• Invasive – native to Europe.
  • This European defoliator feeds on a variety of trees.
  • The first detection of spongy moth in Ontario occurred in 1969; however, widespread defoliation did not occur until 1981.
  • Outbreaks of this pest are cyclical, typically occurring about every seven to ten years. In Ontario, major outbreaks have peaked in 1985, 1991, 2002 and 2008. The most recent outbreak that peaked in 2021 was the largest on record in Ontario.

“Invasive” refers to a species that has moved outside of its native habitat and threatens the new environment, economy or society by disrupting local ecosystems.

"Defoliator" refers to species that eat leaves.

Host species

Hosts range from oak (Quercus), birch (Betula) and aspen (Populus) in the north, to various hardwoods such as sugar maple (Acer saccharum), American beech (Fagus grandifolia) and softwoods such as eastern white pine (Pinus strobus), balsam fir (Abies balsamea) and Colorado blue spruce (Picea pungens) in Southern Ontario.

Characteristics and life cycle

  • Overwinters in the egg stage often on the bark of trees.
  • In spring, eggs hatch and larvae ascend the trees to feed on the new foliage.
  • Initially, feeding occurs during the day, but as the caterpillars mature feeding occurs mainly at night — often this can delay the detection of infestations.
  • Mature larvae are 50 mm long, dark coloured, hairy, with a double row of five pairs of blue spots, followed by a double row of six pairs of red spots, down the back.
  • Feeding is completed in July.
  • Male moths are light brown and slender-bodied, while females are white, heavy-bodied and flightless.

Two dark, hairy caterpillars with blue spots at the front and red spots at the back eating a leaf.

Gypsy moth adults laying eggs

Symptoms and damage

  • Larvae chew holes in leaves or devour entire leaves.
  • In late July, spongy moth egg masses can be observed on the trunks and branches of infected trees.
  • Understory shrubs and plants may also be affected.
  • During severe outbreaks, trees and shrubs are completely defoliated over large areas; despite the trees’ ability to produce a new crop of leaves over the summer, the damage causes substantial growth loss.
  • Defoliation makes trees more susceptible to secondary pests, drought, and poor growing conditions.

Gypsy moth defoliation

Control measures

You can take a localized approach to manage spongy moths on your property. Control options depend on the spongy moth life stage and the time of year.

Learn how to protect trees on your property.

Spongy moth life stage and control options


Image of the spongy moth life stage and control options
TimingLife stageControl options
August to mid-AprilEgg massesRemove egg masses and discard
Mid-April to mid-MayEarly stage caterpillarApply biological pesticide
Mid-May to JuneLate stage caterpillarAttach burlap bands and discard larvae
June to mid-JulyPupaeRemove by hand and discard
July to AugustAdult mothShort-lived; focus on other stages

Natural controls

Natural predators and pathogens are the main reason the spongy moth outbreak in Ontario is collapsing.

Nuclear polyhedrosis virus (NPV)

  • NPV is a viral infection known to kill spongy moth larvae once the virus builds up in a population.
  • It can spread quickly from infected larvae to non-infected larvae, killing them.
  • Dead larvae hanging on a tree in an upside-down “V” show the virus is at work in the population.

Entomophaga maimaiga

  • A fungus known to cause spongy moth populations to collapse.
  • Cool, wet conditions provide an ideal environment for its spread.
  • Dead larvae hanging vertically along tree trunks that appear brittle and desiccated show that this fungus is active.

Predation and parasitism

  • Birds, mammals and other insects are known to prey on spongy moth.
  • A species of wasp called Ooencyrtus kuvanae is known to parasitize spongy moth eggs and can reduce populations.

Cold weather

  • Extended days of extreme cold (−20 degrees Celsius) may kill overwintering larvae in exposed egg masses.

Regulation and monitoring

  • Spongy moth is a regulated pest by the Canadian Food Inspection Agency (CFIA). The CFIA is responsible for establishing and maintaining standards to prevent the introduction and spread of plant pests in Canada.
  • Forest health is monitored every year by the ministry. We conduct ground and aerial surveys to map major forest health disturbances on the landscape. When pest populations reach outbreak level, we may complete pest specific forecast surveys to help predict defoliation for future years.

State of spongy moth in 2023

We began aerial forest health surveys in early July. We based flight lines on:

  • public reports of potentially infested areas
  • survey data from previous years
  • knowledge of previously infested areas

After the aerial surveys, we did ground verification surveys. This included collecting spongy moth specimen samples to confirm species identification in the laboratory.

Areas affected by spongy moth defoliation in 2023

Defoliation caused by spongy moth in Ontario decreased to 2,529 hectares in 2023, down from 58,031 hectares in 2022. This area of moderate to severe defoliation occurred in one district in the Southern Region.

Visit ministry work centres to see a map of our districts.

Defoliation caused by spongy moth was most severe in forest stands containing species of:

  • oak
  • maple
  • poplar
  • willow
  • other broadleaf trees

Southern region

In 2023, the extent of defoliation decreased in all districts in the Southern Region.

Small areas totalling 2,529 hectares of moderate to severe defoliation were recorded in the Aylmer Guelph district, down from 20,215 hectares in 2022.

Map of areas defoliated by spongy moth in 2023 across Ontario

Map of part of Southern Ontario, showing spongy moth defoliation areas scattered between Niagara and west of London north of Lake Erie.
Enlarge Map

Spongy moth populations in 2024

Aerial surveys and reported observations in 2023 showed that widespread spongy moth defoliation is not likely in 2024. Because of this, we decided that egg mass surveys to forecast spongy moth populations in 2024 are not needed.

Although the spongy moth is considered an invasive species regulated by federal quarantine legislation, it has evolved to a state of naturalization. This means the spongy moth population may have periodic outbreaks, which is what occurred in 2020 and 2021. As part of the quarantine legislation, you are not allowed to move firewood from regulated areas where spongy moth has been found.

Forecasting defoliation: Egg mass survey protocol

To forecast spongy moth defoliation, we use a modified Kaladar plot (MKP) protocol at select locations across the defoliated area. This protocol considers:

  • egg mass location in the stand
  • the proportion of new egg masses relative to old ones

The total number of egg masses per hectare is used to forecast spongy moth defoliation for the following year:

  • severe defoliation (more than 75% of forest stand) is projected in areas with more than 6,175 egg masses per hectare
  • moderate defoliation (40% to 75% of forest stand) is projected in areas with 1,236 to 6,175 egg masses per hectare
  • light defoliation (1% to 40% of forest stand) is projected in areas with 1 to 1,235 egg masses per hectare
  • nil defoliation (0% of forest stand) is projected in areas with 0 egg masses per hectare

Complete an egg mass survey

To complete your own egg mass survey on your property, you can request the modified Kaladar plot (MKP) protocols from us by emailing info.mnrfscience@ontario.ca.

Contact us

Visit our forest health conditions page to read our annual report summaries.

For more information, email us at info.mnrfscience@ontario.ca.