The structures people commonly describe as “cocoons” hanging in trees are rarely the true pupal stage of an insect. Instead, these visible objects are typically protective shelters, nests, or egg masses constructed by various moth larvae or other insects. These structures are built from silk, plant material, or a hardened, foamy substance, serving the purpose of defense, temperature regulation, or overwintering. Identifying the specific type of structure is the first step toward understanding the insect responsible, its life cycle, and its potential impact on the host tree.
Large Communal Silken Tents
Large, conspicuous silken tents appearing in the forks of tree branches or enveloping the ends of limbs are the communal nests of certain species of caterpillars. The Eastern Tent Caterpillar, for instance, constructs a dense, silvery-gray web in the crotches of branches, often in cherry or apple trees, noticeable in early spring. These social caterpillars reside together in the tent for shelter and protection from predators and inclement weather. The silk helps them regulate their body temperature; they congregate on the outer layers to warm up in the sun, which is necessary for digestion during the cool spring months.
Another frequent builder of these large webs is the Fall Webworm, but its tents are typically found at the tips of branches and appear later in the growing season. Unlike the Eastern Tent Caterpillar, which leaves the tent to feed, the Fall Webworm expands its nest to completely enclose the leaves it consumes. Both species of larvae lay down silk trails marked with pheromones when foraging for food. While these communal nests can defoliate portions of a tree, most healthy deciduous trees are able to recover and produce a new flush of leaves later in the season.
Individual Hanging Bags
The structures that most closely resemble a traditional hanging sack or bag are made by Bagworms. The Bagworm larva creates an individual, spindle-shaped case constructed from silk and camouflaged with small pieces of leaves, twigs, and bark from its host plant. This unique design allows the larva to carry its home with it as it moves and feeds, with only its head and front legs emerging from the top opening. As the caterpillar grows, it continuously enlarges the bag, which can reach up to two inches in length when fully mature.
The life cycle of the Bagworm features a distinct sexual dimorphism. The adult male emerges from its bag as a winged moth, but the adult female remains a wingless, legless, caterpillar-like creature permanently encased inside her bag. After mating, the female lays hundreds of eggs within the protective bag she built as a larva and then dies. The eggs overwinter there until they hatch the following spring. Bagworms are particularly damaging to evergreen trees like juniper and arborvitae because evergreens cannot easily replace the needles consumed, and severe, repeated infestations can result in the death of entire branches or the whole tree.
Other Common Tree Structures That Resemble Cocoons
Not all cocoon-like objects on trees are silken nests constructed by moths. For instance, the Praying Mantis lays its eggs in a large, hardened, light brown mass called an ootheca, often attached to twigs or stems. This structure is initially a frothy secretion that quickly hardens into a dense, foam-like shell. The shell insulates and protects the eggs throughout the winter until the nymphs hatch in the spring.
Another common sight is the egg mass of the Spongy Moth, which appears as a fuzzy, tan or buff-colored patch, usually found on tree bark or under branches. This mass is a collection of hundreds of eggs covered with hairs from the female moth’s abdomen. Unlike the silken tents, this is strictly a non-moving egg repository and not a larval shelter. A third type of structure often mistaken for an insect case is a gall, triggered by an insect, mite, or fungus. The insect or mite develops inside the plant material itself, not a separate, constructed cocoon.