About the Spongy Moth
The spongy moth (Lymantria dispar), formerly known as the gypsy moth, is an invasive forest pest native to Europe. It was introduced to Massachusetts in 1869 as an experiment to help provide silk for the textile industry. This exotic insect has spread throughout the northeastern United States and into the upper Midwest.
Spongy moth populations rise and fall in cycles of about 10-15 years. During outbreak years, spongy moths are a serious threat to forest health and can be a major nuisance to residents.
Humans can easily transport this pest to new locations. Several life stages of the spongy moth, including eggs, caterpillars, and pupae, can hitchhike on firewood, outdoor equipment, and vehicles.
Life Stages & Identification
Spongy moth has one generation per year and develops through four life stages:
- Egg mass: Females lay tan or buff-colored masses protected by a light-colored almost hairy covering, about the size of a quarter, containing up to 1,000 eggs. Eggs are laid starting in late summer or fall and hatch the following spring. Egg masses may be found on trees, firewood, outdoor furniture, vehicles, and other surfaces.
- Caterpillar (larva): Active in spring; Newly hatched caterpillars are black and very fuzzy. Mature caterpillars grow up to 2.5 inches long and are identified by five pairs of blue spots followed by six pairs of red spots along the back. They typically feed in the treetops at night, but migrate down the trunk to the ground each day.
- Pupa (cocoon): Dark brown, hairy cocoons about 2 inches long. This stage lasts 10–14 days. Pupae attach themselves to a vertical surface with strands of silk. They are usually found in crevices on tree trunks or on larger branches.
- Adult moth: Present from July through August. Males are brown with black markings and are often seen flying around in summer and fall. Females are creamy white with brown markings that resemble an inverted V pointing toward the head; they are larger than males and can’t fly.
Look-Alikes: Spongy Moth vs. Eastern Tent Caterpillar
Many caterpillars observed in Iowa are native species, most commonly the Eastern tent caterpillar. Tent caterpillars look similar to spongy moths and also feed in the spring. Unlike spongy moths, tent caterpillars are native species and are an important food source for a number of birds and other wildlife species.
Key differences:
- Spongy moth caterpillar: Five pairs of blue spots followed by six pairs of red spots; does not build silk tents; can cause severe defoliation of tree leaves.
- Eastern tent caterpillar: Solid white stripe down the back; builds large silk tents in branch crotches; usually causes limited damage.
Impacts
Spongy moth is one of the most destructive pests to trees and shrubs in the U.S. They defoliate millions of acres of forests and urban trees each year.
- Caterpillars feed on more than 300 species of trees and shrubs, with a strong preference for oaks.
- Repeated defoliation weakens trees and increases susceptibility to diseases and other insects. Older or stressed trees may die after a single severe defoliation event.
- Large numbers of caterpillars and falling frass (insect droppings) can make outdoor activities unpleasant.
- Caterpillar hairs may cause allergic reactions or skin irritation in sensitive individuals.
Current Location
Spongy moth is established throughout much of the northeastern United States and parts of the upper Midwest. Iowa lies within a transition zone where populations are actively monitored to slow westward spread.
- Natural spread: Young caterpillars can disperse by “ballooning” on silken threads carried by the wind.
- Human-assisted spread: Egg masses can be transported long distances on firewood, vehicles, campers, outdoor furniture, and nursery stock.
- Ongoing surveys help determine where spongy moth populations are present and where management actions may be needed.
Signs and Symptoms of Infestation
Spongy moth damage is most noticeable during the growing season, particularly in late spring and early summer when caterpillars are actively feeding. Early detection is critical to limit defoliation and reduce long-term tree stress and mortality.
What to look for:
- Leaves may be partially or completely eaten, often starting at the edges and progressing inward.
- Trees may be partially or completely bare of leaves in late spring or early summer, especially oaks.
- Large numbers of hairy caterpillars with paired blue and red spots feeding on leaves, trunks, and nearby surfaces.
- Buff to tan, felt-like egg masses attached to tree bark, firewood, outdoor equipment, and structures.
- Accumulation of dark, pellet-like droppings (frass) beneath trees; heavy infestations may sound like falling rain.
- Silk threads hanging from caterpillars as they disperse.
- Presence of pupae in bark crevices or sheltered areas in mid-summer.
- Adult moths present in mid- to late summer; white, flightless females and smaller brown flying males.
Preferred Host Trees
- Primary hosts: Oaks (especially white and red oak species)
- Other preferred hosts: Aspen, birch, basswood, willow, apple, hawthorn
Occasional hosts: Maple, elm, ash, cherry, pine, spruce and other conifers (typically during high population outbreaks)