removals disrupting recreational areas and incurring high costs for tree treatment, replacement, and removal, highlighting broader social and economic impacts of pest infestations (Ville de Montreal, 2023). The economic impact of the EAB alone is expected to cost Canadian municipalities $2 billion in treatment, tree replacement, and removal over the coming decades (NRCan, 2018; Vogt, Hauer & Fischer, 2015).

The Asian Longhorned Beetle (ALB) (Anoplophora glabripennis), detected in Toronto in 2003, poses a significant threat to hardwood trees in urban areas (Haak et al., 2009). It kills trees by boring into their trunks and branches, leading to structural failure. Efforts to eradicate the beetle in Toronto have involved the removal of over 25,000 trees, affecting local biodiversity and the aesthetic value of green spaces (Wilson and Smith, 2017). These ALB populations have been successfully eradicated (NRCan, 2024a).

The Spongy Moth (Lymantria dispar) is another pest that defoliates various tree species, weakening them and making them susceptible to other stresses. In 2020, southern Ontario experienced one of the largest spongy moth outbreaks, leading to significant defoliation and stress on Oak (Quercus spp), Maple (Acer spp), and other tree species (Invasive Species Centre, 2024b). The Woolly Adelgid (Adelges tsugae), an invasive insect, threatens forests in eastern Canada and remnants of native forests in urban areas. It feeds on hemlock trees (Tsuga canadeanis ), causing them to decline and eventually die, which could have cascading effects on forest ecosystems (Dreistadt, Dahlsten & Frankie, 1990).

Diseases

The Dutch Elm Disease (DED), introduced to Canada in the mid-20th century, devastated elm populations across the east and continues to spread across the country (Government of Saskatchewan, n.d.). Winnipeg, known as the “City of Elms,” has lost thousands of elms to this fungal disease. The loss of these trees has not only reduced urban canopy cover but has also affected the aesthetic and cultural value of the city (Hildahl, 1977). Chestnut Blight, caused by Cryphonectria parasitica, a fungus introduced from Asia, has eradicated native chestnut trees (genus Castanea) throughout the Carolinian zone. This has reduced biodiversity, changed forest communities, and impacted wildlife that once relied on chestnuts as a food source. Tree diseases also impact the Canadian economy, particularly in regions where these species are abundant (Invasive Species Centre, 2024a).

Although not yet widespread in Canada, Sudden Oak Death (SOD), caused by the invasive pathogen water mold (Phytophthora ramorum), poses a significant threat to Canadian urban forests. The potential introduction of SOD could lead to widespread oak tree mortality, significantly altering the landscape and biodiversity of urban forests in Canadian cities (Braddy, 2023). Another emerging threat recently detected in Ontario, Oak Wilt, is a fungal disease caused by Ceratocystic fagacearum that blocks the water-conducting vessels of oak trees, causing them to wilt and die. These diseases have the potential to cause widespread oak tree mortality, highlighting the need for thorough monitoring and rapid response strategies (Forest Pathology, 2024).

Management and Mitigation Strategies

The decline of native tree species across Canada due to pests and diseases has had many negative impacts and significant impacts on forests, biodiversity, ecological functions, and the Canadian economy. The decline of species populations like the American chestnut, black ash (Fraxainus nigra), and hemlock contributes to the loss of genetic diversity of urban forests, making them more vulnerable to future threats. Managing the impact of invasive species is costly ecologically and economically. Ecological cost is the loss of species, biodiversity, habitat, and ecological and cultural functions, while the economic cost of dealing with the impact, managing the impacts, and loss of ecological services is also significant (Crystal-Ornelas et al., 2021). For example, municipalities and conservation groups in Ontario spend approximately $50.8 million annually on managing invasive species (Invasive Species Centre, 2023). The cost of dealing with EAB’s impact and management in urban streets could reach around $1.38 billion by 2035 (Hope et al., 2020). 

The management and mitigation of insect and disease infestations in Canadian urban forests require a comprehensive approach that includes monitoring, cultural practices, biological and chemical controls, public education, and promoting diversity (Hotte et al., 2015). Monitoring and Early Detection is one method that employs regular inspections and the use of traps, which are crucial for the early detection of pests and rapid intervention. This approach has been effective in managing pests like the ALB and EAB in Toronto and Montreal (NRCan, 2018; Ville to Montreal, 2023), as early detection and eradication stopped the spread of the invasive insect. The use of natural predators (biological control) and the judicious application of insecticides (chemical control) can also help manage pest populations. For example, Montreal’s response to the EAB has included a combination of tree removal, biological controls, and public education to limit the spread (Les Amis de la Montagne, 2022). 

Urban forest management plans for Canadian cities have recognized invasive species issues, preventing and managing their impact. For example, the City of Calgary’s strategic management plan incorporates several invasive species strategies: early detection and response, research and development of new eradication strategies, inventorying, prevention, community outreach, and integrated weed management that uses biological, chemical and cultural control methods to eliminate or prevent priority invasive species (City of Calgary, 2020). Additionally, the plan recognizes that maintaining tree health through proper watering, mulching, pruning, and other cultural practices increases tree resilience against pests and diseases [see chapter: Tree Maintenance].

Removing and properly disposing of infected or infested trees and plant materials are vital to preventing the spread of pests and diseases. Federal quarantines and local initiatives, such as the “ash-free” zone in southwestern Ontario, have been slowing the spread of pests like the EAB (NRCan, 2018; MacFarlane and Meyer, 2005). Raising awareness about urban forest health and involving the community in monitoring efforts are also essential for effective pest and disease management. Public education campaigns in cities like Montreal and Toronto have been critical in controlling the spread of pests like the EAB and ALB (Les Amis de la Montagne, 2018). The Ontario Ministry of Agriculture, Food and Rural Affairs also emphasizes integrated pest management practices that focus on prevention and cultural methods (OMAFRA, 2012). Finally, promoting species and genetic diversity in urban forests reduces the risk of widespread damage from any single pest or disease. This strategy is important for mitigating the effects of invasive species like the spongy moth and sudden oak death, but also for other tree species and urban forest management in general (Braddy, 2023).