Eastern tent caterpillar: what the extension services record

This page describes what the named sources say. It is not a diagnosis of any tree and does not say what should be done with one. The eastern tent caterpillar (Malacosoma americanum) is a native North American insect, and none of the three pages read lists it as regulated or notifiable. Each section names its source, all read on October 9, 2026. The insect has a row in insect pests of trees, and its look-alike in late summer has its own page, fall webworm.

Tents, and how they differ from fall webworm

Penn State Extension says the caterpillars are best known for the silk tents they build together in the crotches and forks of branches in spring and early summer. It sets these apart from fall webworm tents, which sit at branch tips in late summer and fall. NC State Extension makes the same point from the other side: the caterpillar that builds nests in trees later in summer is the fall webworm. The Morton Arboretum says the caterpillars feed away from the tent by day and return to it for protection on rainy days and at night, and that colonies of 200 to 300 rest in the tents at night or on cloudy, rainy days.

Caterpillars, cocoons and moths as described

Penn State describes young caterpillars as dark with two thin yellow stripes along the back, and mature ones as about two inches long with a white median stripe flanked by two yellow stripes on a body mottled blue, yellow and black. It says they can be mistaken for forest tent caterpillars, which show a row of white dots on the back in place of a solid line and build no silk tents. NC State gives almost 2 inches and the Arboretum up to 2 1/2 inches, with white stripes bordered by pale blue spots. Penn State and NC State agree the cocoon is white or yellowish-white silk, spun in dry, protected places. The Arboretum gives about 1 inch, with a dark brown pupa inside that remains for roughly two weeks, and says cocoons are formed on tree trunks, fence posts and other hard structures. The moths are tan to dark brown with two oblique white or pale stripes on the forewings.

Egg masses

Penn State says the eggs are black masses wrapping small twigs of host plants, and that they overwinter as eggs. NC State says the eggs are laid in masses around twigs under a foamy secretion that dries into a firm brown covering, and that the larvae develop inside but do not leave the mass until the following spring. The Arboretum describes each mass as about 1/2 inch long, oval, shiny and brown to gray, with as many as 300 eggs, and Penn State gives 150 to 350. Penn State also notes that the smooth, shiny egg mass can be compared with black knot fungus, which is rough and knotty, on the same black cherry tree.

Hosts

The sources agree on the rose family. Penn State says the caterpillars preferentially feed on cherry, crabapple and apple and occasionally on other deciduous shrubs and trees. NC State names black cherry, crabapple and apple as the usual hosts, with many other trees occasionally infested. The Arboretum says rose-family trees are the primary hosts, but that a number of ornamental tree species can be attacked. Because the caterpillars hatch as buds break, NC State and Penn State both say a heavily fed tree can look as if it never leafed out.

Life cycle

Penn State and NC State give one generation a year. Penn State says caterpillars hatch in early spring and mature in early summer, when they leave the tent to pupate; it adds that wandering mature caterpillars stop feeding and no longer threaten plants. Adults emerge in late June and July and lay the egg masses, according to Penn State, and by late June according to the Arboretum. The Arboretum says caterpillars leave the tree by the end of May.

Range and outbreaks

Penn State, on a page updated September 8, 2025, says the insect has been observed in the United States since 1646 and that outbreaks frequently occur at eight to ten year intervals. It places the range across eastern North America: the northeastern United States and neighbouring Canada, south to Florida and west as far as the Dakotas and Texas, with scattered records in Colorado and Montana. It describes the species as native and attacked by many natural enemies, so that outbreaks are periodic and short-lived, and it considers damage generally aesthetic rather than a health threat to most trees. The Arboretum says severe or total loss of leaves can leave a tree weaker and open to other insects and diseases.

Animal and human health notes

Penn State says the caterpillars are covered in long defensive hairs that may irritate the eyes or mouth. NC State says pregnant mares that eat the caterpillars may abort, a condition it calls mare reproductive loss syndrome. This page gives no medical or veterinary advice: the sources are the place to read these notes in full.

Regulated status and reporting

None of the three pages names a regulation, quarantine or report line for the insect. This page gives no treatment steps, product names or rates. For who may apply pesticides see pesticide licences.