Spongy moth (LDD)

Spongy moth, or Lymantria dispar dispar (LDD), is an insect formerly known as “Gypsy Moth”. It feeds on the foliage of trees during the caterpillar phase of its lifecycle.

The City of Ottawa has been focused on the progression of a cyclical spongy moth outbreak since the first signs of increasing population in 2020. Through continued surveying efforts and correspondence with partners, such as the Ontario Ministry of Northern Development, Mines, Natural Resources, and Forestry, the City has been able to take appropriate steps to communicate current best practices and outbreak status updates to residents. 

The information collected through the surveys allowed the City to identify areas that were likely to experience high levels of spongy moth activity in the spring and summer of 2022. Forestry staff coordinated with community groups to provide the materials and instruction necessary for the installation of burlap traps on affected trees. Thank you to the community associations and groups that participated in the City’s burlap initiative.  

The City of Ottawa will continue to monitor the spongy moth population and exchange information with its partners to ensure appropriate action is taken and up-to-date information is made available to residents.  

Residents may see large groups of caterpillars on tree trunks and branches and loss of leaves (defoliation) on trees during the spring because of this insect.

On their properties, residents can consider the following to reduce tree damage:

Report spongy moth sightings

Residents can report spongy moth sightings using the City’s online reporting tool. Input received from residents helps to inform and coordinate monitoring efforts across Ottawa. City staff can use the information collected to supplement the City’s egg mass surveys, conducted during the fall and winter seasons, and to determine the level of the infestation in specific areas of the city.

Egg mass removal (August to May)

Survey your property for egg masses and scrape them off surfaces into soapy water to destroy them.

Resources:

• How to identify and remove spongy moth egg masses

Btk application (May to June)

Btk (Bacillus thuringiensis kurstaki) is a biopesticide available for purchase from local hardware stores to use against spongy moth. This product affects the caterpillars’ digestive system and should only be applied between May and early June when spongy moths are still in the larval (caterpillar) stage.

Hand pick caterpillars (May to July)

Handpicking caterpillars is most effective on small newly planted trees, shrubs, and plants infested with spongy moth. If possible, gently shake the tree so caterpillars fall from leaves. Thoroughly inspect the remaining foliage, branches, and trunk for caterpillars. Wearing gloves, pick them off your tree. Fallen and collected caterpillars should be placed and left to soak in soapy water to destroy them. Caution: the caterpillars have long hairs that can cause skin irritation, so it is recommended to wear long-sleeves and gloves.

Burlap banding (May to September)

By mid-June, spongy moth caterpillars will grow to about 2.5 cm in length and move down tree trunks. Residents can help to reduce the number of caterpillars on trees by trapping them with burlap. Burlap banding can be folded in half and attached to tree trunks using a string around the trunk. Caterpillars will congregate in the burlap, which can be removed and disposed of in the City’s curbside collection.

Resources:

• How to install burlap banding for spongy moth 

Spongy moth is an invasive insect native to Europe. It is currently established in much of eastern Canada. The caterpillar stage of this insect feeds on the leaves and foliage of many hardwood and softwood trees and can feed on over 300 plant species including landscape and garden plants. It is a cyclical (generally every 7-10 years) nuisance pest, that causes concern when it defoliates trees. The drastic defoliation that can occur can appear to be a severe impact on tree health. However, without other compounding factors such as drought, it rarely has an impact on long term tree health.

Lifecycle overview:

• Overwinter in the egg stage often on the bark of trees
• Hatch and larvae ascend the trees to feed on the new foliage in spring.
• Feed during the day; as caterpillars mature, feeding occurs mainly at night.
• Mature into larvae that are roughly 50 mm long, dark-coloured, hairy, with a double row of five pairs blue spots, followed by a double row of six pairs red spots down the back.
• Male moths are light brown and slender-bodied, while females are white and heavy-bodied.
• Complete their feeding in July-August.

Tree impacts:

• Larvae chew holes in leaves or devour entire leaves.
• In late July, spongy egg masses can be observed on the trunks and branches of infected trees and on items such as outdoor equipment, trailers, and vehicles.
• During severe outbreaks, trees and shrubs are completely defoliated over large areas; despite the trees’ ability to produce a new crop of leaves over the summer, the damage causes significant growth loss.
• Understory shrubs and plants may also be affected.

Recent observations based on a defoliation survey performed in August 2022 suggest that spongy moth populations are decreasing. Defoliation is less severe overall in comparison to what was observed in the summer of 2021. Additionally, the number of caterpillars, adult moths, and new egg masses is also lower than what was observed in previous years. 

It is important to note that outbreaks of spongy moths tend to be cyclical. This means that populations rise and fall due to a number of natural factors. Outbreaks usually occur every 7-10 years and have been observed for many years in the Ottawa area. 

The first recorded outbreak in North America occurred in New England in 1889. The first record of spongy moth in Ontario was in 1969 on Wolfe Island, near Kingston. The insect is now naturalized in Ontario and was first noted in the Ottawa area in the early 1980’s. Cyclical outbreaks of spongy moth have occurred since, and typically occur every seven to ten years.

What is spongy moth?

Spongy moth is an introduced, defoliating insect that is a pest of trees and forests in eastern North America. The caterpillar, or larva stage of the insect, eats the leaves of trees, making them more susceptible to disease and damage from other insects.

How much damage can spongy moth cause to trees?

Tree damage depends on the degree of infestation, past defoliations, and the tree's vulnerability. Defoliation can range in severity from light to almost complete defoliation. If the tree has been weakened or stressed by other conditions and attacked repeatedly in recent years, severe defoliation can result in the death of the tree; however, most trees will recover from a spongy moth infestation.

Will my tree die because of a spongy moth outbreak?

Very unlikely. If you have a deciduous tree, even if your tree is completely defoliated it will likely re-foliate by August. Spongy moth is rarely a primary contributor to tree death. However, spongy moth may be a contributor to tree death if other events also impact the health of a tree such as drought, disease, further defoliation by other insects such as Forest Tent Caterpillar, etc. Keep in mind that older trees have likely experienced at least one significant spongy moth outbreak in their lifetime already.

Do natural controls of spongy moth exist?

Yes. There are a wide variety of insects and other arthropods, birds and small mammals that eat substantial numbers of gypsy moth eggs, caterpillars, pupae, and adults. Of the more than 40 known predatory bird species in Canada that feed on spongy moth, some of the best known are the blue jay, American robin, black-capped chickadee, and gray catbird. Tiger beetles, stinkbugs, ants, the yellow jacket, and the bald-faced hornet are some of the better-known insects that help control low residual spongy moth populations in between outbreaks. Among small mammals, shrews, moles, and mice are effective predators. There are more than 100 insect parasites of the spongy moth egg, larval and pupal stages.

What has collapsed spongy moth outbreaks in the past?

Spongy moth has several natural predators. However, during outbreak periods when the spongy moth population reaches epidemic levels, there are historically two natural controls that ultimately collapse the spongy moth population:

1. Nuclear Polyhedrosis Virus (NPV)
2. Entomophaga maimaiga fungus

How can I see that natural controls are beginning to effect spongy moth?

Overwintering egg masses that are healthy are typically about the size of a two-dollar coin. However, once a combination of NPV and fungus impact the population, the less healthy females will lay egg masses that are smaller in size. When the average egg mass size is roughly the size of a dime, that is an indication that the population is generally unhealthy, and a mass die off is imminent.

Can cold winter temperatures affect the population of spongy moth?

There is almost certainly some mortality of spongy moth egg masses in Ottawa during very cold temperatures. However, even very cold temperatures are unlikely to lead to the collapse of the outbreak. Temperatures of -25 C can start to kill embryos of spongy moth. However, to have significant mortality, those temperatures must be sustained for periods of 3-5 days. Snow can also act as an insulating layer and egg masses below snow will be protected from very cold temperatures.

What kinds of trees are most affected by the spongy moth?

The caterpillars prefer the leaves of deciduous hardwood trees like maple, elm, and oak. Oak appears to be the preferred species in Ottawa. It will also feed on apple, alder, birch, poplar, and willow trees. As the caterpillar matures and runs out of foliage of its preferred species, it will begin to feed on more than 300 vegetative species, like conifers such as pine and spruce.

Why hasn’t the City done a broad aerial spray to manage spongy moth? Will it happen?

At this stage of the infestation, the City is not considering an aerial spray program. City staff are monitoring defoliation and coordinating surveys to estimate insect populations. Based on the survey data and in consultation with experts and partners across Ontario, staff are developing future response plans for spongy moth that will include consideration of all available tools, including pesticide