During summer and fall, large silken webs begin to appear and envelope tree branches. These webs, which contain larvae (also called caterpillars) and leaves, are created by a native defoliator called the fall webworm (Hyphantria cunea). Fall webworm caterpillars are capable of defoliating trees in natural and managed landscapes in both rural and urban locations throughout South Carolina. The caterpillars will eventually turn into small, unassuming white moths. Despite the nests being unsightly and alarming, they are unlikely to cause long-term damage to the tree.
Identification and Life Cycle
Adult fall webworms are white moths; some individuals may have black spots on the wings (Fig. 1). Adult females lay hundreds of eggs in masses on the underside of leaves during late spring to early summer (Fig. 2). The eggs hatch approximately one week later. Caterpillars are green to yellow to tan in color, with two parallel rows of black dots down their backs. They are also covered with fine hairs, which can be irritating to some people. The transition from caterpillars to pupae takes 4-8 weeks. Caterpillars spin thin cocoons and pupate in bark crevices, in leaf litter, on the forest floor, or just under the soil surface. There can be 2 to 5 generations per year in the southern United States.
Caterpillars are gregarious (meaning they tend to stay and feed together) and start to build silken webs as soon as they start feeding. Each web is likely made up of caterpillars from the same egg mass. In the beginning, webs are only large enough to encompass a single leaf (Fig. 3). As the caterpillars mature, their webs grow larger, envelop more leaves (until they cover the entire terminal branches), and become more conspicuous (Fig. 4). Larvae can regulate the temperature inside the webs, typically keeping it above 122 °F. As the webs grow larger, leaves encompassed by the webs are consumed, leaving only leaf veins (Fig. 5). In most cases, defoliation is limited to a branch or two, but in smaller trees, complete defoliation may occur if the entire tree is covered in webbing (Fig. 7). When the infestation is extraordinarily severe, the webs can spread to multiple parts of a tree and to surrounding trees (Fig. 6).
Fall webworms have been recorded feeding on over 300 hardwood tree and shrub species. The most common host species in the eastern United States are pecan, hickory, walnut, persimmon, maple, and sweetgum. In addition to their preference for certain species, other factors, such as the amount of foliage available on a tree and the abundance of a particular tree species in an area, also play a role in determining the tree species attacked and the severity of infestation. Adult females prefer to lay eggs where there is an abundance of new leaf growth and on tree species that are relatively common.
Management and Control
Fall webworms have not been reported to cause significant damage to forests or natural areas; however, they have much more pronounced effects on urban and orchard landscapes. Scouting is important to find webworm webs before they grow very large and consume a great deal of foliage. A common cultural control method is to remove the affected limb(s). If this is not feasible due to tree height or cosmetic issues, the web may be knocked down or torn open. Fall webworms have many natural enemies (e.g., birds, wasps, parasitoids, and diseases) that will consume or kill the caterpillars or pupae. These natural enemies can easily approach the caterpillars when the web, which provides a protective barrier, is torn open. In orchards, mass trapping using pheromones or mating disruption can be effective management methods; however, these tactics are costly and are best used in small, isolated areas where the effect on other moth species is limited. Many synthetic insecticides, as well as biorational pesticides such as Bacillus thuringiensis (Bt) formulations, provide adequate control. Spray timing is important because treatments need to occur soon after eggs hatch and before the webs become extensive. Any chemical applications must be done following label directions and should target foliage surrounding the web.