Spongy Moth
Spongy Moth (previously called European Gypsy Moth or by it’s scientific name Lymantria Dispar dispar/LDD) is a non – native invasive insect from Europe that was brought to North America in the 1860’s. First introduced to Massachusetts, it had spread to Ontario by 1969 causing it’s first widespread defoliation event in the province by 1981.
Spongy Moth larvae (caterpillars) feeds on a wide range of deciduous and coniferous trees. They prefer oak, birch, poplar and willow trees but will also feed on maple, beech, elm, pine, spruce and fir. During the larval (feeding) stage of the lifecycle in late May and June, substantial defoliation of trees may occur during peak years. The larvae pupate in late June to early July and emerge as adults in late July to early August. Only the larval stage of Spongy Moth feeds on leaves.
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.
A population increase was observed in parts of Simcoe County in 2019 and widespread defoliation was evident throughout many parts of Ontario in 2020. Insect damage continued to be seen in many areas in 2021.
Simcoe County has completed egg mass sampling in 2019, 2020 and 2021. Egg mass surveys count the overwintering egg masses of the species to help forecast the population potential the following year. Egg mass surveys were completed in 2021 with a total of 84 survey plots in 37 different county forests. Defoliation forecasting for 2022 from these egg mass surveys forecasted 11% of plots could see severe defoliation, 35% of plots could see moderate defoliation and 54% of plots could see light to no defoliation. The presence of Nucleopolyhedrosis virus (NPV) and/or Entomophaga maimaiga fungus were noted in most survey plots and insect mortality was noted. These pathogens are important in the collapse of the population.
In 2022 it was evident that the Spongy Moth population collapse had occurred in most parts of Simcoe County. In 2022, there were very few new Spongy Moth egg masses observed and little defoliation due to spongy moth.
The Ontario Ministry of Northern Development, Mines, Natural Resources and Forestry (MNDMNRF) completes annual forest health monitoring and provides provincial updates. The most recent Spongy Moth map from the MNDMNRF is available here.
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 Spongy Moth 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 Spongy Moth. Residents are remained that the care and maintenance of privately owned trees is the responsibility of the landowner.
For residents or landowners wanting to manage Spongy Moth impacts, several resources are available below.