Fall Webworm

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Pest Identification & Damage

Common Name(s): Fall Webworm

Scientific Name: Hyphantria cunea (Drury), Lepidoptera: Family Arctiidae.

Identifying Characteristics for Damaging Stage(s):

The white to pale yellow, fuzzy, black-spotted larva stage is damaging because the worms are equipped with chewing mouthparts, enabling them to strip green tissue from leaves. The worms are usually present in large numbers and living in a protective silken web.

Damage/Nature and Symptoms:

Leaves are skeletonized and stripped by feeding larvae. Typically damage occurs to one or two branches that become webbed together.

Distribution and Life Cycle

Distribution:

This common insect is generally distributed throughout the U.S., southern Canada and Mexico. It is one of few American insects that have gone the other way, i.e. were accidentally introduced into Europe, where it is also now a common pest. It infests over a hundred different species of trees and shrubs, including fruit trees and ornamentals.

Summary of Life Cycle:

This insect winters in the pupa stage, above ground, but below protective debris, old leaves and other fallen materials. Webworm moths are white, with black spots and emerge beginning the following spring, but extending into mid-summer. Moths mate and females deposit large egg masses on terminal leaves of individual branches. Larvae which emerge from eggs produce some light webbing which is the beginning of their protective "sack", which may hold as many as 400 feeding caterpillars. As the worms grow in size, the web becomes larger and may cover two or more individual branches. The web is very messy and contains cast caterpillar skins and large amounts of dark-colored excrement pellets, or "frass". Larvae feed for 4 - 6 weeks, before dropping to the soil surface to pupate. In the South, up to 4 generations may occur and probably just 2 occur in the midwest. Larvae have the unusual habit of twitching in unison when they are threatened, a habit that supposedly frightens away predators and parasites.

Management Methods:

Inspection/Survey Methods:

Periodically examine tree branches for evidence of infestation, including webbing and fuzzy caterpillars.

Non-Chemical Management Strategies:

If easily accessible, simply strip webbing and worms from infested branches and knock them to the ground. Few will find their way back to feed on the tree. Remember that the loss of the leaves of a few branches is not likely to seriously harm the tree. Webs are a nuisance and are not attractive to look at, and these may be pruned from the tree as well.

Chemical Management Strategies:

Nearly any insecticide that is approved for use on trees and shrubs should easily control the larvae by simply wetting the webbing with a spray mixture.

For the latest information on available chemical controls, consult the Pesticide Selection Guide.

References:

Metcalf, C. L., W. P. Flint and R. L. Metcalf, 1962. Destructive and Useful Insects. 4th Ed.: McGraw-Hill Book Co., New York NY. 1087 p.