European Gypsy Moth (EGM) is a non – native invasive insect from Europe that was brought to North America in the 1860's. First established in Massachusetts, it had spread to Ontario by 1969.
EGM larvae (caterpillars) feeds on a wide range of deciduous and some coniferous trees, however a component of
oak is key and is generally required to drive significant outbreaks. During the larval (feeding) stage
of the lifecycle in late May and June, substantial defoliation of trees may occur during peak years.
Populations of this species are cyclical with population surges approximately every 7-10 years. When populations rapidly rise, they are historically followed by a crash. This population crash is due to mortality from a host
specific virus or fungus.
An outbreak was observed in parts of Simcoe County in 2019 and particularly severe and widespread
defoliation was evident in 2020. The most noticeable impacts have been seen in Tiny, Penetanguishene, Midland, Tay and Adjala-Tosorontio with smaller impacts in many other areas within the county.
Simcoe County completed egg mass sampling in fall 2019 and again in fall of 2020 within several County Forests. Egg mass surveys count the overwintering egg masses of the species to help forecast the population potential the following year. Egg mass surveys in 2020 showed that population levels were increasing in some areas and decreasing in others. The presence of Nucleopolyhedrosis virus (NPV) and/or Entomophaga maimaiga fungus were noted in most survey plots and insect mortality was noted. However, it is anticipated that some areas will see moderate to severe defoliation in 2021. 2020 egg mass survey results are available here: SCF 2020 Gypsy Moth Egg Mass Survey Results.
The Ontario Ministry of Natural Resources and Forestry (MNRF) has competed mapping showing areas with European Gypsy Moth defoliation in 2020. The MNRF mapping illustrates a general outbreak across many areas of the province. 2020 mapping is available here: MNRF 2020 European Gypsy Moth Defoliation Map
Stress to defoliated trees will occur, and mortality to some deciduous trees may result after several
successive years of defoliation. Severe defoliation of coniferous trees may result in mortality after just one
season. However, as population levels typically collapse back to low densities within 1-3 years of
outbreak, no substantive or long-lasting impacts to overall forest health has resulted historically.
It is recognized that peak EGM outbreaks represent a significant nuisance factor to affected
residents, and the potential for any tree mortality on private property is impactful and concerning to
landowners. Maintaining healthy trees and forests is an important defense against Gypsy Moth.
If residents or landowners want to manage European Gypsy Moth impacts, several resources are available below.
(Source: MNRF, https://files.ontario.ca/mnrf-foresthealthconditionsontario2018.pdf)
For further information, please visit:
Homeowners Management Guide to European Gypsy Moth
Landowners Management Guide to European Gypsy Moth
Ministry of Natural Resources and Forestry - European Gypsy Moth
To contact a Certified Arborist or Professional Forster, please visit:
International Society of Arboriculture
Ontario Professional Foresters Association
