Caterpillars typically feed in groups, chewing small holes in leaves, and progress to feeding from the outer edge of the leaf toward the center. They rest on branches and trunks during the day, but when populations are dense, they feed continually day and night until the tree is stripped. They do not make or inhabit tents or webs.

When populations are high, entire forests may be defoliated, but trees are rarely killed unless they are already in a weakened condition. However, consecutive years of defoliation make them more susceptible to insect and disease problems, which can potentially lead to death.

The spongy moth goes through four stages of development – egg, larvae (caterpillar), pupa, and moth. It has one generation a year. The insect overwinters as eggs. They are laid in a single mass. Each mass contains up to 1,000 eggs. The masses are about 1 1/2 inches long and covered with velvety, buff or cream-colored hairs from the abdomen of the female moth. The eggs hatch in spring, usually in May in northern Illinois. Each of the tiny larvae (caterpillars) spins a silken thread suspended from a leaf, where winds can disperse the larvae several hundred feet, spreading the population.

The mature caterpillars are up to 2 1/2 inches long, dark in color, with long hairs along the body.  They are distinctly marked by five pairs of blue bumps and six pairs of red bumps along their back. About mid-June, the full-grown caterpillars stop feeding and find a sheltered place to pupate.

In early July, the adult male moth will emerge from its pupal case and begin searching for the females. The female moth also emerges from her pupal case but is unable to fly. The moths do not feed in the adult stage.

The male moth is brown with black markings and has a wingspan of 1 1/2 inches. The female moth has a wingspan of 2 to 2 1/2 inches and is white or cream-colored with black, wavy markings on the wings. Male moths are attracted to a mating pheromone that the female moths release. Once the eggs are fertilized, the female moth lays her egg mass under loose bark, in woodpiles, on outdoor furniture, or any other concealed location. The moths die and the eggs do not hatch until the following spring.

Cultural management:

Examine outdoor furniture, firewood, and vehicles (even the wheel wells), for pupae and egg masses. Scrape egg masses from their location, remove other life stages by hand, and soak them in a container of warm soapy water overnight. The hairs of the spongy moth can cause allergic (skin rashes) or respiratory reactions. Wear gloves, protective clothing, and a dust mask when handling.

Community and governmental agencies can release pheromone flakes. These flakes mimic the natural mating pheromone released by the female moth and can be used to confuse the male moths and disrupt mating.

Biological management:

The spongy moth can be attacked by various predators, parasites, and pathogens. There is a nucleopolyhedrosis virus (NPV) that is present in some areas and it can kill the caterpillar. The caterpillar ingests the virus and it ruptures the caterpillars cells, resulting in death. The dead caterpillars can be seen hanging on the tree in an inverted-V position.

Around 1990, the fungus Entomophaga maimaiga was documented in North America. This fungus can infect and kill caterpillars. This is more common in wet weather in early spring.  Caterpillars killed by this fungus can be seen on trees, head down, the body thin and shriveled.

Chemical management:

There are insecticides that can be used to treat spongy moth caterpillars. On large trees, treatment is best done by certified arborists. Spongy moth caterpillars are most effectively treated when they are very small and they are often found in the higher branches of the tree at that time. The NPV virus is produced as a treatment for spongy moth caterpillars, but is generally available only to programs sponsored by the USDA Forest Service. This product is very specific to spongy moth and does not kill other insects. The bacterial insecticide known as Btk (Bacillus thuringiensis var. kurstaki) is also sprayed to kill spongy moth caterpillars. This insecticide targets caterpillars only, but that does include caterpillars of beneficial insects that may be out feeding at the same time. The product has a short period of activity, usually less than a week.

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For more information, contact The Morton Arboretum Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).