• 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 2022

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 2022

Defoliation caused by spongy moth in Ontario decreased to 58,031 hectares in 2022, down from almost 1.8 million hectares in 2021. This area included both light and moderate to severe defoliation in the Southern Region. Most defoliation occurred in Southwestern Ontario in the Aylmer and Guelph districts.

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 2022, the extent and severity of defoliation decreased in all districts in the Southern Region.

Most of the moderate to severe defoliation was recorded in the southwestern districts including Aylmer and Guelph:

  • Aylmer District had the largest area of moderate to severe defoliation of 17,057 hectares, a decrease from 120,487 hectares in 2021
  • Guelph District had areas of moderate to severe defoliation of 3,158 hectares, a decrease from 112,978 hectares in 2021

Large areas of light defoliation were recorded in southwestern districts including Aylmer, Guelph and Midhurst.

In Southeastern Ontario, small, localized areas of moderate to severe defoliation were recorded in the following districts, which all had less defoliation than in 2021:

  • Peterborough District defoliation totalled 942 hectares
  • Kemptville District defoliation totalled 685 hectares
  • Pembroke District defoliation totalled 452 hectares

Map of areas defoliated by spongy moth in 2022 across Ontario

Map of Northeast and Southern Ontario, showing spongy moth defoliation areas mostly in southwestern Ontario.
Enlarge Map

Spongy moth populations in 2023

In 2022, we conducted surveys to forecast anticipated spongy moth populations in 2023. The forecast is based on fall egg mass density, which is the number of egg masses on trees in a given area. We use this data to help forecast defoliation. Survey locations were selected to:

  • support operational surveys in 2023
  • confirm the state of the current outbreak

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.

2022 egg mass survey results

Based on egg mass surveys conducted in late fall in all districts across the Southern Region, defoliation in 2023 is forecast to be:

  • severe in areas of Aylmer, Guelph, Parry Sound and Bancroft districts
  • moderate in areas of Kemptville, Bancroft, Aurora and Aylmer districts
  • light in areas of Midhurst, Bancroft and Parry Sound districts
  • nil (none) in areas of Pembroke and Bancroft districts

During the survey, we observed egg mass predation (attacks) by birds and small mammals. At some egg mass sample sites, we also found evidence of parasitism. For example, small pinholes in egg masses indicated the presence of the tiny parasitic wasp, Ooencyrtus kuvanae.

These predators and parasites can help to reduce spongy moth populations, but we won’t know their effects until the spongy moth larvae emerge in the spring.

Map of projected 2023 spongy moth defoliation

This map shows:

  • spongy moth damage in 2022, represented by shaded areas
  • the defoliation forecast for 2023 (represented by dots, which are egg mass survey locations)
Map of southern Ontario showing distribution of projected 2023 spongy moth defoliation
Enlarge Map

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.