You may have noticed glistening webs suspended in tree canopies all over Georgia. These webbed masses are nests of the fall webworm, one of our native insects. Webworm populations are high this year. Rather than having a few webs in a tree canopy here and there, we see canopies loaded with many webs, and some small trees losing all of their leaves.
Webworms live throughout the mainland United States and feed on over 400 tree species in forests, yards, and fruit and nut orchards. Host trees include pecan, hickory, walnut, persimmon, maple, sweet gum, river birch, etc. While they can be unattractive in ornamental settings, webworms do not cause major damage in non-commercial settings. Healthy deciduous trees can tolerate defoliation later in the year and will resprout new foliage in the spring.
Fall webworm may have up to 5 generations a year in the south, resulting in inactive webs on the same trees as fresh webs that are loaded with caterpillars. There is one species of webworm (Hyphantria cunea), and it has two color forms, the red-headed and black-headed webworm. Their name refers to the color of the caterpillar head capsule. Red-headed webworm adults are white with brown spots, and it is the species most often encountered in the southern U.S. Caterpillar color may vary, but in general, red-headed larvae are yellowish-tan and have long white hairs, orange bumps, and a dark stripe along their back.
In the spring females will lay egg masses on the lower surface of leaves. Caterpillars hatch and begin to feed on leaves. They develop the silken web that encloses branch ends and foliage. The webbing increases in size as the caterpillars grow and develop. Fall webworms prefer new foliage that is exposed to sunlight. The webs contain caterpillars, the foliage they feed on, and little black pellets of frass, or insect waste. More mature caterpillars leave the nest at dusk and feed on foliage, returning to the nest before daybreak, which explains the bare branches adjacent to the webbed masses.
Mature caterpillars leave the nest and spin cocoons in crevices in tree bark, under stones, or in the soil. Adults emerge, and the cycle begins again – up to four more times a year in the south.
Webworm populations are naturally controlled by native predators and parasites. Predators include birds, spiders, parasitic wasps, and small mammals. Usually when a minor pest has a major outbreak it is due to weather conditions that affect either the pest insect or their predators and parasites.
Webworms are usually not a serious pest in forests, but they can be problematic in nut- and fruit-bearing tree orchards. In the southeast they are usually most conspicuous in pecan trees. Webworms can be a nuisance in ornamental settings because the defoliation can be unsightly. While webworm damage may be disturbing to homeowners, healthy mature deciduous trees will recover from some defoliation this late in the summer. Smaller trees can be more susceptible to serious damage.
Often insect pests, including fall webworm, are not noticed until they begin causing damage. However, the best time to attempt pest control is when insects are younger and more susceptible, before damage is apparent. This is especially true for fall webworm, because the webbing acts as a partial barrier, protecting the caterpillars from insecticide sprays. Managing fall webworm most effectively involves routinely inspecting trees to detect the pest and act before it becomes a major problem.
Pruning out the webs is a possibility, but the tree will be left with unsightly gaps in the canopy. Another method to reduce nests is to remove them with a stick and destroy the caterpillars. Even just disrupting the web can be effective to make younger caterpillars more vulnerable to natural enemies. This method is possible for smaller trees but would be difficult for taller trees.
Insecticides can be used when the caterpillars are young. Control will be less successful as nest size increase, because penetration of the entire nest can be difficult. Current pesticide recommendations are available from your county Extension office and the UGA Georgia Pest Management Handbook. Be sure to carefully read the entire pesticide label before applying any pesticide.
Since healthy trees recover from webworm defoliation, the best approach is to leave webworms alone in non-commercial settings. Webworm activity can be observed as an interesting part of our native insect diversity. In addition, the webworm nests are an accessible opportunity for science education for children, so take the kiddos out in the yard and open up a nest.
Author: Elizabeth Benton
Forest Health Specialist