Tree pests and diseases

The City continually monitors and responds to forest health threats such as pests and diseases. Tree planting and maintenance take species vulnerability into consideration to maximize forest resilience. The pests and diseases listed below are actively managed or monitored.

Emerald ash borer

The emerald ash borer (EAB) is a highly destructive, non-native, wood-boring beetle that affects all species of ash (Fraxinus spp.) trees, ultimately killing them. The City has monitored its presence since 2002. Now established, EAB is expected to destroy all of Guelph’s untreated ash trees.


The City is not actively monitoring EAB populations. However, we continue to inspect and attend to ash trees that could impact our rights-of-way, property and public safety.

If you have an ash tree on your property, please have it inspected by a qualified tree care professional, and consider treating or removing the tree.

Signs of emerald ash borer can include loss of leaves and dead branches in upper part of the tree, vertical cracks in the tree trunk, S-shaped tunnels under the bark (filled with sawdust), and small D-shaped holes.

Treating emerald ash borer

TreeAzin® is recommended by the Ontario Ministry of Natural Resources and the Canadian Forest Service as the best option for treating against emerald ash borer.

Treatment must begin before the tree is heavily infested. TreeAzin® must be injected by a licensed pesticide applicator using a special device, and injections are recommended every two years to keep the tree healthy.

Removing trees infested with emerald ash borer

Trees affected by emerald ash borer will eventually die, and should be removed before they become a safety risk. As the property owner, you are responsible for removing and disposing of dead, hazardous limbs and trees. To have infested limbs or trees removed safely, contact a qualified tree care professional.


Recovery from the negative impacts of EAB means planting of a diversity of new trees when ash trees are removed from streets parks, and forested areas such as Mitchell Wood and Marksam Park. The work in forested areas also includes control and removal of invasive buckthorn.

Hemlock Woolly Adelgid

Hemlock Woolly Adelgid (HWA, Adelges tsugae) is an invasive species of aphid-like insect that attacks and kills hemlock trees, first detected in Ontario in 2012.


Guelph has a small hemlock population and therefore a low risk of impact to the overall tree canopy cover due to HWA. However, the City will monitor local hemlock populations and reports of detections and take appropriate action to prevent negative impacts to hemlock trees in Guelph.

Oak wilt

The first known case of oak wilt (Bretziella fagacearum), a non-native fungus, has recently been detected in southern Ontario. Oak wilt impacts all species of oak, especially red oak. It is a fungal pathogen which restricts the flow of water and nutrients leading to defoliation. The disease can progress rapidly and cause trees to die within a year.

Oak wilt is spread by sap beetles that carry the oak wilt fungal spores on their bodies from infected trees to healthy ones as they move around in search of food.

Signs and symptoms can include

  • leaves with brown discoloration starting along the leaf edges that moves inward towards the middle margin;
  • early leaf drop, wilting, and defoliation starting at top of tree; and
  • vertical cracks in the bark, with underlying sweet, scented fungus.


Guelph has a small oak population and therefore low risk of impact to the overall tree canopy cover due to oak wilt. However, the City will monitor local oak populations and reports of detections and take appropriate action to prevent negative impacts to oak trees in Guelph.

The City will only prune oaks September through March, unless exceptional circumstances such as potential safety hazards occur.

You can help protect oak trees by

  • checking your oak trees for oak wilt and contacting a qualified tree care professional for help,
  • avoiding pruning oak trees from April to October and
  • not moving firewood.

Spongy moth

Spongy moth (Lymantria dispar dispar) is an invasive species introduced to Canada and the United States in 1869.

The larvae feed on the leaves of over 300 tree species, threatening to deforest urban tree canopies if left unaddressed. Spongy moth larvae are commonly identified by the red and blue dots along its back. Young caterpillars are black or brown and the size of a thumbnail.


Active monitoring for spongy moth is no longer necessary in Guelph. Populations have significantly decreased due to natural controls in our environment. Little to no defoliation is expected in Guelph at this time.

What to do with spongy moths on your trees

If you do still find spongy moth on your trees, there are several best management practices you can use to defend your private trees against spongy moth to prevent loss of leaves and tree death

  • install sticky bands or burlap around tree trunks
  • hand pick larvae
  • spray with BTK products (a naturally occurring bacteria found in hardware stores).
  • install moth traps
  • scrape egg masses from trees and other surfaces before the end of April
  • hire a licensed exterminator to treat trees with insecticide, like TreeAzin©, used to control Emerald Ash Borer

Spotted lanternfly

The spotted lanternfly (SLF, Lycorma delicatula) is an invasive insect native to China recently detected in Southern Ontario. The Tree of Heaven (Ailanthus altissima) is the preferred host plant by the adult SLF, although over 100 potential host native hardwood and fruit trees have been identified.

The risk of SLF to the city’s canopy cover is moderate due to the extensive list of species that could be affected. The City will monitor local tree Tree of Heaven populations and reports of detections and take appropriate action to prevent negative impacts to trees in Guelph.


Termites are not typically a threat to living trees. However, they feed on dead wood which can damage and weaken live trees as well as spread to your home. Guelph’s termite management program helps to control the local termite population and prevent damage to wood structures. Since 2007, the program has proven effective in reducing termite populations.

Other tree pests and diseases

Please contact us if you suspect a tree pest, disease or invasive plant is on City property. Residents should also report all possible detections of invasive pests, plants and diseases to the Canadian Food Inspection Agency immediately.

In addition to those listed above, the City actively monitors for reports and signs of other native and invasive insects and diseases that could threaten our urban forest.

If we suspect a tree pest or disease is present on City property, we conduct a tree inspection. If an invasive pest or disease is confirmed, we report the matter to the Canadian Food Inspection Agency.

In cases when the health of a tree is threatened, we take appropriate action.

Most tree pests and diseases like tent caterpillars, cankerworms, aphids or box elder bugs, are native, non-invasive and do not significantly impact tree health or structure.

Removing trees from your property

If your property is larger than 0.2 hectares (0.5 acres), please learn more about Guelph’s Tree Bylaws and permits before removing any trees measuring at least 10 centimetres in diameter at 1.4 metres above the ground. If the trees are infested with emerald ash borer, you may be eligible for an exemption.

Finding a tree care professional

For more information

[email protected]