The spongy moth, previously known as the gypsy moth, is an invasive insect recognized for its destructive impact on North American forests. Its caterpillars are significant defoliating pests, feeding on a wide variety of trees and shrubs. Understanding their habitats and geographic range is important for managing their spread and mitigating environmental impact.
Geographic Distribution
The spongy moth is native to Europe, Asia, and North Africa. It was accidentally introduced to North America in 1869 by a French scientist in Massachusetts attempting to breed a hardier silkworm. Escaped moths established a population, leading to the first recorded defoliation in 1889 in Medford, Massachusetts.
Since then, the spongy moth has spread significantly. It is now widely established in the northeastern United States and eastern Canada, including Ontario, Quebec, New Brunswick, Prince Edward Island, and Nova Scotia. Isolated infestations have also occurred in western states like Utah, Oregon, Washington, and California, where eradication efforts are often undertaken. Over 595 million hectares in North America are considered climatically suitable for establishment.
Preferred Environments
Within their geographic ranges, spongy moths thrive in specific environments, particularly forests. They show a strong preference for deciduous and mixed forests. Oak species are a favored host, but their diet is broad, encompassing over 300 tree and shrub species.
Beyond oaks, they also frequently infest maples, birch, alder, hawthorn, apple, and willow trees. While primarily feeding on hardwoods, they can also defoliate softwoods like pine, spruce, and hemlock, especially during outbreaks or when preferred hosts are limited. Spongy moths can adapt to various landscapes, including urban and suburban settings, where they can infest trees in yards, parks, and along streets.
Life Stage Locations
The locations of spongy moths vary significantly across their distinct life stages. Female moths lay their eggs in fuzzy, tan-colored masses, typically containing 500 to 1,000 eggs. These egg masses are often found in protected areas on tree bark, particularly in crevices, but can also be laid on various outdoor objects.
Once hatched in the spring, tiny larvae (caterpillars) climb trees to feed on leaves. Younger larvae often disperse by “ballooning,” carried by wind on silk threads to new locations. As they grow, older caterpillars primarily feed at night and seek sheltered spots during the day, such as bark crevices, leaf litter, or under objects like picnic tables. Pupae, the transitional stage, can be found in sheltered locations like bark crevices, under branches, or on outdoor items. Adult moths emerge from pupae, with males flying in search of mates and flightless females remaining near their pupation sites, often on tree trunks, waiting to lay eggs.
Range Expansion
Spongy moths expand their range through both natural and human-assisted dispersal. Natural spread primarily occurs through “ballooning,” where young larvae, less than 0.25 inches long, produce silk strands and are carried by wind currents. This allows young caterpillars to travel up to half a mile from their hatching site, though most land closer.
Human activities contribute significantly to long-distance movement. Transporting infested materials, particularly firewood, is a major factor in their spread. Egg masses, containing hundreds of eggs, readily attach to vehicles, trailers, outdoor equipment, and nursery stock, allowing them to hitchhike across vast distances. Public awareness and inspection of outdoor items before moving them are important for limiting this human-assisted spread.