Foresters have noticed an increase in the prevalence of egg masses of the spongy moth (Lymantria dispar dispar), at some forest sites in southern Ontario over the past few years. This recently renamed, longstanding invader is native to Europe. Spongy moth was accidentally introduced in Massachusetts in 1869 by a French naturalist trying to cross it with North American silkworms in order to start a silk industry in the US. Some of the moths escaped and a population quickly established, with the first major outbreak occurring in 1889. Populations of spongy moth were first detected in eastern Canada in 1924 and then again in 1936 but an egg mass removal campaign is thought to have eradicated these early populations. A few decades later, it was detected again in Canada, in Quebec in 1955 and then Ontario in 1969. Since then, the moth has spread and become more problematic. The larvae feed on tree and shrub leaves, impacting the health of the plant when infestations are severe or when feeding occurs in successive years.

The closely related Asian gypsy moth (Lymantria dispar asiatica or L. dispar japonica), is not known to occur in Canada though is occasionally intercepted.

Read more about spongy moth predictions for 2021.

Watch a webinar on spongy moth status for Ontario 2021

Biology
The moth overwinters in the egg stage. Egg masses (sidebar) may be seen on trunks of trees, and other objects, from late summer until the next spring. Each egg mass may contain between 100 and 1000 eggs, depending on how severe the infestation is. Eggs hatch around May, when host tree leaves start to emerge. Newly hatched larvae can be seen on the egg masses for the first few days, then climb up the tree or shrub to feed on buds and new leaves. During the first three instars (developmental stages), spongy moth larvae feed during the day. Older larvae feed at night, descending the tree each day to rest on the trunk or the ground and climb back up the next day to resume feeding. This habit can make it difficult to detect infestations at early stages but allows for them to be readily trapped (see below).

About eight weeks later, in late July, larvae pupate, either on the tree trunk or near the ground. About two weeks after that, they begin to emerge as adults, with the males emerging first. Females are flightless, and crawl upward releasing a pheromone that enables the males to locate them. Females lay their eggs in masses on tree trunks, or other surfaces, in late July.

Trees at risk
Spongy moths prefers red and white oak, along with poplar and white birch, but feeds on 300 species of trees and shrubs. Other main hosts include beech, fruit trees, ash, balsam fir, basswood, hawthorn, eastern hemlock, alder, birch, hickory, larch, linden, pine, maple, spruce, elm and willow.

During severe outbreaks, large sections of trees or shrubs can be defoliated, this can cause significant growth loss to the tree. Trees defoliated in successive years may die or become weak and susceptible to other pests or pathogens.

See a full list of potential host trees

Current range
In Canada, spongy moth is established in Ontario, Quebec, New Brunswick, Nova Scotia and Prince Edward Island. In the US, it is established from the New England states to Virginia, West Virginia, Ohio and Wisconsin.

See a map of the spongy moth range in eastern North America

Detection

Fuzzy, tan-coloured egg masses (side bar) can be seen from late July through to the following spring. The spongy appearance of the egg mass gives the insect its new common name. Look for them on tree trunks or branches, sides or under eaves of buildings, on outdoor furniture, trailers, boats, fences, firewood or swing sets.

Larvae are as small as 3mm long in their first instar, and up to 70mm in length in their last instar. Older larvae are especially distinctive with five pairs of blue dots on the front segments of its body and six pairs of red on the back segments (sidebar), they are sometimes found resting on the trunk of the tree.

Adult females are white with wavy dark bands on the front wings and have larger bodies than males. Males are a grey-brown with darker wing markings, their antennae are feathery (sidebar).

What homeowners can do
May to July: Hand pick, or brush, caterpillars from smaller trees and shrubs. Use gloves to handle them as their hairs can cause skin irritation. Put fallen and collected caterpillars in a container with soapy water and leave them for a few days.

May to September: Trap caterpillars as they move up the tree trunk to feed. Secure a piece of burlap or other cloth around the trunk by tying it midway and allow the top section to overhang the bottom section (sidebar). Check under the overhanging cloth daily for caterpillars and put them in a container of soapy water for a few days. Full instructions here: here

August to May: Remove any visible egg masses by scraping them off with a knife and dropping then into a bucket of hot water and bleach.

Insecticides can be used on the foliage to protect trees, however, chose a pesticide registered for spongy moth in your province and use it safely, or consult a forester, arborist or your garden centre for options.

Read more about how to use pesticides safely