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A Tale of Two Caterpillars
by Clare Daughtrey | July 2025
Early one morning on a cool day in the middle of April, I found myself traversing the winding dirt trail of the Long Path heading north towards Stateline Lookout. As I glanced upwards, I noticed in the distance a dense white substance in a tree a few yards away. I traveled a bit off trail to get a better look, and found it to be the web tent of Malacosoma americanum, or the aptly named eastern tent caterpillar.
These caterpillars were first observed in the United States in 1646. Governor John Winthrop in the Massachusetts Bay Colony wrote in 1646:
Great harm was done in corn … in this month by a caterpillar, like a black worm about an inch and a half long … the churches kept a day of humiliation, and presently after the caterpillars vanished away.
They build their nests in the forks of smaller tree branches in the spring and early summer, and use them as a home base where they are well protected from predators. Caterpillars that hatch from the same egg mass will remain together to form a colony. In early mornings and late evenings, they will emerge from their tent to feed on leaves. As the larvae feed and grow, they increase the size of the web until it is a foot or more in length. In the summertime, caterpillars will eventually leave this tent to find a place to build their cocoons. Although the eastern tent caterpillar is a native species, they are considered a pest due to their destructive tendencies.
Caterpillars hatch in the spring as early as March. Young individuals are dark colored with two thin yellow stripes that run parallel along its back. Mature caterpillars have a white stripe in between the two yellow stripes, with the rest of the body a blotchy blue, and are about two inches long. Both young and adult caterpillars are covered in white or yellow defensive hairs. There is no visible difference between adult moths; both are light brown and fluffy, with two horizontal stripes on each wing.
Eastern tents aren’t the only caterpillar of note in the eastern United States. The more destructive Lymantria dispar, otherwise known as the spongy moth (formally known as the gypsy moth) has been a nuisance since the late nineteenth century. Native to Eurasia, the spongy moth had been a pest there long before its arrival in the United States.
Spongy month caterpillars can be observed in their larval phase beginning in early to mid-May. Young caterpillars are black and hairy, while older individuals are a grey or yellow color with bristly hairs. Unique to the spongy moth is a color pattern along their backs consisting of five pairs of blue dots succeeded by six pairs of red dots. Mature caterpillars are about 1 ½ - 2 ½ inches long. Adult moths can be observed beginning in July. Male moths are brown with a dark brown wing pattern, while female moths are white, larger than the males, and do not fly.
Spongy moths are now considered “naturalized,” meaning they will always be around in our ecosystem. Like most caterpillar species, including the eastern tent caterpillar, they undergo population increases and decreases in cycles. The frequency of outbreaks, or sharp spikes in number, varies from state to state but tends to happen every 10 - 15 years for the spongy moth and every 8 - 10 years for the eastern tent moth. Outbreaks end when predator and disease populations increase to levels where they can once again control the population. Caterpillar populations will in turn die down, and the cycle repeats.
Distribution of spongy moths happens through various means. The most common method is “hitchhiking,” in which female moths lay eggs on surfaces outdoors, such as furniture, firewood, and cars, and the eggs hatch in new places when these objects are moved. Each egg mass can contain up to 1,000 eggs and is covered with a yellowish hair-like substance. Eastern tent caterpillars are black, shiny masses that are generally laid on small twigs of their host plants where, unlike the spongy moths, they sit stationary while they overwinter. Each egg mass can contain approximately between 150 - 300 eggs.
The spongy moth is a rare case in which we know both how and where the species was introduced to North America. Frenchman Étienne Léopold Trouvelot was an astronomer, artist, and an amateur entomologist (one who studies insects). In the late 1860s, Trouvelot moved to Medford, Massachusetts, and there received a shipment of spongy moth worms from a colleague overseas. His objective? To create a hybrid silkworm by crossbreeding the spongy moth with the domestic silkmoth to create a more efficient worm for silk production. The traditional domestic silkmoth was struck with disease — while the Civil War affected the availability of cotton — leaving an opportunity for an entrepreneur to fill that need. Unfortunately, several specimens escaped from Trouvelot’s captivity, and thus, the trouble began.
Within ten years, the damage had been done. Trees in surrounding neighborhoods were suffering from severe defoliation, meaning they were partially or completely bare of leaves. A forest heavily infested with spongy moth caterpillars will look barren and almost winter-like during the warmer seasons. One factor that makes this caterpillar so destructive is that although the oak is its preferred host, it feeds on more than 300 species of shrubs and trees. If deciduous trees are limited or spongy moth populations are high, they will even consume coniferous trees such as spruce and pine.
Eastern tent caterpillars are also responsible for tree defoliation, but mature caterpillars who are nearing ready to spin a cocoon and pupate do not feed at all. These caterpillars cause the greatest concern when they begin their quest to pupate, and in large numbers can become quite the nuisance (their nests are also quite unsightly!). This species prefers rosaceous trees such as cherry, apple, and crabapple, but will also consume deciduous shrubs and trees if needed.
Defoliation is an issue because it can reduce the overall health of a tree, affecting photosynthesis and leaving it susceptible to further stressors such as drought, disease, and other pests. Fortunately, once the caterpillars become moths, a defoliated tree that is otherwise healthy will grow enough leaves to return to its initial state within a few weeks. If there are no other stressors present, deciduous trees can generally survive two to three consecutive seasons of defoliation. Because host trees are present throughout the United States, this means that all states are at risk for infestation. The spongy moth is responsible for the defoliation of millions of acres throughout the northeastern United States, and is also present in Illinois, Indiana, Michigan, Minnesota, Ohio, Virginia, West Virginia, and Wisconsin. The eastern tent caterpillar is also present throughout the eastern United States as well as further west into Texas and the Dakotas.
Fortunately, there are means to deal with the havoc wreaked by these pests. The most effective method of prevention is awareness — check for egg masses on trees and outdoor objects, and report them to state or federal agriculture officials if discovered. Self-inspection checklists are available online from the U.S. Department of Agriculture. Follow quarantine regulations in your area, if applicable. If you are unsure if you live in an active quarantine area, you can check the official U.S. Department of Agriculture website for a map of infected areas. Before traveling to and from an infested location, check your belongings for egg masses, caterpillars, and adult moths. Buy only local firewood and do not move it outside of the area in which it will be burned. Observe any local, state, or federal moth restrictions. Comply with agriculture officials surveying for the insects. Traps can also be placed around the trunk of a tree to prevent caterpillars from reaching the canopy.
If you come into contact with any spongy moth egg masses, scrape them off with a sharp tool, such as a knife or credit card. Dispose of egg masses and any other stages of life in a container of hot soapy water, or in a sealed plastic bag. For the eastern tent caterpillar, egg masses can also be removed. Their silk nests can be destroyed using a stick, but should never be burnt, as the fire will cause more damage to the tree than the caterpillars would.
The U.S. Department of Agriculture and other organizations are working to discern the extent of infestations throughout the country, eliminate populations, and reduce spread. By keeping an eye out for the spongy moth caterpillar and eastern tent caterpillar and taking action, you too can have an impact!
Citations & Further Reading
Eastern Tent Caterpillars:
History of New England Volume 2 by John Winthrop, 1630-1649
Eastern Tent Caterpillar by the University of Kentucky's Ric Bessin
Tent Caterpillars by the Maine Department of Conservation
Spongy Moths:
Spongy Moth Checklist, US Department of Agriculture, 2023
Spongy Moth Information by the US Animal and Plant Health Inspection Service
Spongy Moth Alien Forest Pest Explorer by Perdue University