It is common to observe various “cocoon-looking things” in trees. These structures are not always true cocoons, but rather silken nests or shelters crafted by insects.
Understanding Tree “Cocoon-Like” Structures
Many cocoon-like structures in trees are not true cocoons. A true cocoon is a silken casing spun by a moth larva (caterpillar) before it pupates, serving as a protective covering for the pupa. These structures can vary in material, color, and placement.
The structures observed in trees are more frequently communal silken nests or shelters built by groups of caterpillars. These structures serve multiple purposes, including protection from predators and harsh weather conditions. They also function as communal feeding sites, allowing caterpillars to feed within the web’s safety.
Identifying Common Types of Tree Nests and Shelters
Several insect types create distinctive silken structures in trees, each with unique characteristics.
Tent Caterpillars
Tent caterpillars construct prominent, tent-like silk nests. Nests are found in tree branch crotches or forks, especially in spring as leaves emerge. The caterpillars, often dark-bodied with a white stripe or dots, congregate within these nests for shelter, venturing out to feed. They enlarge nests as they grow, the silk providing protection from predators and temperature fluctuations.
Fall Webworms
Fall webworms create messy, expansive webs that envelop entire branches and their foliage, appearing from late summer into early fall. These webs, which can grow several feet across, contain caterpillars, excrement, and chewed leaf fragments. Unlike tent caterpillars, fall webworms feed inside their protective webbing, expanding it to enclose more leaves as they consume foliage. The caterpillars are hairy and can be yellowish-green with a black head or tan with a red head.
Bagworms
Bagworms construct distinctive, spindle-shaped bags that hang from branches. These bags are made of silk and bits of the host plant, camouflaging the caterpillar inside. The caterpillar carries its bag as it feeds, gradually enlarging it throughout the summer. In late summer, the bag is permanently attached to a twig for pupation. Bagworms are a significant pest, particularly for evergreens like arborvitae, cedar, and juniper, but they can also affect deciduous trees.
Spongy Moth Caterpillars
Spongy moth caterpillars do not build silken tents or nests in trees. They are characterized by hairy bodies with five pairs of blue dots followed by six pairs of red dots along their backs. While they do not create webs for shelter, young caterpillars can spin silken threads to “balloon” on wind currents, spreading to new locations. These caterpillars defoliate a wide variety of trees, especially oaks.
Assessing Impact and Deciding on Action
The impact of these insect structures on tree health varies by species, infestation severity, and tree health. Healthy mature trees often withstand defoliation without long-term damage, recovering by producing new leaves. Repeated or severe infestations can weaken trees, making them more susceptible to other stressors or diseases.
Evergreen trees are particularly vulnerable to bagworm damage, as they do not readily regrow needles and can suffer permanent harm or even death from heavy infestations. Deciduous trees, while often recovering from defoliation, may experience reduced growth or stress.
Intervention may be considered for young, recently planted, or high-value ornamental trees, or in cases of severe infestations. Small nests can be removed by hand, particularly early in the season when smaller. Bagworm bags, for instance, can be snipped off and destroyed. Pruning out branches containing smaller tent caterpillar or fall webworm nests can also be effective. Some “cocoon-looking things” might be from beneficial insects, so not all structures require removal.