So Long Spongy Moth

Close-up of spongy moth on tree bark.

A Population Crash… For Now

If you’ve gone hiking recently, you’ve probably seen the spongy moth and its effects on the trees. The spongy moth, otherwise referred to as the LDD moth (Lymantria dispar dispar) is a hairy, red and blue caterpillar that kills trees. This invasive pest from Eurasia feeds on the leaves of many types of trees but prefers maple, oak, poplar and birch.

In the past, this pest typically went unnoticed but became a household name in 2021 after an outbreak decimated trees across Ontario.

Using ecological monitoring data, CVC has measured the outbreak’s damage to trees and projected population levels for this year.

Spongy Moth Population Trends

Outbreaks of spongy moth are cyclical. Most years, numbers are low and spongy moths go largely unnoticed. But, every seven to 10 years the population spikes and becomes damaging.

Using data from CVC’s Integrated Watershed Monitoring Program (IWMP), the damage to ecosystems from spongy moth can be measured. IWMP tracks ecosystem health in the Credit River Watershed by monitoring forests, wetlands, streams and groundwater. IWMP has been collecting tree health data in forest and wetland ecosystems since 2009. Using data from 30 forests and 23 wetlands, we can tell the story of the 2021 spongy moth outbreak.

The spongy moth population was small and stable from 2009 to 2018, causing little stress to trees. In 2019, we observed more damage to leaves and a higher spongy moth population than before, signaling a potential outbreak emerging.

A line graph that shows the annual percentage of trees infested with spongy moth from 2009 to 2022. Up until 2018 there was between zero and two per cent. 2019 saw a sharp increase to nine per cent, followed by another sharp increase in 2021 to 20 per cent. The numbers dropped in 2022 back down to two per cent.
Percentage of trees in CVC’s Integrated Watershed Monitoring Program (IMWP) long-term monitoring stations with signs of spongy moth infestation (eggs, caterpillars, or moths).

This trend continued over the next two years, where there was a population spike and massive defoliation across the watershed. 2021 was the largest spongy moth outbreak ever recorded in Ontario. Ninety-one per cent of IWMP monitoring stations were infested in 2021 and 24 per cent of monitored deciduous trees were defoliated.

Caterpillars on a tree trunk.
Spongy moth caterpillars covering a tree stem during the 2021 outbreak.

The population crashed in 2022, signaling the end of the outbreak and a return to pre-outbreak levels. This crash was caused by viruses, fungi and parasitoid wasps that act as natural population controls. These controls end outbreaks by killing spongy moths when the population gets high.

Egg mass on tree bark.
Spongy moth eggs that were killed by a parasitoid wasp (Ooencyrtus kuvanae) in 2022. Some young caterpillars are still hatching but many were killed before they could hatch.

Few monitored trees died as a result of the 2021 spongy moth outbreak. Most trees in the forests and wetlands recovered well, as they have energy reserves to withstand temporary stresses such as the spongy moth.

Population Projection for 2023

In addition to IWMP’s monitoring, CVC collects data to project spongy moth populations through the Invasive Species Monitoring Program (ISMP). Through this program, we monitor many kinds of invasive species across the watershed and recently added spongy moth population projections after the large 2021 outbreak.

For the past two years, we have monitored spongy moth populations in 30 forests across the watershed. This involves monitoring the amount of spongy moth eggs in standardized plots over the winter to predict the severity of the following year’s defoliation.

Based on this survey data, our ISMP team predicts this year there will be low spongy moth populations and defoliation rates. This was a large decrease from 2022, where we predicted 53 per cent of sites would be moderately or severely defoliated.

Although this pest may go unnoticed for now, another outbreak will likely occur within the next decade. In the meantime, we will continue to monitor pests like spongy moth to keep track of known stressors and emerging threats in our watershed while measuring their impact.

Learn more about spongy moth and threats facing trees in our natural areas.

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By Joe Gabriel, Technician, Watershed Monitoring

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