The appearance of silken nests in trees often signals a problem requiring immediate attention. While the webbing and defoliation can look alarming, the infestation is rarely caused by true silkworms (Bombyx mori), which are domesticated and do not aggressively infest trees. The visible damage is typically the work of common native moth caterpillars that construct protective communal nests while feeding on leaves. Taking prompt, targeted action is the most effective way to manage these pests and ensure your trees remain vibrant.
Identifying the Real Pest
The caterpillars responsible for tree webbing are almost always one of two common species: the Eastern Tent Caterpillar (Malacosoma americanum) or the Fall Webworm (Hyphantria cunea). Correctly identifying them is the first step toward effective removal, as they are distinguished primarily by the timing of their activity and the location of their nests.
Eastern Tent Caterpillars are active in the early spring, emerging as tree leaves begin to bud. Their characteristic tents are dense, tightly woven silk structures built in the crotches or forks of branches near the main trunk. The caterpillars leave the nest during the day to feed, returning to the silk structure for shelter at night or during inclement weather.
Fall Webworms appear later in the growing season, typically from mid-summer through the fall. These caterpillars spin a looser, dirtier-looking web that always encloses the ends of branches and the foliage they are feeding on. Unlike the tent caterpillar, the larvae remain and feed entirely within the protective structure of their web.
Immediate Physical Removal
For smaller trees or accessible nests, physical removal offers the most immediate, chemical-free control method. The goal is to remove the entire silk nest along with the caterpillars inside. Pruning the affected branch is effective if the nest is on a small, non-essential branch, which is often the case with Fall Webworm infestations at the tips of limbs.
When pruning is not an option, especially for Eastern Tent Caterpillars nested in major branch crotches, the web can be manually pulled out or wound around a long stick. The best time to perform this removal is in the early morning or late evening, as the caterpillars are typically gathered inside their nests during these cooler periods. Once removed, the nest must be destroyed, usually by dropping the mass into a bucket of soapy water for at least 48 hours before disposal.
Care should be taken to avoid tearing the web and allowing the caterpillars to escape onto the tree trunk or ground, where they can survive and re-infest. Never attempt to burn the nests while they are attached to the tree, as this practice causes severe damage to the tree’s bark and underlying tissue. For nests that are too high to reach, a strong jet of water from a hose can sometimes break apart the webbing, exposing the caterpillars to natural predators.
Biological and Targeted Chemical Control
When physical removal is impractical due to the size of the tree or the number of webs, targeted application methods are necessary. Biological control is a recommended approach, primarily using products containing the naturally occurring bacterium Bacillus thuringiensis subspecies kurstaki (Btk). Btk is a selective insecticide that must be ingested by the caterpillar to be effective, posing little threat to beneficial insects like bees, birds, or other wildlife.
The timing of Btk application is important because it is most effective on small, young larvae and degrades quickly in sunlight. Since the caterpillars must eat the treated foliage, the spray must be applied thoroughly to the leaves around the nest when the larvae are actively feeding. Because Btk takes several days to be lethal, repeat applications may be necessary, particularly if rain washes the product away.
Chemical insecticides, such as those containing spinosad or pyrethrins, should be considered a last resort for heavy, inaccessible infestations. These products are generally more broad-spectrum than Btk and can harm beneficial insects, so they must be applied with caution. They are most effective when the caterpillars are young and actively feeding outside of a dense web. Any chemical application should follow the product label instructions precisely to ensure effectiveness and safety.
Long-Term Monitoring and Prevention
Effective long-term management focuses on monitoring for signs of future infestations and maintaining overall tree health. A proactive step is to inspect trees for egg masses during the dormant season, from late fall through early spring. Eastern Tent Caterpillars lay their eggs in shiny, dark brown or black masses that encircle small twigs, resembling a hardened foam or shell.
These masses can be scraped off the twig with a dull knife or sandpaper and destroyed by soaking them in soapy water. This action eliminates hundreds of potential caterpillars before they hatch in the spring, significantly reducing the pest population. Fall Webworms generally overwinter as pupae in the soil or leaf litter, making their egg masses harder to find and remove.
Beyond targeting the pests directly, maintaining a healthy tree is a form of prevention. Trees that are properly watered, fertilized, and pruned are less susceptible to severe damage from defoliating pests. Annual inspections during the key hatching periods—early spring for tent caterpillars and mid-summer for webworms—allow for the earliest detection and intervention, which is the easiest and most successful time to manage an infestation.