Gypsy Moth Infestation

Left: Light colored Egg Masses on tree trunk. (Brown shapes are last year's pupae) Right: Removal technique.

April 16, 2021

Gypsy moth egg masses have been found on trees in our neighbourhood.

When sightings were reported in Ottawa last year, we remained hopeful that these highly destructive and invasive pests—which pose a significant threat to trees—would be slow to spread to our area.

The buff-coloured egg masses are easily identified on tree trunks.  Each mass contains hundreds of eggs, which if left undisturbed can develop into hundreds of black caterpillars that crawl up a tree to feed in the leafy canopy at night, before making their way back down again to hide on the tree trunk during the day.  Trees can be partially or fully defoliated as a result, creating significant stress which increases their risk of injury or death. Evergreen trees which are completely defoliated rarely survive.

The late stage, larger caterpillars can strip a tree bare in a matter of days.  The current outbreak which has now reached Ottawa involves "numbers not seen in 30 years, stripping leaves from sugar maples, oaks and evergreens so quickly you can almost watch the forest canopy disappear" according to this Ottawa Citizen article of July 2020.

The caterpillars reportedly prefer oak trees, sugar maples and evergreens including balsam, hemlock, white pine and spruce — but will also eat apple, beech, birch, black locust, butternut, cherry and elm.  When hanging from branches, the caterpillars can spread by wind, thereby infesting a wider area, including the trees next door, before finally maturing into adult moths. 

What to Look For

Egg masses are buff-coloured or light tan in colour, and are generally found on tree trunks (especially in bark crevices) anywhere from ground level upward, and on the underside of tree branches.  The masses can appear singly or in clusters, and can also be found on buildings and outdoor furniture. (Check beneath window sills, soffits, decks, wheelbarrows, and around garden sheds.) Egg masses can also be found on firewood and on vertical stone surfaces.

What to Do if Found

Timely removal of egg masses before they hatch is the best defense:

  1. Find a plastic container for collecting the egg masses. (Empty yogurt or margarine containers can work well.)
  2. Find a dull scraping tool, such as palette or putty knife. Even a dull kitchen knife will do.
  3. Find gloves to protect your hands (such as work gloves or nitrile disposables).  
  4. Hold the empty container tightly against the tree surface, beneath the egg mass.
  5. Insert the knife at the top edge of the mass. Gently work the knife to lift the mass away from the tree and into the container below. 
  6. The mass may break off in one piece, or break into "crumbs". Collect as much of the mass as possible; even "small crumbs" may develop into caterpillars.
  7. To destroy the eggs masses, place them in a container with a mixture of dish-washing soap and water, and leave covered for 2 days or more to kill the eggs. 
  8. Then filter the residue and dispose of it in the green bin. (Do not place in garden compost.)

If collecting egg masses from flatter surfaces, a flat plastic lid with a straight edge cut across, or other flat surface may be more effective for initial collection before depositing the egg masses in your container.

CLICK HERE for a YouTube video demonstration posted by the City of London, Ontario.

How to Remove Caterpillars:

  1. Wrap a 2 ft strip of burlap around a tree trunk at chest height. Hold it in place temporarily with tape. 
  2. Tie a string or twine around the middle of the burlap and fold the top portion down to form a skirt. When the caterpillars move down the tree seeking shade in the heat of day, or move up the tree at night to feed, the skirt will trap many caterpillars as they climb.  
  3. Find gloves to protect your hands (work gloves or nitrile disposables). The long hairs on the caterpillars contain histamines that can cause allergic reactions.
  4. Carefully collect the caterpillars every day and put them into a soap and water solution.
  5. Leave them for two days or more until they are dead.  
  6. Filter the residue (or decant the water) and dispose of it in the green bin. Do not put it into garden compost.

CLICK HERE for a YouTube video demonstration posted by the City of Toronto.


Visit the following websites for more information on gypsy moth control:

Ontario's Invading Species Awareness Program | Gypsy Moth

Ontario Nature | Gypsy Moth Invasion

Province of Ontario | Gypsy Moth

City of Ottawa | Gypsy Moths


Related Reading:

Gypsy Moth Infestation Another 'Cataclysmic Insult' to Eastern Ontario Forests | Ottawa Citizen, May 12, 2021





News Archive