Spongy Moth

Healthy trees can typically survive three to five years of heavy moth feeding. A healthy tree will usually be able to regrow their leaves later in the season. However, if the trees are stressed due to drought, disease, fungi, or other insects, this loss of leaves can deplete the tree’s energy reserves and eventually result in death. Unfortunately, conifer trees (though not the moth’s favourite meal) can die after only one year of very heavy feeding.

Several local species will eat the caterpillars or pupae. Birds such as blue jays and orioles will eat the caterpillars, and chickadees will feed on the egg masses. Mice, chipmunks, skunks, voles and other small mammals will eat the pupae or larvae.

Besides wildlife that eat spongy moth, there are some other natural control methods including a virus, fungus, and parasitic wasps and flies. The nuclear polyhedrosis virus (NPV) infects the caterpillars, causing them to die. The virus spreads through contact between caterpillars and through the feces of birds that eat dead or dying infected caterpillars. The effectiveness of the virus is dependent on high caterpillar density. Interestingly, when caterpillars die due to the NPV they hang in an upside down “v” shape. The fungus Entomophaga maimaiga overwinters in soil and infects the caterpillars, resulting in their death. It needs cool, wet weather to persist and be effective. These natural controls are part of why the population declines after a few years.

These outbreaks occur every 5 to 10 years and usually last between two and four seasons.

Outbreaks generally collapse due to the combined effects of natural controls: a fungus (Entomophaga maimaiga), a virus (Ld-Nucleopolyhedral virus), and parasitic wasps and flies. The effectiveness of these natural enemies is dependent on weather conditions and the size of spongy moth populations.

Please note: the hairs on spongy moth caterpillars and in egg masses can cause irritation or allergic reactions in some people. Consider using gloves and other protective wear whenever contact is possible.

There are several ways that you can try to help your trees affected by this moth. A combination of these methods works best. You may not get rid of all the caterpillars, but you can reduce the amount of damage they cause. If there are a few trees on your property that you’re concerned about, concentrate your efforts on them. If you have a larger woodlot some damage to a few trees is not likely to have a big effect on the forest in the long term.

If you’re worried about the trees on your property, you can try the methods listed below or consult a local tree service company:

• Keeping your tree healthy (year-round): This can mean watering in dry periods, adding mulch to the soil around its base, and making sure its root zone is clear of heavy objects or things preventing rain from getting into the soil.
• Egg mass scraping (August to late April): Until the caterpillars start emerging in late April, the oval-shaped, tan-coloured egg masses can be scraped off trees and other surfaces into a container. Fill the container with soapy water, and make sure you stir the egg masses and break them up if possible. Let them sit for two days in a sealed container to kill the eggs and then put them in the garbage. Scraping egg masses onto the ground will not kill them. Spongy moth egg masses can be found on almost any surface, including the side of your house. Watch the finding and removing spongy moth egg masses video. 
• Spraying Btk (Planning: winter before you want to spray, Spraying: May to June): Btk (Bacillus thuringiensis kurstaki) is a biopesticide that can kill moths in the caterpillar stage when sprayed on the leaves of affected trees. The caterpillar needs to eat the bacteria off the leaf to be killed. Various Btk products are publicly available, however it should be applied by a registered pesticide applicator. This can be done through ground sprays or aerial sprays for larger forested areas. Consult a local tree service company for ground spray options and research for aerial spray options well in advance of the spring season. Note: Pesticides of any kind should only be used with the greatest caution. Always make sure to carefully read the label and follow instructions for the proper timing and dosage of application. Visit Health Canada to read about the safety of Btk. This spraying can be done for individual trees or for large scale infestations.
• Trunk wrapping (May to June): When the caterpillars are present wrap a wide strip of burlap around the trunk, tie it around the middle with string and then fold down the top to create a pocket. Caterpillars will gather there for shelter during the day, creating easy access for catching and disposing them. Watch the spongy burlap banding traps video.
• Hand picking caterpillars and pupa (May to July): Collect caterpillars and pupa from trees or other surfaces and place in soapy water (make sure the solution is well mixed). After a couple of days, discard the caterpillars and pupa in the garbage. Wear gloves to ensure your skin is not irritated by the caterpillar hairs.
• Set out pheromone traps (July to August): Pheromone traps can be used to attract and trap adult male spongy moths to stop them from mating with females. Although this is generally used in monitoring spongy moth populations, it can reduce egg mass loading in small areas.

When a population boom occurs, spongy moth caterpillars often eat all the leaves on their host trees. Once this happens, the caterpillars search for more food by crawling around or dropping down on silk lines. Windy conditions help blow them out of a tree canopy and onto anything that’s nearby.

This, in part, is why the trunk wrapping method works. Caterpillars climb up trees looking for leaves to eat and climb down to seek shelter in the heat of the day.

There is a product TreeAzin (manufactured by Bioforest) that can provide trees protection against spongy moth. It’s a systemic insecticide that is injected into a tree. When used for spongy moth, it can severely decrease the amount of defoliation on a treated tree.

Treatments must be done by a qualified professional with special equipment, can be very costly, and require holes to be drilled into the base of each tree. It must be done pre-emptive to the caterpillar outbreak and therefore takes planning to be effective as it targets the early life stages of the caterpillar. Learn more information from Bioforest, and see their list of service providers.

2021 Actions

In response to the severity of spongy moth outbreak and defoliation impacts in 2021, CVC did targeted spongy moth outreach efforts, collaborated with watershed municipalities and formalized our spongy moth egg mass monitoring program.

Outreach and Education

As watershed residents noticed the impacts of spongy moth throughout the spring and summer of 2021, CVC responded with the following outreach and education actions:

• Responded to spongy moth-related landowner and media enquiries
• Co-hosted a free webinar (on spongy moth life cycle, identification and management) with Toronto and Region Conservation Authority.
• Released four blogs: Another Year of Invasive Moths, How to be a Good Neighbour, Managing LDD Moths, LDD Moth Update

Monitoring

As part of a larger initiative to create a formalized integrated pest management framework, CVC initiated a spongy moth egg mass monitoring program in 2021. We completed 29 surveys across 22 conservation areas focusing on vulnerable forest communities characterized by spongy moth preferred hosts (oak, maple and poplar) throughout the watershed.

Seven survey sites reported egg mass numbers high enough to predict severe defoliation in 2022 however, distribution of those sites was patchy and spread across the upper two-thirds of the watershed. CVC staff anecdotally reported a decrease in egg mass numbers in 2021 as well as increased presence of parasitic insects, nuclear polyhedrosis virus (NPV) and the fungus Entomophaga maimaiga during the growing season.

2022 Actions

We observed the natural controls taking effect and the spongy moth populations severely declined in 2022. CVC continued to respond to the spongy moth outbreak with the following outreach, education, partnership and management actions:

Outreach and Partnerships

We continued to provide outreach and education to watershed residents and partners including:

• Engaged with social media posts and provided technical information when requested.
• Expanded our spongy moth webinar partnership to include Toronto and Region Conservation Authority, Conservation Halton, Hamilton Conservation Authority and Lake Simcoe Region Conservation Authority. Watch the webinar recording.
• Collaborated with our partnering municipalities and local conservation authorities through working groups and sharing of technical knowledge and data.

Monitoring

CVC staff conducted spongy moth egg mass surveys at the same 29 priority vulnerable forest communities across 22 conservation areas throughout the fall.

A drastic change was observed when compared to the 2021 surveys. All of the survey sites had little to no egg masses, with only five sites having egg masses. Therefore, all sites were predicted to have light or no defoliation in 2023.

Management

Due to the widespread distribution of the sites expected to have heavy defoliation (based on the 2021 egg mass surveys), CVC did not conduct aerial sprays for spongy moth at our conservation areas in 2022. However, we did support the City of Mississauga in their spongy moth management program as it related to CVC owned lands in Mississauga.

Additionally, we investigated the potential for localized control efforts such as tree banding and egg mass scraping to protect priority trees in conservation areas where significant defoliation was predicted. Upon further assessment, these areas showed less than expected defoliation, and the localized control efforts were deemed unnecessary.

For current outbreak status and actions see “Current Update – Spring 2023” section.