The conspicuous, spider-web-like structures draped across tree branches are generally not the work of spiders, but rather the collective effort of insect larvae, specifically caterpillars. These silken nests are a telltale sign of communal moth larvae, which use the webbing as a protective shelter while they feed on the tree’s foliage. The alarming appearance of these “tents” often leads to the mistaken belief that the tree is under severe attack. This phenomenon is a natural occurrence, most often involving two distinct species of native moth, each with its own preferred season and nesting style.
Identifying the Primary Culprits
The two most common architects of these large silken structures are the Eastern Tent Caterpillar (Malacosoma americanum) and the Fall Webworm (Hyphantria cunea), both of which are the larval stages of moths. Eastern Tent Caterpillars are active in the early spring, coinciding with the initial leaf-out of trees. They are characterized by dark bodies with a distinct white stripe running down the center of their backs, growing up to two inches long. They have sparse, light-colored hairs, and the adult moth is reddish-brown with two oblique whitish stripes across its forewings.
The Fall Webworm is a late-season pest, appearing from mid-summer into the fall. Its larvae have a fuzzier appearance due to long, silky hairs. Their body color varies from pale yellow to green or brown, often featuring a broad dark stripe down the back and yellowish stripes along the sides, and they grow to about one to one-and-a-half inches. The adult moth is typically pure white, sometimes featuring small dark spots. The Eastern Tent Caterpillar favors trees in the rose family (e.g., wild cherry, apple), while the Fall Webworm is polyphagous, feeding on over 600 species of deciduous trees, including pecan and walnut.
Distinguishing the Different Web Structures
Identifying the insect responsible often depends on the season, the nest location, and the web’s physical appearance. Eastern Tent Caterpillars build their nests in the spring, placing them in the crotches or forks of branches, closer to the main trunk. These tents are densely woven with a thick, cottony texture, serving as a communal shelter where the caterpillars congregate during the day or in poor weather. The caterpillars emerge to feed on nearby leaves, leaving a silk trail to guide them back.
The Fall Webworm constructs its nests later in the year (mid-July through September) specifically at the tips or ends of branches. Their webs are typically thinner, more loosely woven, and messier, often enclosing the foliage they are actively feeding on. Unlike the Eastern Tent Caterpillar, Fall Webworm larvae remain within the tent as they feed, continually expanding the webbing to encompass new leaves. True spiders, such as the sheet web spider, create smaller, flatter webs that do not enclose entire sections of foliage.
Impact on Tree Health
The main concern resulting from these infestations is defoliation, which occurs as the larvae consume the tree’s leaves. For healthy, established trees, a single season of defoliation by either species is usually not a serious health concern. The tree typically produces a new flush of leaves and recovers quickly, especially since Eastern Tent Caterpillar defoliation happens early in the spring. The damage is often considered more of an aesthetic issue because the large, unsightly webs and loss of leaves can be alarming.
Damage becomes genuinely harmful when a tree experiences repeated, heavy defoliation over several consecutive years. This stress depletes the tree’s energy reserves, weakening its defenses and making it vulnerable to other pests, diseases, or environmental stressors like drought. Young, newly planted, or already stressed trees are the most susceptible to significant damage, as they have fewer energy reserves to cope with foliage loss. In these cases, control measures are necessary to ensure the tree’s long-term survival.
Safe Removal and Management
Management of these web-making caterpillars usually involves simple, non-chemical methods targeting the physical structures. The most effective approach for both species is physical removal, which involves pruning the branch holding the nest and destroying the entire structure. Using a long stick or pole to wrap up the silken web and pull it out is an alternative for nests that are out of reach or on branches that should not be pruned. Once removed, the nest and caterpillars should be dropped into soapy water or sealed in a bag for disposal.
Timing the removal is important to ensure the maximum number of caterpillars are captured within the web. Eastern Tent Caterpillars typically retreat to their thick, centralized tents at night, on cloudy days, or during cool weather, making these the ideal times for removal. Fall Webworms remain inside their nests as they feed, so removal can be done anytime the nest is visible.
Chemical control, such as using an insecticide containing Bacillus thuringiensis var. kurstaki (Btk), is an organic option. Btk is most effective when applied to very young caterpillars that have just hatched. For healthy trees, natural predators like birds and parasitic wasps often keep the caterpillar populations in check, making intervention unnecessary in most instances.