Invertebrates
Spongy Moth
Lymantria dispar · also called gypsy moth
In a stand of oaks stripped nearly bare by a spongy moth outbreak, spongy tan egg masses cling to tree trunks by the hundreds, a quiet reminder that this entire invasion traces back to a nineteenth-century silk experiment that never should have gotten loose.
Overview
The spongy moth, Lymantria dispar, is an invasive defoliating insect established across forest habitat throughout eastern North America after an accidental escape from a failed silk-breeding experiment in the 1860s. Its cyclical, forest-stripping outbreaks and flightless females that spread almost entirely through human transport make it one of the most consequential invasive insects in North American forestry.
An escape that never stopped spreading
More than a century and a half after a handful of moths slipped out of a Massachusetts breeding experiment, their descendants continue working steadily westward across the continent, one egg mass hitching a ride at a time.
Status
Least Concern, increasing — continuing to expand its range gradually westward as an established invasive forest pest across North America.
Spongy Moth: common questions
How did the spongy moth arrive in North America?
The spongy moth was deliberately imported to Massachusetts in the 1860s as part of an experimental attempt to breed a hardier silk-producing moth by crossing it with domestic silkworms, but some individuals escaped captivity, and the species has since spread and established itself as a significant invasive forest pest across eastern North America.
Why was the species renamed from gypsy moth to spongy moth?
The common name was changed from an earlier term considered an ethnic slur to spongy moth, a name instead describing the distinctive spongy, tan-coloured, hair-covered egg mass that a female lays, a renaming carried out by the organisation responsible for standardising common insect names.
Can spongy moths fly long distances to spread?
Not directly — a female spongy moth has small, non-functional wings and cannot fly at all, so long-distance spread happens mainly through egg masses accidentally transported on vehicles, firewood, and outdoor equipment rather than through the moths flying to new areas themselves.
Why do spongy moth outbreaks eventually end on their own?
Spongy moth outbreaks typically collapse after a few years due to a combination of a naturally occurring fungus, a specific virus that spreads rapidly through dense caterpillar populations, and increased predation, after which populations remain low for a period before the cycle eventually builds toward another outbreak.