Spongy Moth Activity in Moore Park
Many residents have recently expressed concern about caterpillars feeding on oak trees throughout Moore Park. We reached out to the City of Toronto’s Urban Forestry Division for an update on the current situation and the City’s management approach.
According to Urban Forestry, Moore Park is experiencing localized, moderate spongy moth activity, but current population levels remain below the threshold for aerial spraying. The City has been monitoring the neighbourhood, manually removing egg masses, and treating selected City-owned trees while continuing to assess conditions throughout the season.
The response below, provided by Urban Forestry, explains how the City determines when intervention is necessary, why aerial spraying is considered a last resort, and how natural predators play an important role in controlling future spongy moth populations.
City of Toronto Response
Good Morning Chris,
Councillor Saxe office asked that I connect with you regarding your concern with caterpillars stripping oak trees.
Urban Forestry has a unit dedicated to monitoring and managing pest and disease threats to the urban forest. Spongy moth has been present in Toronto and southern Ontario for decades, with the last major infestation in Toronto occurring in 2019. Staff survey known “hot spots” every year to look for signs of defoliation and then return to these areas (and others where feeding has been reported through the on-line tool) through the summer and fall to conduct egg mass counts. The surveys focus on City owned trees, but staff may note egg masses on private trees where visible (and again we take into account reports from residents via on-line reporting). Thank you for taking the time to complete the survey as these help inform our decision for the following year with respect to treatment.
As noted earlier, the last outbreak was in 2019 and since then our surveys had not indicated a buildup of spongy moth until 2025. Last summer we did note the presence of egg masses on Oak trees in the Moore Park area, but the population was moderate and localized and did not meet the threshold at which we would consider an aerial spray. The accepted technical threshold is 1,250 egg masses per hectare – where the hectare is defined as an area with a high concentration of mature oak trees. In urban environments, factors such as tree size, location, stand density and degree of canopy cover are taken into consideration. In response to what we observed, staff did manually remove egg masses in the winter and inject 43 City trees in the fall specifically in the Moore Park area in addition to similar work in other areas.
Urban Forestry uses an Integrated Pest management approach employing a combination of manual egg mass removal, injections and the application of pesticides as a last resort. The objectives of our management are:
- reduce spongy moth populations to minimize severe defoliation and mortality of vulnerable trees, particularly high-risk oak species
- minimize pesticide impact on non-target insect species by focusing on oak trees and using selective, low-impact treatments for resistant and immune tree species
- focus aerial spraying and timing for maximum treatment efficiency and effectiveness
- Allow Nucleopolyhedrosis virus (NPV), Entomophaga maimaiga fungus and parasitic insect populations to build in order to ensure large scale spongy moth populations to collapse
As noted in the last bullet above, there are predators in the environment that feed on spongy moth larvae. Allowing the populations of these predators to increase is critical to the long- term management of spongy moth (staff are currently seeing dead caterpillars due to infection by predation). Conducting a spray program when spongy moth counts are low to moderate and localized, negatively impacts the predator population by reducing the size of the host population. Eliminating natural predators through aerial spraying removes them as a control for Spongy Moth and allows the Spongy Moth population to rebound much faster – which in turn necessitates more spraying. For these reasons, Best Management Practices recommend avoiding an aerial spray until the egg mass counts exceed the threshold level, with widespread populations and strong evidence of natural predation.
A further point to note is that an aerial spray involves the use of a registered pesticide product. While spray areas focus on local oak dominated areas, it is inevitable that resident properties adjacent will be subject to receiving non-target application of pesticide – on homes, cars, trees, pools and play structures. While we use pesticides as a last resort, there are still many people who object to their use for a variety of reasons and their viewpoints are taken into consideration in our management response.
Staff from Urban Forestry consult with forestry staff in other municipalities and higher levels of government as well as the Invasive Species Centre and academic institutions to stay current with respect to research and best management practices for forest pests and diseases. We try to convey that information to the public through social media, the Urban Forestry webpage, Councillor notifications, and other means. I can assure you that we are monitoring the situation and using the best information available to manage this forest threat to the long-term benefit of the urban forest natural environment and residents. As we move through 2026 and into 2027, we will continue to monitor and update Councillors.
Kind Regards,
Quinn Dunn (She/They)
Policy and Program Advisor
Environment, Climate and Forestry Division
City of Toronto