The Spongy Moth in Connecticut: An Overview

Trees at risk  
Identification  
Life cycle and behavior  
Outbreaks and response  
How to help  
Resources 

Spongy moth caterpillar.

The spongy moth (Lymantria dispar) was first discovered in North America around 1868. It has been in Connecticut since 1905 and is now established in the forests of southern New England. Its natural range is in Eurasia. The spongy moth is well-known for the damage it does to trees. Normally, the spongy moth population in Connecticut is low, and its damage is fairly restricted. However, when it reaches outbreak status, it becomes a significant problem.

"Spongy moth" has been formally adopted as the new common name for the moth species Lymantria dispar (formerly known as the gypsy moth). The new name is based on the insect’s sponge-like egg masses. You can learn more about the name change from the Entomological Society of America.

In the 1860s, several entrepreneurs were interested in developing a silk industry in the United States. These young men, well-skilled in entomology, were experimenting with various species of moths that might work as a source of silk fiber.

In 1868 or 1869, one of these men in Medford, MA, named Etienne L. Trouvelot, received a shipment of potential silk moth candidates from a colleague in France. The shipment included some spongy moth eggs. Mr. Trouvelot began rearing these spongy moths in his backyard, and some escaped. That is where the story of the spongy moth begins in the United States.

In 1910, a fungus, Entomphaga maimaiga, was released in the Boston area. Although it took some time to become effective, this fungus now controls the spongy moth population, making the spongy moth a minor issue in most years. This fungus requires moisture to activate, and drought years are more likely to have spongy moth outbreaks.  

Trees at risk  

Oaks are the preferred host tree for the spongy moth, but the insect also readily consumes beech, birch, elm, maple, and most other hardwoods. During heavy infestations, it will also consume pine, spruce, and hemlock. Spongy moth tends to avoid ash and tulip poplar. 

Identification

Female spongy moth laying eggs.

Male spongy moth.

The spongy moth caterpillar is hairy with five pairs of blue spots followed by six pairs of red spots along its back. The female moths are white and flightless, and the male moths are brown.

Signs of an infestation include:

• Clusters of tan, fuzzy egg masses on tree trunks, branches, or nearby surfaces.
• Noticeable defoliation, often starting at the top of the tree.
• Caterpillars crawling on trunks or hanging from silk threads 

Similar Moths of Concern 

Some moth species can be mistaken for the spongy moth, including:

Browntail moth caterpillar. 

The brown-tail moth (Euproctis chrysorrhoea) is a pest that goes into occasional outbreak status on the islands along the coast of Maine and on Cape Cod. It is in the same subfamily as the spongy moth.

Winter moth adult. 

Though not closely related to the spongy moth, the winter moth (Operophtera brumata) is established in southeastern Connecticut. The damage caused by the winter moth is very similar to that caused by the spongy moth. Starting in 2006, the parasitoid fly Cyzenis albicans was released in New London County. By 2016, winter moth larval densities had dropped significantly. By 2022, no significant winter moth damage was observed.

Life cycle and behavior 

Spongy moth eggs hatch between late April and mid-May. The larval, or caterpillar, stage typically lasts about 7 weeks. The larvae are most active in May and June. The larvae pupate in late June or early July and remain in this stage for 1 to 2 weeks before emerging as adults.

The spongy moth undergoes a complete metamorphosis, changing from egg to larva (caterpillar) to pupa to adult.

Spongy moth pupae.

The caterpillar molts 3 or 4 times as it grows. The period between each molt is known as an instar. First instar larvae are very small and can be difficult to spot. Fourth and fifth-instar larvae are large, with an infamous appetite to match. The majority of tree defoliation is caused by the later instars.

Spongy moth larvae in the first instar stage. Photo credit: Bill McNee, Wisconsin Dept. of Natural Resources.  

During outbreaks, fifth-instar caterpillars can strip trees bare of their leaves by the end of June. The caterpillars also descend from trees and crawl on lawns. Their droppings can fill gutters and cover outdoor spaces.

Adult spongy moths do not feed. The adults live for about 2 weeks, solely to reproduce. Though they cannot fly, adult females have wings. Males can fly and have large, feathery antennae that pick up the scent of the female’s pheromones.

Spongy moth male and female, mating. 

Following mating, the female spongy moth lays between 600 and 1,000 eggs in a teardrop-shaped mass covered in light brown silk. Egg masses will be placed in a wide range of places, including out in the open, under bark flaps, on the sides of houses, under eaves, etc. Each female produces only one egg mass in her lifetime. Males mate multiple times. After mating, both the female and male moths die.

After laying eggs, female spongy moths die and drop to the ground. 

Outbreaks and response

At low population levels, the spongy moth poses little threat to trees and forests. A complex combination of controls, including predators and parasites, effectively keeps spongy moth populations low.

Occasionally, a spongy moth outbreak will occur. Examples of outbreaks in Connecticut include 1971 and 1981. In 1971, defoliation reached over 650,000 acres, while in 1981, that number reached 1.5 million acres. That was nearly 80% of the state's 1.9 million acres of forestland. A wide range of tree species were stripped clean, and the woods were turned bare due to the extensive feeding of these insects. Arborists were hard-pressed to meet the demands of an increasingly frantic public, as the caterpillars covered houses, roads, and sidewalks. The spongy moth caterpillars were even blamed for traffic accidents, as people would skid on their massed numbers in the roadways. 

Oak mortality caused by spongy moth and secondary stressors. Photo credit: Beth Bernard. 

Mast and mice: One population control that exists is the ecological interplay between mast production and mice. "Mast" refers to the seeds of forest trees, such as acorns, beech nuts, and hickory nuts. In the forest, mast is the primary food for a variety of wildlife, including white-footed mice (Peromyscus leucopus). When the spongy moth population is at a reduced level, these mice are able to control the moth population by feeding on larvae and pupae.

Mice eating spongy moth larvae.

When the mast crop declines, the mouse population declines as well. When this happens, the spongy moth population grows. 

Because the spongy moth is not a favored food of the white-footed mouse, a growing spongy moth population does not necessarily translate into an increase in the number of mice. Once the spongy moth population grows past a point where mice can keep it in check, it tends to grow rapidly until it reaches outbreak status.

NPV virus: A virus also controls spongy moth population levels. At low spongy moth population levels, a virus called NPV often exists at low levels within the moth population. Two to 3 years after the spongy moth population reaches outbreak level, the NPV virus spreads widely, infecting and killing spongy moth caterpillars in great numbers.

Past pesticide use: Early efforts to control spongy moth were notable for their aggressiveness, as well as their ultimate failure. These efforts encouraged reliance on pesticides, including DDT. Following World War II, DDT was widely used to curb the spread of the spongy moth. The goal was to "hold the line" at the Hudson River in order to prevent westward spread. The use of DDT was eventually prohibited in the United States.

Fungal control: Another factor in spongy moth population levels is the emergence of the maimaiga fungus (Entomophaga maimaiga), which significantly changed the spongy moth outbreak dynamic. This fungus is originally from Japan. It was released several times in the U.S. as a potential biological control agent, starting in 1910. After each trial, its effects on spongy moths appeared to be minimal. That is, until 1989. In that year, surprisingly, this fungus emerged from oblivion and immediately began killing spongy moth larvae on a wide scale.

The first known occurrence of this fungus’s effectiveness was in Connecticut. The spring of 1989 was exceptionally wet across the state, which led to the activation of the fungus. Since that year, the fungus re-emerges annually in Connecticut and persists in the soil and leaf litter as a resting spore. In spring, if rainfall is sufficient, the fungus is activated.

As they reach the third instar stage, spongy moth caterpillars spend a significant amount of time on the ground as they seek shelter during the hotter parts of the day. When the caterpillars descend to the ground, they become infected by the maimaiga fungus.

After infection, caterpillars begin to climb back up a tree before succumbing to the fungus, as seen in this video. As these dead caterpillars decompose on the tree, the fungus releases a second, wind-borne type of spore that spreads the disease even further among the surrounding spongy moth caterpillars. This allows the fungus to spread rapidly through the population.

Spongy moth caterpillars killed by the maimaiga fungus hang dead on tree bark, often in an inverted V shape.

At the end of the spongy moth season, the fungus produces a resting spore that accumulates in the soil and leaf litter. These resting spores remain viable in the soil and leaf litter for at least 10 years.

Precipitation is key: This cycle of infection by the maimaiga fungus has kept the spongy moth population in Connecticut generally at a low level. When precipitation is limited, outbreaks occur because the fungus lacks sufficient moisture to activate. For example, the drought in Connecticut in 2015 and 2016 triggered a multi-year outbreak that caused extensive defoliation in eastern Connecticut.

Secondary stressors: Significant oak mortality continued into 2018 due to the one-two punch of defoliation and drought having significantly weakened the trees. This set them up for further damage and even death from secondary pests and pathogens, such as the two-lined chestnut borer and the honey fungus, also known as shoestring root rot or amillaria fungus. Oak mortality from these secondary pathogens continued into 2019 and 2020. 

Another outbreak occurred in 2021 in northwestern Connecticut, centered around Sharon. This outbreak caused complete defoliation of up to 30,000 acres of oak, beech, and aspen, and heavy defoliation of red maple and birch. Spring and early summer rains in 2022 ended the outbreak.  

How to help

1. Learn more about what you can do to protect your trees from spongy moth in Spongy Moth: Information for Tree and Woodland Owners.
2. For forests, the best defense against pests like spongy moth is forest management that supports diverse species, age classes, and forest composition. For more information about the overall health of your forested land, contact your Service Forester or consult a private Connecticut Certified Forester.
3. If you own or manage urban or landscape trees affected by the spongy moth, contact a Licensed Arborist who can answer your questions.
4. Because the spongy moth is widespread in Connecticut, it is not necessary to report spongy moth sightings. You can still ask questions by contacting the CAES State Entomologist: CAES.StateEntomologist@ct.gov.    

Resources

Spongy Moth: Information for Tree and Woodland Owners 
CAES Spongy Moth Fact Sheet  
U.S.D.A. Forest Service Spongy Moth   
U.S.D.A. APHIS Spongy Moth   
Cornell Mushroom Blog: Entomophaga maimaiga

 

Content last updated June 2026.