Spongy moth, Lymantria dispar, formerly known as gypsy moth, is a native pest of Europe and was accidentally introduced to Massachusetts in 1869. Since then, its population has spread throughout the Northeast and into parts of the upper Midwest. The caterpillar is the damaging stage of this insect, feasting on the foliage of more than 300 species of trees and shrubs, with a strong preference for oaks. Repeated defoliation can weaken trees leaving them vulnerable to attack by diseases and other insects. That, in turn, can lead to tree mortality. Older and less vigorous trees may be killed by a single defoliation. During outbreaks, caterpillars can cause noticeable defoliation in woodland areas.

Besides being a pest of trees, spongy moth can also be a “people” problem. Mass quantities of caterpillars, along with their excrement of digested leaf material raining down from the above tree canopy can impact enjoying outdoor activities. The hairs of the caterpillar can also cause allergic reactions in sensitive individuals.

Every year, more than 4,000 traps are set throughout Iowa to survey for spongy moth. The traps have a sticky substance inside and are baited with a synthetic female spongy moth sex pheromone to draw in the male moths if present in the area. Trapping for spongy moth in Iowa is a collaborative effort including Iowa Department of Natural Resources (IDNR), USDA Animal and Plant Health Inspection Service (APHIS), and Iowa Department of Agriculture & Land Stewardship (IDALS).

Along with 10 other states, Iowa participates in the Slow-the-Spread (STS) program which includes intensive trapping along the naturally advancing spongy moth front. Generally infested areas in southwest Wisconsin continue to approach the northeast border of Iowa. The strategy of STS is to limit the rate of spread from infested to non-infested areas by means of a transition zone through early detection and targeted treatment. Outside the transition area, trap placement includes high-risk areas such as campgrounds, sawmills, and garden centers throughout Iowa where an introduction may occur by human-assisted dispersal. Trapping surveys are vital for the early detection of this invasive pest.

More information about the STS program can be found at National Slow The Spread Program.

Data collected from annual trapping surveys is used for determining treatment areas the following year. Iowa has occasionally used mating disruption treatments to suppress low level, localized spongy moth populations since joining the STS program over a decade ago. Aerial mating disruption treatments typically occur toward the end of June to prevent male moths from finding and mating with the flightless females. The mating disruption treatments are specific to the male spongy moth.

Aerial management options also include Bacillus thuringiensis var. kurstaki (Btk) to control young spongy moth caterpillars following egg hatch in the spring. Btk is a naturally occurring soil bacterium that has been formulated into a commercial biological insecticide for spongy moth control. Soon after consuming Btk from host leaf tissue, the caterpillars stop feeding and die within a short period of time. Btk rapidly breaks down in the environment, so a second application typically takes place 7–10 days later to target any later hatching spongy moth caterpillars.

The public is notified by IDALS of proposed treatments within the STS zone as part of the environmental assessment process. Only at the end of the review process is the environmental assessment signed off on by the U.S. Forest Service. More on proposed treatments and maps, or the latest on scheduled aerial treatments will be made available during treatment years. Iowa has no treatments planned for 2025.

Spongy moth undergoes four developmental life stages: egg, larva (caterpillar), pupa, and adult. There is one generation per year. If you live in Iowa and suspect spongy moth in any life stage, please don’t delay and let us know by going to our Contact us page. Early detection of this invasive species is very useful. Please email photos if possible.

Females lay tan or buff-colored egg masses. A single egg mass is about the size of a quarter and may contain up to 1,000 eggs. Egg masses are found on tree trunks, in sheltered branch locations, and virtually anything kept outdoors. Females are not particularly choosy and will lay egg masses on outdoor furniture, campers, firewood, vehicles, etc. Egg masses are laid during the summer and do not hatch until the following spring.

The caterpillars, or larvae, emerge in the spring. The caterpillars go through a series of progressive molts by which they increase in size, growing up to 2 ½ inches long. The initial stage is small enough where caterpillars drop down on silken threads to be transported to new locations on wind currents. Mature caterpillars may consume up to one square foot of foliage a day and are distinctive in appearance with five pairs of blue spots and six pairs of red spots.

The caterpillars will stop feeding and enter the pupal stage before emerging as winged adults 10 to 14 days later. Pupae are approximately 2 inches long, dark brown and covered with hairs. Pupae may be found exposed on trees, the sides of buildings, and also in hard to find places.

Male moths are mottled brown (dark brown to tan) while the females are creamy white. Both have wavy black markings on their wings. Despite having wings, the larger female moths do not fly. Male moths are very capable of flight and follow the pheromone scent emitted by the female moth by using their feather-like antennae. Adult moths do not feed; their only function is to mate. In Iowa, adult moths are present from July through August.

• USDA APHIS - Spongy Moth
• Federal Spongy Moth Quarantine Interactive Map
• Iowa State University - Spongy Moth
• 'Spongy Moth' Adopted as New Common Name for Lymantria dispar