Last year, we saw a large-scale infestation of various types of defoliating caterpillars throughout the city – mainly in the west end of Toronto, and into Mississauga.

It’s likely that we’ll have a similar problem this year, and we’ve seen evidence that it will occur into the core of the city.

There are three main types of caterpillars to look out for. If you catch the signs early enough, there are treatment options to help control the infestation.



Materialscientist at English Wikipedia [CC BY-SA 3.0 ( or GFDL (], from Wikimedia Commons

Gypsy moth caterpillars will feed on almost all species of trees – from broadleaf to evergreens. It particularly likes Oaks, Basswoods, and Apple trees.

The egg mass is a white to brown fuzzy patch about the size of a toonie and can be found on tree trunks and tree branches. Larva will hatch towards the end of May and being to feed on the foliage of the host tree. The caterpillar has blue and red spots and a hairy appearance to its body.

After about 7 weeks of feeding, the caterpillars will pupate in a brown/black case attached to the trunk or branch of the tree.

It should be noted that a tree can be completely defoliated during the feeding period of the larva.

By Dendroica cerulea (Flickr: Gypsy moth egg sacs) [CC BY 2.0 (], via Wikimedia Commons

How to Control it

Manually Remove Egg Masses

Before the caterpillars hatch (in winter or early spring), you can manually scrape the egg masses off the lower parts of the tree and dispose of them.

BTK Spray Treatment

If the tree is not too large, you can get a BTK treatment applied. This is a bacterial pathogen product that can be sprayed on the canopy of the tree during the feeding period of the larva. This is a time-sensitive treatment that must be done during the early stages of the caterpillar in order to be effective – when the caterpillar attempts to eat the treated foliage, it will quickly die.

BTK specifically targets caterpillars and is not harmful to other animals or insects.

TreeAzin Injection

For larger trees, like mature oaks, you can inject TreeAzin into the tree, which will act as a systemic insecticide that will move into the foliage of the tree where it will kill the caterpillars. Trees should be injected as soon as foliage has emerged. The caterpillars will die when they eat the foliage.

Tree Banding

You can do two rounds of tree banding – which consists of sticky traps being wrapped around the tree. The first round can be done just before the larva period. A burlap belt is installed over the top of the sticky trap in order to create a shaded area. When caterpillars descend the tree during the day to seek shaded shelter, they will crawl under the burlap and become stuck in the trap.

The second round of banding is done later in the season to trap the female moths. The female cannot fly, so when walking up the tree trunk to find a place to lay their eggs, it will become trapped. The trapped female will release a pheromone attracting other male moths to the trap. This breaks the life cycle for the following year.



Image From:

Cankerworm will feed on many species of trees. It particularly likes Oak and Beech.

Flightless female cankerworm moths emerge from the soil in fall. They will crawl up the tree in order to lay its eggs along twigs or bark crevices. These are small and difficult to spot.

Larva will hatch in mid-late spring and begin feeding on the foliage of the host tree. The larva is long and skinny and moves in an inch-worm motion. They can also be transported great distances to other trees by the wind.

After approximately 5 weeks of feeding, the caterpillar will fall to the soil in order to pupate underground.

How to Control it

  • BTK (see above)
  • Tree Banding (see above)
    • Note: For cankerworm, only one round of banding is necessary. You can place the sticky traps around the tree in fall (October). The flightless female moth will emerge from the soil and begin to crawl up the tree in order to lay its eggs. It will become trapped on the sticky band, breaking the life cycle for the following year.


Nematodes are a tiny, worm-like organism that can be applied to the soil of the affected tree. In the summer, it will kill and eat the pupating cankerworm in the soil, breaking the life cycle for the following year.



By Katja Schulz from Washington, D. C., USA (Eastern Tent Caterpillar) [CC BY 2.0 (], via Wikimedia Commons

Eastern Tent Caterpillars will feed on many species of broadleaf trees. It particularly likes to feed on Apple and Cherry trees.

The moth lays its eggs in small shiny masses that encircle the twigs of the tree. They are very small and difficult to spot.

The larva will hatch in spring and begin feeding on the foliage of the host tree. The caterpillars have a distinctive blue spots with yellow stripes. After hatching, the caterpillars will gather at a branch fork and begin to construct their webbing shelter (tents) which will contribute to expand during the larva period. From the tent, they will venture out into the canopy to feed on the foliage. It is common for a host tree to have several tents.

After approximately 5 weeks of feeding, the caterpillar will leave the host tree in search of a sheltering object (something like a fence post) to form a white cocoon and pupate. Moths will then emerge from the cocoons in July to start the whole process again.

By Esc861 [Public domain], from Wikimedia Commons

How to Control It

  • BTK (See above)
  • TreeAzin (See above)

Remove Tents

You can cut or scrape the tents out of the tree in the early stages when the caterpillars are young and congregating together in the tents.



We always try to be proactive with our care and stop these problems before they get out of hand. However, the scale of infestations are often difficult to predict. If your tree suffers defoliation from one of these critters, there are ways to keep your tree healthy and mitigate the stress of defoliation.

Our stress mitigation is all about ensuring that the tree has enough water, oxygen and nutrients to stay healthy and to support new growth after an infestation. We can help with this through biostimulant treatments in the soil, mycorrhizae inoculations, and vertical mulching and soil aeration. 

Your tree needs to have the tools to bounce back to health after an infestation. Through proactive care, you can minimize the long-term damage caused by pests like defoliating caterpillars.

Give your arborist a call if you see signs of caterpillars on your tree.

Be Good to Your Trees.

                         Because Your Trees are Good to You.


Kristie Nairn, BA, MA, MMst
Director of Marketing, Cohen & Master Tree and Shrub Services Ltd.