Fall Webworms

SCIENTIFIC NAME

Hyphantria cunea

APPEARANCE

Adult coloration varies from completely white to white with black spots. In general, fall webworms in the northern portion of their range are all white, while those in the southern part of their range are white with dark spots. Their wingspan is about 3 1/2 inches.

The female moth lays an egg mass that is covered by white hairs that detach from the female's body and contain several hundred yellowish eggs. Young larvae have two sets of black markings down their bodies and are pale yellow. When fully mature, larvae are about an inch long, covered with white hairs that emerge from black and orange "wart" looking structures on the larvae's back. Larvae can also be distinguished by their black or red heads. If a web is disturbed, all larvae in the web go into a jerking-type of movement, thought to be a defensive mechanism to scare off predators.

Fall webworm webs are spun by the larvae and cover the foliage at the end of a tree or shrub branch. An important way to distinguish fall webworms from other so called tent-making defoliating moths is the caterpillars begin building their webs at the ends of the branches, not from the crotch of the tree. Each web may contain several hundred larvae, leaf fragments, larval cast skins and feces.

BEHAVIOR, DIET & HABITS

Fall webworm larvae feed on almost any kind of tree, except conifers. Their preferred hosts include mulberry, hickory, walnut, sweetgum, willow, oak, poplar, ash, apple and other fruit trees. Fall webworms are more of an aesthetic nuisance than a threat to their host's long-term health. They develop via complete metamorphosis, meaning they have four very different life stages – egg, larva (grub), pupa (cocoon) and adult. Fall webworms overwinter in the pupal stage under ground litter, in cracks and crevices and in the soil.

Adult moths first show up in the spring and summer months, depending on their location, mate and the females lay masses of eggs on the underside of leaves. About one week after egg laying, larvae hatch, begin making a silk web over the leaves they are feeding on and enlarge the web as they grow and continue to consume more and more foliage. Although the pupae leave the web to molt, it is common for the larval web to remain in the tree during the winter months. Fall webworms generally do not kill trees since their maximum defoliation takes place shortly before trees drop their leaves in the fall and become dormant.

REPRODUCTION

Southern populations of fall webworms may complete four generations per year, while in the north they will usually complete only one or two generations per year.

SIGNS OF AN INFESTATION

Appearance of webs located on the outside edges of the tree's branches.

DISTRIBUTION

Fall webworms are found throughout the United States.

OTHER INFORMATION

Since fall webworms may impact both the health and impact and appearance of ornamental trees, at times control efforts may be required. Therefore, it may be prudent to have an inspection and management recommendations provided by your pest management professional (PMP). Among some of the management options are:

  • Do nothing since in most cases tree damage is limited primarily to affecting only the appearance of the tree, not the death of the tree.
  • Removing and destroying the portions of tree branches that are affected by the insect's web.
  • Contacting your PMP for recommendations about chemical and biological insecticide control.