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The Spongy Moth

ALERT!!! Caterpillars have seemingly appeared out of nowhere and spread like wildfire across the eastern United States, tormenting innocent young children with their fuzzy bodies and spindly legs that cause rashes and itchiness. You may have seen them crawling around, eating leaves, or even defying the laws of gravity by floating through the air.

Schools around the country, specifically in the northeastern portion of the United States, have issued warnings to their students about the dangers these small insects pose including rashes, welts, and irritated patches of redness. Some schools have gone as far as spraying pesticides across school grounds to prevent the caterpillars from reproducing. Despite these efforts, the spongy caterpillars persist and continue to haunt children and adults alike.

History & Characteristics

Like many other caterpillar species, spongy caterpillars eventually turn into butterflies or moths. These moths are referred to as “spongy moths” because they lay sponge-like egg masses. Originally being a native species of France, the Lymantria dispar caterpillars were first introduced to North America in 1869. Their rapid spread across the US combined with the significant damage they cause to plants and animals alike have prompted scientists to classify these caterpillars as an invasive species.

Although they cause extensive damage, these tiny, black caterpillars are not particularly special. They only grow to about 2.5 inches and their only unique trait is being excessively hairy. Their hairs, also called setae, typically carry histamines which cause itchy rashes when they come into contact with human skin. Hundreds of cases of severe rashes have been reported across the United States as a result of contact with the histamines on the hairs of the caterpillars, sometimes lasting up to two weeks. Disturbingly enough, setae tend to stick to articles of clothing which can cause prolonged allergic reactions to the histamines.

Life Cycle

Egg Stage

The life cycle of the spongy moth, sometimes referred to as the gypsy moth, begins in late summer when female moths lay their eggs. These eggs are deposited in tan-colored masses that resemble fuzzy patches, often found on tree trunks, branches, rocks, and even man-made structures like fences and outdoor furniture. Each egg mass contains between 500 and 1,000 eggs and is covered with a protective layer of hair-like setae from the female’s abdomen. This covering provides insulation and protection from predators and harsh winter conditions.

During the egg stage, the spongy moth’s impact on humans and the environment is relatively low, but this period is crucial for controlling the population. This is also the time when people should check their outdoor furniture for eggs because, if left unchecked, these egg masses can quickly turn into thousands of caterpillars covering every square inch of your back deck.

Caterpillar Stage

This is the stage that most people are familiar with. People typically fail to notice caterpillar eggs, but as temperatures rise in late April/early May and the eggs hatch into larvae, they become much more recognizable.

When the larvae first hatch, they are small and dark-colored but eventually develop distinctive blue and red spots along their backs with long hairs protruding from their bodies. Mature spongy caterpillars can reach lengths of 2.5 inches (6.35 cm) and are easily identifiable by their striking coloration and hairy bodies.

The larval stage is the most destructive phase of the spongy moth’s life cycle. These larvae have voracious appetites and feed on the leaves of over 500 different tree species, usually preferring oak, maple, birch, or willow.

During heavy infestations, these caterpillars will sometimes completely defoliate trees, causing significant stress and making them more susceptible to disease, other pests, and environmental stressors. In some cases, repeated defoliation can lead to tree mortality, altering forest compositions and reducing biodiversity. Additionally, the caterpillars are a nuisance in residential areas, often covering trees, patios, and outdoor furniture. On top of this, they rudely leave behind frass (insect droppings) and partially eaten leaves for residents to clean up.

Pupal Stage

After feeding for 6–8 weeks, the larvae enter the pupal stage, usually in late June/early July. The caterpillars spin silk cocoons in sheltered locations (i.e. under tree bark, leaf litter, man-made structures). Inside these cocoons, they undergo a transformation to pupae: hardened, dark brown, cylindrical pods.

During the pupal stage, the spongy moth is immobile and does not feed, therefore having no direct environmental impact. However, this stage is critical for the insect’s development, as it undergoes metamorphosis, changing from a destructive caterpillar into an adult capable of reproducing. The pupal stage lasts 10-14 days, providing a brief window for targeted control measures such as the application of biological insecticides or physical removal.

Adult Stage

In mid to late summer, adult moths emerge from their cocoons. Male spongy moths are brown, with feather-like antennae and a wingspan of about 1.5 inches (3.8 cm). They are strong fliers and capable of traveling significant distances to locate potential mates. Female spongy moths are larger, with a wingspan of up to 2 inches (5 cm), and white with black markings. Unlike males, females are flightless and rely on their appearance and pheromones to attract mates. The primary goal of the adult stage of the spongy moth is reproduction. Typically, the adult moths live no more than a week.

While adults do not feed and therefore do not cause direct damage to vegetation, their role in reproduction ensures the continuation of the species and the increases the potential risk of future infestations. After mating, females lay their eggs, often in the same location as the previous generation, and begin the life cycle anew. This can lead to cyclical population explosions and subsequent defoliation events which have long-term impacts on forest health and ecosystem stability.

Fortunately, these terrible outbreaks do not occur every year. In fact, populations vary greatly during the spongy moth life cycle, with some years seeing very minor damage and others experiencing heavy damage due to large infestations of caterpillars causing significant leaf damage and tree defoliation. This cycle repeats every 10–15 years with the outbreaks typically ceasing by natural means, like disease and predators. But, don’t get too happy, this doesn’t mean they aren’t an issue — they will continue to wreak havoc on our environment if we let them be.

What Can You Do?

Encountering spongy caterpillars can be an unsettling experience, especially given their potential to cause significant environmental damage. However, there are several steps you can take individually or within your community to reduce their impact on the environment. The methods described below address reducing spongy moth populations throughout their entire life cycle although there are other methods that target specific stages.

Manual Removal

Firstly, manual removal of spongy moth egg masses is a highly effective method in controlling their population. During fall and winter, egg masses can be found on tree trunks, branches, outdoor furniture, and other surfaces. Using a scraper or a stiff brush, these egg masses can be carefully scraped off into a container filled with soapy water to kill the eggs. It’s important not to simply crush the egg masses or drop them on the ground, as this can allow some of the eggs to survive and hatch in the spring.

Biological Controls

Secondly, biological controls are an effective, but more involved, way of managing spongy moth populations. One effective biological control is the application of Bacillus thuringiensis var. kurstaki (Btk), a naturally occurring bacterium that targets caterpillars. When ingested by spongy moth larvae, Btk disrupts their digestive systems, causing them to stop feeding and eventually die. Btk is selective and poses minimal risk to humans, pets, and other wildlife, making it a suitable option for environmentally-friendly pest control.

Protecting Predators

Moreover, promoting and protecting natural predators of the spongy moth can contribute to controlling their numbers. Birds, small mammals, and certain insect species prey on spongy moths at various stages of their life cycle.

Some of their predators are mice, chipmunks, shrews, voles, skunks, raccoons, and squirrels. Bird predators include orioles, starlings, robins, crows, black-capped chickadees, nuthatches and blue jays. However, the recent population explosions of the spongy moth have made it difficult for their predators to keep their populations in check. Encourage these predators to hunt spongy caterpillars by helping maintain diverse and healthy habitats for them!

Spreading Awareness

Finally, public awareness and community involvement are crucial elements in the fight against spongy moth infestations. Educating neighbors, schools, and local organizations about identifying and managing spongy moths can give rise to coordinated efforts that are more effective than individual actions. Communities can organize egg mass scraping events, fundraise to purchase organisms for biological control, and work together to implement other preventative measures.

By taking these steps, individuals and communities can do their part in reducing the impact of spongy moths on the environment. While it may seem impossible to eradicate them completely, concerted, persistent efforts can help protect forests, maintain biodiversity, and minimize the damage caused by this invasive species.



The spongy moth, scientifically known as Lymantria dispar, is an invasive species introduced to North America in 1869 that has caused significant environmental damage since the 1880s. These insects emerge periodically in large numbers, defoliating trees through excessive leaf consumption and causing histamine-induced rashes in humans.

To mitigate their environmental impact, individuals can manually remove egg masses, use biological controls like Btk, and help natural predators like birds and small mammals thrive. Community awareness and involvement are crucial in coordinating efforts to manage spongy moth infestations — there’s always something you can do to help!

Stay updated and active by following the Environmental Defense Initiative on Medium and our social media platforms!

Author: Maggie Yang

Editor: Emma Mazzotta


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