Tree pests

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.

Ash trees on your property

If you have an ash tree on your property, please have it inspected by a certified arborist, and consider treating or removing the tree. Visit Ontario Trees for help identifying ash tree species.

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 required every two years to keep the tree healthy.

Removing trees infested with emerald ash borer

Trees affected by emerald ash borer will eventually die, but you don’t have to remove them until 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 certified arborist or forester.

We encourage you to replace any tree you remove with a native tree species.

If your property is property 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 tree(s) are infested with emerald ash borer, you may be eligible for an exemption.

Signs of emerald ash borer

Signs of emerald ash borer usually become obvious when a tree is heavily infested:

  • Loss of leaves and dead branches in upper part of the tree,
  • Yellowing leaves,
  • Adult beetles feeding on leaves,
  • Unusually high woodpecker activity,
  • Long shoots growing from the tree trunk,
  • Vertical cracks in the tree trunk,
  • S-shaped tunnels under the bark (filled with sawdust), and
  • Small D-shaped holes

Not all ash trees that are losing their leaves are a result of EAB. Some instances may be a matter of general ash decline.

Ash trees on City property

Knowing about 10,000 City-owned trees will eventually die, Guelph’s plan to manage EAB focuses on reducing the risk of personal injury and property damage, minimizing damage to our urban forest, and preparing for recovery.

The principles of Guelph’s emerald ash borer management plan are to:

  • Ensure safety of people and property.
  • Help people understand options for treating or removing ash trees on their properties.
  • Treat as many City-owned ash trees as feasible; only invest in treatment for the very best candidates.
  • Leave affected City-owned ash trees standing as long as they are safe to stand, to maintain environmental benefits for as long as possible.
  • Prepare for recovery before ash-tree decline, if possible.
  • Protect natural areas from ecological degradation caused by EAB infestation.


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.

Spongy Moth (Lymantria dispar dispar)

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

Spongy moth was previously known as “gypsy moth” (and more recently as LDD moth). The previous common name was changed by the Entomological Society of Canada in 2022, due to its derogatory slur against Romani people.

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


We’re monitoring trees in our parks and natural areas for spongy moth moth egg masses.

An egg mass survey was completed in November 2021, to follow up the survey completed in January 2021. The survey determined the loss in leaves to oak and Norway maple trees in spring/summer 2022 won’t cause significant harm to Guelph’s canopy.

With fairly low numbers of egg masses across Guelph, we will not be using aerial spray programs or pesticides to manage caterpillars. A smaller, more targeted approach to managing these moths in Guelph will help prevent defoliation in some areas of the city in 2022.

What to do with spongy moths on your trees

There are a number of 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.

For more information on egg masses in your neighbourhood refer to the Defoliation Risk map.

Other tree pests and diseases

Please contact us if you suspect a tree pest or disease on City property.

In addition to Guelph’s emerald ash borer management plan, the City actively monitors for signs of other 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 would take appropriate action.

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

For more information

[email protected]
519-822-1260 extension 5626