Spongy moth is an invasive pest from Europe that was accidently introduced in Massachusetts in 1869. The caterpillar is the damaging stage of the insect feasting on more than 300 species of trees and shrubs, with oak leaves being their favorite food. When populations reach outbreak proportions the caterpillars can completely defoliate forested areas. Repeated defoliation can weaken trees leaving them vulnerable to attack by diseases and other insects. Older and less vigorous trees may be killed by a single defoliation.

Besides being a pest of trees, the caterpillars can also be a “people” problem. Caterpillars drop excrement of digested leaf material while feeding above in tree canopy, making it difficult to enjoy the outdoors in yards and recreation areas. The hairs of the caterpillar can also cause allergic reactions in sensitive individuals.

Since its introduction to Massachusetts, the range of spongy moth has expanded across much of the northeast and parts of the upper midwest. The advancing front continues to advance west, threatening Iowa. Besides natural dispersal, spongy moth can quickly move long-distances by "hitchhiking" on vehicles, campers, firewood and many outdoor items. Females often lay masses on such articles which are then transported long distances by people.

For more than a decade Iowa has been part of a national program called Slow the Spread (STS) which helps states on the naturally-advancing spong front delay its spread rate, and limit its impacts. The STS programs operates in the “transition zone” between the leading edge of the generally infested zone and the uninfested zone. This zone ranges from North Carolina to Minnesota, relying on detection and targeted control methods to reduce the normal build-up and spread of this species. Areas of northeast Iowa fall within the transition zone. The national strategy is a collaborative effort between state and federal government.

More information about the STS program can be found at The STS Foundation.

Every year, more than 4,000 traps are deployed statewide in Iowa to survey for the spongy moth. The triangular ‘delta traps’ have a sticky substance inside and are baited with a synthetic version of the sex pheromone the females produce. Males fly into the trap looking for a mate and become stuck. Data collected from annual trap surveys is used for determining areas that warrant a more intensive trap survey or treatment the following year.

Nearly half of all the traps are placed in northeast Iowa in the transition zone, ahead of the advancing front of the main spongy moth population. The remaining traps monitor high risk areas such as campgrounds and garden centers throughout the state where an introduction may occur by human-assisted movement. Monitoring for spongy moth around the state is a collaborative effort including Iowa Department of Natural Resources, Iowa Department of Agriculture & Land Stewardship, USDA-APHIS-PPQ and cooperators.

Aerial treatments are used to suppress low-density spongy moth populations within the Slow the Spread’s transition zone. Trapping survey data is used in part to help identify outlier populations where treatment could most strategically delay advancement. Previous year’s trapping data is also taken into consideration when prioritizing treatment sites. Since populations of spongy moth are low in Iowa, treatments do not necessarily occur every year. More information about treatments, and up-to-date treatment plans will be provided during treatment years. There are no treatments planned in 2023.

Mating disruption is most commonly used by the STS program for managing low level spongy moth populations. Aerial application of the female spongy moth pheromone is applied over an area to disrupt mating. A mating disruptor is designed to stick to tree foliage in urban and woodland areas.

Link: Questions and Answers - Gypsy Moth Mating Disruption (.pdf)

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 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.

• Iowa State University - Gypsy Moth
• Gypsy Moth: Potential Impact on Iowa (.pdf dated 2018)
• Questions and Answers - Mating Disruption as a Gypsy Moth Management Option (.pdf dated 2019)
• Federal Spongy Moth Quarantine Interactive Map
• Before Moving, Check for the Spongy Moth (if from quarantine area)
• USDA - European Spongy Moth