The Rise and Fall of the Spongy Moth

Good news! It appears that the invasive spongy moth (formerly known as European Gypsy Moth or LDD moth) has retreated in the Toronto region after a multi-year outbreak that started in 2018.

spongy moth caterpillar
Spongy moth caterpillar

This encouraging information has been supported by a team of biologists from Toronto and Region Conservation Authority (TRCA), who have been collecting long-term presence and abundance data on the spongy moth – as well as for 13 other invasive pests and diseases – in regional forest monitoring plots since 2008.

Based on this long-term data, it appears that the trees most affected by the outbreak are recovering well, with only a few having succumbed. There were also fewer observations of live spongy moth adults, caterpillars, and egg masses on the trees in 2022.


two adult spongy moths mating
The spongy moth, or Lymantria dispar dispar, is an invasive insect brought to North America from Europe in the late 1800s by a silk farmer. Spongy moth caterpillars defoliate host trees rapidly – mostly hardwood species such as oak, birch, poplar, willow, and maple. In this photo you can see two spongy moths mating: a female (white) and a male (brown). Just below this pair are a spongy moth cocoon and caterpillar.

What Are The Data Telling Us?

For the last five years, TRCA biologists have recorded the number of forest monitoring plots that have spongy moth, and the level of severity of defoliation in affected trees. The years 2020 and 2021 saw a large jump, compared to 2018-2019, in the number of trees at all levels of defoliation severity.

In 2022, TRCA noted a decline in the number of trees and level of defoliation severity caused by the spongy moth.

MEASURE 2018 2019 2020 2021 2022
Number of trees affected by spongy moth
(all severity levels)
21 71 155 306 140
Percentage of regional monitoring plots affected by spongy moth 27 64 73 91 73
Average severity of defoliation
(all affected trees in all plots)
0.81 2.14 1.77 1.70 1.24
Average percent canopy cover for all plots 91 88 90 83 91

The table above displays the 2018-2022 data from 22 regional forest long-term monitoring plots for the presence of spongy moth on trees, in monitoring plots, and the average severity of defoliation.

TRCA added a severity level to the tree pest and disease monitoring protocol in 2017. Severity is determined using a 4-level system (0-3) with 3 being the most severe, meaning the percentage of leaves affected on the tree is greater than 51%.

In TRCA’s jurisdiction, the most severe defoliation was observed in Peel Region in 2020, with York Region and Toronto more affected in 2021. It was reported by the National Forestry Database that hungry spongy moth caterpillars damaged 583,157 hectares of forests in Ontario in 2020.

The COVID-19 pandemic was difficult enough for people forced to spend large amounts of time indoors. To step outside for fresh air, only to encounter leafless trees covered in invasive spongy moth caterpillars, added further to the stress of this period.

chart showing the number of trees at each level of defoliation severity from 22 TRCA forest long term monitoring plots for years 2018 to 2022
The number of trees at each level of defoliation severity from 22 TRCA forest long term monitoring plots for years 2018 to 2022.
defoliation in the Greater Toronto Area caused by the spongy moth
defoliation in the Greater Toronto Area caused by the spongy moth

The photos above from TRCA forest monitoring plot FV-11 (West Gormley) in July 2021 show severe defoliation. Typically, this plot is shaded and dark during the summer months. Here, the canopy cover appears closer to that of early May.

What Have We Learned from All This?

We can thank the many natural and introduced predators, parasitoids – insects that lay their eggs inside other insects – and diseases for this recent decline in spongy moth populations.

Key biocontrol agents helping to control spongy moths include the virus Lymantria dispar multiple nucleopolyhedrovirus, the fungal disease Entomophaga maimaiga, and two tiny parasitoid wasps called Cotesia melanoscela and Ooencyrtus kuvanae. Outbreaks crash when insect mortality catches up with population size. READ MORE.

parasitoid wasp Ooencyrtus kuvanae
Ooencyrtus kuvanae, a parasitoid wasp that specializes on the eggs of spongy moth. It was introduced in the beginning of the 20th century, and later repeatedly introduced in the 1970s and 1980s. This spongy moth parasitoid has been doing quite well in the Toronto region. Photo taken at Happy Valley, 2021.

Overall, the leaves in the forest monitoring plots have grown back. Thankfully many trees can survive a few years of defoliation, using stored reserves to grow new leaves.

In the most impacted forest monitoring plots in Toronto region, our data shows that the average canopy cover measured in 2022 almost equals the average canopy cover measured before the spongy moth outbreak.

Furthermore, only a slight increase in all exotic plants combined has been observed in the forest understory, with the invasive European buckthorn being most dominant.

Forests can withstand occasional disturbances such as insect outbreaks, but their resilience is strengthened by having a diversity of healthy trees that can tolerate different stressors. Continuous long-term monitoring in our regional forests enables conservation managers to track the health and function of these important ecosystems over time.

What Can You Do?

  • If you have seen a spongy moth or any other invasive species in the wild, please contact the toll-free Invading Species Hotline at 1-800-563-7711 or visit EDDMapS to report a sighting. Visit Ontario’s Invading Species Awareness Program to learn what you can do to stop their spread.
  • Visit TRCA’s spongy moth webpage for information and management resources to help homeowners protect trees on their properties from spongy moth infestation.
  • Spongy moths can be found throughout southern Canada, across the eastern and central United States, and most of the western states. Populations have been found in southern Ontario, New Brunswick, Nova Scotia, and British Columbia. It is predicted that climate change will likely increase the frequency, intensity, and range of these cyclical outbreaks. Learn how TRCA is acting on climate change and what you can do to help!
  • TRCA frequently hosts webinars on the environmental impact of invasive species, and organizes local invasive species removal events. Visit TRCA’s events calendar to find out what’s happening in your community.

Scientific data collection and research informs decisions affecting the natural areas and watercourses within our region. Connect with TRCA online.