The spongy moth (Lymantria dispar dispar) is a non-native, invasive forest pest that was introduced to North America from Europe in 1869.
The spongy moth has also formerly been known as European Gypsy Moth.
It was first detected in Ontario in 1969 and has quickly spread across southern Ontario during the 1980’s. Spongy moth is unfortunately considered a well-established regional pest in southern Ontario.
The caterpillars of the spongy moth are dark and hairy. They have five blue dot pairs and six red dot pairs on their back. They go through four to five "molting" events where they shed their skin and each time, they get bigger.
A single spongy moth caterpillar can eat an average of one square metre of leaves. This can be devastating to trees.
Spongy moth in London
The City takes an integrated pest management approach to respond to spongy moth. This means the City uses different management techniques to address the pest population starting with the least harmful to the environment.
The City conducts surveying to monitor the population of spongy moth in London, which is important because spongy moth population density varies widely over time. Both the level of defoliation from the feeding of spongy moth caterpillars, as well as the abundance and size of its egg masses are monitored.
Defoliation is used to determine how resilient the trees will be to future defoliation. Sequential years deplete the tree’s resources, and increase the risk of trees dying. High levels of defoliation in one year can prompt additional control measures to prevent tree mortality.
Egg masses are also counted and measured, which provides information on the expected population and defoliation in following spring/summer. Abundant egg masses show a large population of spongy moth, which is cause for concern. The size of the egg masses can be used to judge the health of the population, with large egg masses indicating that the moths were healthy when the eggs were laid, and resources such as food were abundant.
Surveying conducted in 2023 found very few egg masses, which indicates that the population spike has now collapsed, and fewer spongy moth caterpillars are expected in the spring/summer of 2024. The egg masses were large, likely due to little competition between the caterpillars for food, since their population was lower in 2023 than in previous years.
As a result of the survey’s findings, particularly that egg masses were few in number, the City will not be applying treatments in 2024. Monitoring will continue later this year, which will forecast the severity for 2025.
Preventing spongy moth
The earlier and more often the pest’s life cycle can be interrupted, the more successful we will be in managing the pest.
You can take action to remove spongy moth in the winter, prior to eggs hatching in spring. Removing and destroying egg masses in winter reduces the number of hatched caterpillars in spring. Likewise, trapping and destroying both female and male moths in summer reduces the number of moths who then breed and start the egg laying cycle again.
Trapping and removing spongy moth
Residents can take steps to help preserve trees near their home by trapping spongy moths at different times of the year.
Egg stage and destroy egg masses (November to late April)
Survey your property for egg masses and scrape them off surfaces into soapy water to destroy them.
- Place a container below the egg mass to collect them
- Use a scraper tool to remove the egg mass from the surface. Ensure that all eggs are scraped. Try not to leave any residual eggs in bark ridges or crevices
- Empty the contents of your catchment container or bag into a bucket of soapy water
Leave the eggs sitting in the bucket for a day or two, then dispose of the contents
Hand pick caterpillars (May to July)
Gently shake small trees, shrubs and plants so caterpillars fall from their leaves. Thoroughly inspect the remaining foliage, branches and trunk for caterpillars and, using gloves, pick them off. Fallen and collected caterpillars should be placed in soapy water and left to soak for a few days. Then dispose of the contents.
Large caterpillar stage and burlap banding (July to August)
Once the caterpillars grow to about 2.5 cm (an inch) in length by mid-June, they will move down the trunk. Reduce the number of larvae on the trees in your yard by trapping them.
- Wrap and secure a piece of burlap cloth around the trunk of your tree
- Tie twine or rope around the center or slightly below the center of the burlap
- Drape the burlap cloth over the twine or rope so there is an overhang where the caterpillars can crawl underneath to seek shelter during the day
- Check the trap by lifting the overhanging burlap cloth every afternoon and collect any hiding caterpillars
- Put them into a bucket of soapy water for a few days to destroy them, then dispose of the contents
Female moth stage and burlap wrapping (July to August)
Similar to the large caterpillar stage, to trap female moths (which are unable to fly) wrap burlap lower on the trunk of the tree to trap the moth before it crawls up the tree and lays eggs. Once captured, put the moths in a container of soapy water and leave them there for a few days, then dispose of the contents.
Male moth stage and trapping male moths (July to August)
Hang non-toxic pheromone baited traps (available at hardware and nature stores) in trees to attract male moths. The scent draws the male moth where they are then trapped preventing them from mating with female moths and producing eggs. Once captured, put the moths in a container of soapy water and leave them there for a few days, then dispose of the contents.
Frequently asked questions
What kinds of trees are most affected by the spongy moth caterpillar?
Spongy moths prefer the leaves of deciduous hardwood trees like maple, elm and oak. It will also feed on apple, alder, birch, poplar and willow trees. As the caterpillar matures, and population levels increase, it will also begin to attack evergreens such as pine and spruce. Spongy moth don't appear to like ashes, sycamores, butternuts, black walnuts and dogwoods.
How much damage can spongy moth cause to trees?
Tree damage depends on the degree of infestation, past defoliations, the tree's vulnerability and the environment and can range from light to almost complete defoliation. If the tree has been weakened or stressed by other conditions, and attacked repeatedly in recent years, the defoliation can result in the death of the tree.
Does the spongy moth have any natural enemies?
Yes. Predators include other insects like wasps, flies, beetles, ants and spiders as well as birds such as chickadees, blue jays, robins and nuthatches. Animals such as chipmunks, squirrels and raccoons will also prey on the caterpillar.
The wasp that targets the spongy moth is a parasite of the spongy moth egg. It is now commonly found wherever spongy moths are and has become an important natural control of the moth.
Also, spongy moth is susceptible to several naturally occurring diseases caused by bacteria, fungi and a virus. The virus and bacteria escalate when spongy moth populations peak. The spongy moth virus disease is often referred to as “wilt” because dead caterpillars hang in an inverted “V” from tree trunks or foliage.
These natural biological controls contribute the most to keeping levels within a normal range and tend to follow two to three years after the gypsy moth populations peak.
What has the City done to help manage this invasive pest?
The City of London is responsible for managing and maintaining trees on City-owned lands: boulevard trees, trees in public parks, and Environmentally Significant Areas.
In 2008, the City completed a comprehensive Urban Forest Effects Model study which noted that 13% of London's tree canopy would be susceptible to spongy moth.
In 2009, the City performed an aerial spray of a biological pesticide called Btk for spongy moth management. It was applied to a few City-owned parks and an environmentally significant area, not over private property. These locations were targeted for having high infected oak tree populations, combined with a several year drought in the area, which compounded tree decline and death.
Since 2009, the City has taken an integrated pest management approach. This means we are using different management techniques to address the pest population starting with the least harmful to the environment.
In early 2020, City crews removed egg masses from City trees located on boulevards, parks and Environmentally Significant Areas.
In 2021 and 2022, the City performed aerial sprays of the biological pesticide called Btk in five municipal parks for spongy moth management.
In 2023, no aerial spraying was conducted. Surveying conducted in 2023 found that the population spike we were experiencing had collapsed. This was attributed largely to the spray programs in 2021 and 2022.
No active management or spray program was recommended for 2024. The City will however continue to monitor the spongy moth population levels.
What can residents do?
Property owners are responsible for managing trees (and pests) on their property. You are encouraged to monitor trees on your property, look for egg masses in winter, caterpillars in spring, and moths in July and August. Take action to remove spongy moths as often as possible.
Should property owners consider a commercial insecticide to help control the spongy moth population?
During severe infestation an insecticide may be considered a viable option. Homeowners can consider consulting with, and hiring a licensed contractor to apply pesticide sprays or tree injections. Timing of the application and the treatment of the entire canopy is essential to the success of control. You should also be aware that pesticide applications do not produce an instant defense and will not completely eradicate the problem, but can be very effective in reducing the insect population when used appropriately.
Why are there still some egg masses on boulevard trees, after the City has been by?
Arborists have worked to remove the egg masses from City trees to reduce the population. It is not possible to eliminate this pest completely. It is well established in our region, and the City’s overall objective is to reduce the number of spongy moths. The City continues to monitor numbers and will take further action if deemed necessary.
My family has been experiencing rashes that we think come from spongy moth caterpillars. What should we do about this?
The hairs of the spongy moth contain histamine which some people are allergic to. Not everyone will have a reaction if coming in contact with the caterpillar, but it is possible and is a known adverse effect. If you are experiencing any sort of reaction, please contact your family care physician for medical advice.
Why hasn't the City done a broad aerial spray of London to manage spongy moths? Will it happen?
Given the widespread distribution of spongy moth in North America, eradication of the pest is unlikely to be successful. Aerial spraying can be used to reduce its population, but City management nust also keep cost in mind. Other regions or cities may spray if they continue to experience widespread and severe amounts of spongy moth damage. Based on the surveying conducted in 2023, the population spike that we were previously experiencing has collapsed, with widespread damage not expected in 2024. As a result, aerial spraying is not warranted at this time, either on a localized or broad scale.
Additional information and resources
There is more information available online about spongy moth and how you can help protect trees.