The springtime sight of small, worm-like creatures suspended from tree branches by fine, silken threads is a common occurrence across North America. This phenomenon is a shared behavior among the larval stages of several different types of moths, not just a single species. These caterpillars drop from the canopy, often in large numbers, creating a temporary annoyance for anyone walking beneath the trees. Understanding the identity of these aerial acrobats and the purpose of their thread is the first step in addressing their presence in the landscape.
Identifying the Dangling Larvae
The majority of the caterpillars that hang from trees belong to one of two main groups. The most frequent culprits are Cankerworms, commonly referred to as inchworms or loopers. These larvae have a distinctive method of movement, appearing to measure their path by arching their middle body into a loop due to having fewer prolegs (fleshy, unjointed legs) in the middle of their abdomen.
Cankerworms, which include both spring and fall varieties, are typically slender and can range in color from light green to a dark greenish-brown or nearly black. They grow to about one inch in length and begin feeding early in the spring as tree buds open.
A more easily identifiable dangler is the Spongy Moth larva, an invasive species with a distinct appearance. Spongy Moth caterpillars have a noticeable pattern of colored bumps running down their back: five pairs of blue dots followed by six pairs of red dots. These larvae are covered in fine, bristly hairs and can reach up to 2.5 inches long when fully grown.
The Mechanics of Hanging: Why They Drop
The ability of these larvae to hang mid-air is due to their production of silk, which is extruded from a spinneret located on the underside of the head. This silk is produced in modified salivary glands and hardens upon contact with the air to form a durable thread. The process of dangling serves two primary biological functions: defense and dispersal.
When a caterpillar senses vibrations from a predator, such as a predatory wasp or ant, it will instantly drop from the branch as an evasion technique. The silk thread acts as a safety line, allowing the larva to hang suspended until the immediate danger has passed. Once the threat is gone, the caterpillar uses its thoracic legs to climb back up the silk line and resume feeding.
Dispersal, often called ballooning, is a common behavior, especially for the newly hatched larvae of species like the Spongy Moth. These tiny caterpillars spin a long silk thread that catches the wind, allowing them to drift to a new host tree, sometimes traveling significant distances. This wind-aided transport is a primary mechanism for the spread of these defoliating pests across a landscape.
Impact on Trees and Management Strategies
The feeding activity of these defoliating caterpillars, particularly Cankerworms and Spongy Moths, can cause significant damage to the canopy during springtime, resulting in skeletonized leaves or complete defoliation. While a single year of complete defoliation rarely kills a healthy, mature tree, it forces the tree to expend energy reserves to produce a second flush of leaves. Repeated defoliation over two or three successive years can severely weaken or even kill a tree, especially if it is already stressed by drought or disease.
A management strategy for Cankerworms is tree banding, which targets the adult female moth before she lays eggs. Because the female moths of both spring and fall cankerworms are wingless, they must crawl up the tree trunk to reach the canopy. Applying a sticky band around the trunk in the fall and refreshing it in the spring traps the females, preventing them from laying eggs in the upper branches.
A biological control option is the application of Bacillus thuringiensis subspecies kurstaki (Btk), a naturally occurring soil bacterium. Btk is a selective insecticide that must be ingested by the caterpillars while they are small, usually within the first two weeks of feeding. It is largely harmless to beneficial insects, pets, and humans. For widespread or severe outbreaks, consulting a certified arborist for professional application ensures proper timing and coverage.