Spongy moth is an insect native to Europe and Asia that has been severely weakening trees across North America. Spongy moth was introduced to North America in the late 1860’s near Boston and has spread over the past century. Despite the successful use of insect predators, as well as fungal and viral controls, spongy moth populations do occasionally reach outbreak levels and continue to expand their range. Spongy moth caterpillars defoliate host trees, mostly hardwood species, such as: oak, birch, poplar, willow, maple and others. During outbreak years, nearly all broadleaf trees may be completely defoliated, caterpillars appear everywhere, and “frass” (caterpillar droppings) appear to rain from the trees. Adult spongy moths are only seen in mid-summer when temperatures are above freezing. This species is known to infest trees in woodland or suburban areas.
Spongy moths can be found throughout southern Canada, across the eastern and central United States, and most of the western states. Populations have been found in southern Ontario, New Brunswick, Nova Scotia and British Columbia. Each population varies annually and fluctuates with local conditions.
Impacts of Spongy Moth
- Defoliates and kills large amounts of trees, affecting the many benefits provided by trees.
- Economic impacts affect all forest users.
- Caterpillars may chew small holes in leaves or completely strip a canopy, depending on age and population levels.
How to Identify Spongy Moth
- Four development stages: egg, caterpillar, pupa and moth.
- Caterpillars are 5-6 centimetres long with five pairs of blue dots and six pairs of bright red dots along their back.
- Female moths are white with dark markings and cannot fly.
- Male moths are brown and can fly.
- Females are larger than males with a 5 cm wing span, males only span 2.5 centimetres.
- Egg masses are about 4 cm long, tan colored, and can be found on tree trunks, furniture, buildings, etc.
What You Can Do
- Learn how to identify spongy moth during its various life stages.
- Egg masses can be easily controlled by removing and burning or soaking with soap and water mixture.
- A band of either burlap or other cloth product wrapped around the trunk will provide a place for caterpillars to hide during the heat of the day. Check these bands regularly and scrape caterpillars into a container of soapy water.
- Keep your trees healthy and better able to ward off attacks. In urban areas, water trees during dry spells and protect their root zone. In natural areas, good forestry practices will ensure healthier trees that are better able to withstand stresses such as defoliation.
- If you’ve seen an spongy moth or any other invasive species in the wild, please contact the toll-free Invading Species Hotline at 1-800-563-7711 or visit EDDMapS to report a sighting.
OFAH/OMNRF Invading Species Awareness Program. (2021). Spongy Moth. Retrieved from: www.invadingspecies.com.
This factsheet may be reproduced for non-commercial purposes.
Header photo by John H. Ghent, USDA Forest Service, Bugwood.org.