• Invasive – native to Europe.
  • This European defoliator feeds on a wide variety of trees.
  • The first detection of gypsy moth in Ontario occurred in 1969; however, widespread defoliation did not occur until 1981.
  • Established populations exist south of a line from Sault Ste. Marie east to North Bay and Mattawa; a separate infestation exists in New Liskeard.
  • The Ontario distribution coincides with the range of the insect’s preferred hosts of oak; however, no known populations of gypsy moth exist in northern parts of the range of bur oak north of New Liskeard in the northeast region, and west of Thunder Bay to Lake of the Woods in the northwest region.

“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 eats 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) 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 blue spots, followed by a double row of six pairs red spots, down the back.
  • Feeding is completed in July.
  • Male moths are light brown and slender-bodied, while females are white and heavy-bodied.

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

  • Gypsy moth outbreaks occur every 7 to 10 years.
  • Larvae chew holes in leaves or devour entire leaves.
  • In late July, spongy 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 significant 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 gypsy moths on your own property. Each control option will depend on the gypsy moth life stage and the time of year.

Learn how to protect trees on your property.

Gypsy moth life stage and control options

image of the gypsy moth life stage and control options

Timing Life stage Control options
August to mid-April Egg masses Remove egg masses and discard
Mid-April to mid-May Early stage caterpillar Apply biological pesticide
Mid-May to June Late stage caterpillar Attach burlap bands and discard larvae
June to mid-July Pupae Remove by hand and discard
July to August Adult moth Short-lived; focus on other stages

Natural controls

Natural predators and pathogens are the main reason the gypsy moth outbreak in North America is collapsing.

Nuclear polyhedrosis virus (NPV)

  • NPV is a viral infection known to kill gypsy 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” is a sign that the virus is at work in the population.

Entomophaga maimaiga

  • A fungus known to cause gypsy moth populations to collapse.
  • Cool, wet conditions provide an ideal environment for the spread of this fungus.
  • Dead larvae hanging vertically along tree trunks that appear brittle and desiccated is a sign that this fungus has killed gypsy moth larvae.

Predation and parasitism

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

Cold weather

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

Regulation and monitoring

  • Gypsy 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 of Natural Resources and Forestry. 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 gypsy moth in 2020

We began aerial forest health surveys in early July after a delay due to COVID-19 restrictions. We based flight lines on:

  • public reports of potentially infested areas
  • 2019 survey data
  • knowledge of previously infested areas

We did ground verification surveys after the aerial surveys. This included collecting gypsy moth specimen samples to confirm species identification in the laboratory.

Areas affected by gypsy moth defoliation in 2020

Defoliation caused by gypsy moth in Ontario increased from 47,203 hectares in 2019 to 586,385 hectares in 2020.

This area included both light and moderate to severe defoliation mapped in the southern region (561,469 hectares) and 24,916 hectares mapped in the northeast region.

All affected districts reported an increase in area defoliated from 2019. Visit Ministry of Natural Resources and Forestry regional and district offices to see a map of our districts.

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

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

In some areas, gypsy moth defoliated conifer species including eastern white pine.

Southern region

In the southern region, we recorded most of the moderate to severe defoliation in the eastern districts including Peterborough and Bancroft.

The Peterborough District had the largest area of moderate to severe defoliation of 159,578 hectares in 2020, an increase from 409 hectares in 2019.

This concentrated area of defoliation extended to Kemptville District and north to Pembroke District, where we mapped smaller areas of defoliation.

Southwestern Ontario

In southwestern Ontario, areas of moderate to severe defoliation were more fragmented. The area affected totaled 99,387 hectares in the Guelph and Aylmer districts, an increase from 37,551 hectares in 2019.

We recorded defoliation in the Midhurst and Aurora districts (57,356 hectares) and extended to the Parry Sound District, where we mapped smaller areas (2,046 hectares).

Northeast region

In the northeast region, the Sudbury District had the largest area of moderate to severe defoliation of 24,262 ha.

In the North Bay and Sault Ste. Marie districts, we recorded smaller areas of moderate to severe defoliation, but in both cases the total area was higher than in 2019.

Map of areas defoliated by gypsy moth in 2020 across Ontario

Map of northeast and southern Ontario, showing gypsy moth defoliation areas northwest of Sudbury, along the north shore of Lake Huron, and across much of southern Ontario.
Download PDF

Gypsy moth populations in 2021

In 2020 we conducted surveys to forecast anticipated gypsy moth populations in 2021 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 predict defoliation.

Other factors can also contribute to unforeseen gypsy moth population collapse, including:

  • climate
  • fungus
  • viruses
  • parasites

We will not see the effects of these factors until spring 2021, when gypsy moth larvae emerge.

Although the gypsy moth is still considered an invasive species that federal quarantine legislation regulates, it has evolved to a state of naturalization. This means the gypsy moth population may have periodic predictable outbreaks, which is what we saw in 2020.

2020 survey results

Regional impact

Egg mass surveys show that in 2021, defoliation is likely to be severe at all locations sampled in the following districts:

  • Aurora
  • Midhurst
  • Peterborough
  • Bancroft
  • Kemptville

In the Guelph district, four sample locations indicate severe defoliation and one indicates moderate defoliation for 2021.

Species impact

During the survey, we discovered egg mass predation (attacks) by birds and small mammals.

We also found evidence of parasitism at some egg mass sites. For example, small pinholes in egg masses indicated the presence of the tiny parasitic wasp, Ooencyrtus kuvanae.

These predators and parasites will help to reduce gypsy moth populations.

Survey protocol

To predict 2021 gypsy moth defoliation, we used a Modified Kaladar Plot (MKP) protocol at select locations in the southern region. This protocol considers:

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

The total number of egg masses per hectare is used to forecast gypsy 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

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.

Map of projected 2021 gypsy moth defoliation

This map shows gypsy moth damage in 2020 and the forecast for defoliation in 2021.

Map of Southern Ontario showing distribution of projected 2021 gypsy moth defoliation.
Enlarge map

Contact us

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

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

Updated: May 20, 2021
Published: July 07, 2014