Webworms are the larval stage of moths that create large, silken nests over the foliage of trees and shrubs. The most commonly encountered pest is the Fall Webworm (Hyphantria cunea), a native insect found across North America. These caterpillars are noted for their communal feeding habits and the expansive, loosely woven tents they construct on the terminal branches of host plants. Understanding their biology and behavior is the initial step toward effectively managing these late-season defoliators.
Identifying Webworm Larvae and Adults
The adult Fall Webworm moth is a small insect with a wingspan of about 1 to 1.5 inches. Its wings are typically snowy white, though some regional variations may exhibit dark spots on the forewings. These moths emerge from their overwintering pupae in the spring and early summer to mate and lay eggs.
The larvae are the stage responsible for the visible damage and webbing, growing to approximately 1 inch in length. They are conspicuously hairy, with bodies that vary in color from pale yellow or greenish-yellow to a darker gray or brown. Larvae possess groups of long, silky hairs arising from small projections along their bodies, and their head capsules can be either black or reddish-orange, distinguishing two common races.
It is important to differentiate these pests from the Eastern Tent Caterpillar, which also creates silken structures. Webworm nests are typically found at the tips of branches in the late summer and fall, with the caterpillars feeding inside the messier web. Conversely, the Eastern Tent Caterpillar creates a denser, more defined silk tent in the crotches of branches during the spring, and the larvae leave the protection of the tent to feed.
Life Cycle and Web Formation
The life cycle begins when adult female moths deposit masses of several hundred eggs on the undersides of host plant leaves, often covering the cluster with hairs from their own abdomen for protection. These eggs hatch in about a week, and the young larvae immediately begin their communal feeding and web-spinning activities. The caterpillars remain inside the silken enclosure for four to eight weeks as they grow, molting several times.
The large, tent-like silk web serves as a protective shelter for the developing colony of larvae. This communal structure shields the soft-bodied caterpillars from natural enemies, such as birds and parasitic wasps. As the larvae consume the leaves within the nest, they continuously expand the web to incorporate fresh foliage, resulting in the distinctive, increasingly large, and unsightly nests seen at branch ends.
After reaching their full size, the mature larvae abandon the web and crawl down the tree to seek a sheltered location for pupation. They will spin a flimsy cocoon in leaf litter, under loose bark, or in the soil, where they may overwinter as pupae until the following spring. Depending on the geographic location, the Fall Webworm may complete two to four generations per year, with the later generations in late summer and early fall typically producing the largest and most noticeable webs.
Impact on Host Plants
Webworms feed primarily on the leaves of deciduous trees, with over 100 species serving as hosts, including hickory, pecan, walnut, fruit trees, maple, and sweetgum. The caterpillars consume the leaf tissue, often leaving behind only the larger veins and midribs. This feeding damage, known as defoliation, is highly visible because the nests can grow up to three feet across and often appear on the most noticeable outer branches.
While the appearance of large, silk-covered branches can cause concern, the damage is generally considered a cosmetic issue for mature, established trees. The most extensive feeding occurs late in the growing season, typically from late summer into early fall. By this time, the host tree has completed most of its annual growth and stored adequate energy reserves for the winter, meaning defoliation rarely causes long-term harm or tree mortality.
Repeated, heavy infestations over several consecutive years, especially on young or stressed trees, can reduce the tree’s vigor. In these situations, the pests can make the host more susceptible to environmental stressors, diseases, or secondary insect infestations. However, in most residential and landscape settings, the primary issue is the aesthetic disturbance caused by the messy, leaf-filled webs.
Management and Removal Methods
The most effective method for managing webworms, especially for small infestations on accessible branches, is physical removal. Using a pole pruner, the entire webbed branch can be clipped off and destroyed. For higher branches, a stick or long pole can be used to tear open the silken webbing, which immediately exposes the communal colony of larvae to natural predators and the elements.
Encouraging the webworm’s natural enemies provides biological control, as numerous species of parasitic wasps, predatory insects, and birds prey on the larvae. Tearing open the webs significantly increases the vulnerability of the caterpillars to these beneficial organisms. In cases where webs are numerous or too high to reach, chemical treatments may be considered, but should be applied when the larvae are small and the webs are just beginning to form.
Insecticides containing Bacillus thuringiensis (Bt), a naturally occurring bacterium, are recommended for control because they specifically target and paralyze the digestive system of caterpillars. Sprays must penetrate the web to reach the feeding larvae; alternatively, products like Spinosad are also effective against the caterpillars. The product must be targeted directly onto the foliage that the caterpillars are actively consuming within the protected silk nest.