Fall webworm—Hyphantria cunea
Larvae (caterpillars) of this moth (Erebidae: Arctiinae) make prominent, silk tents on shoot terminals. The caterpillars can feed on more than 100 species of woody broadleaves, including birch, cottonwood, elm, fruit and nut trees, liquidambar, maple, mulberry, poplar, sycamore, and willow.
Larvae of various species are called webworms because of the profuse, silk webbing they excrete on foliage where they feed. Fall webworm, mimosa webworm, and others typically occur in webbed colonies on leaves and twigs at the terminal (outer tip) of branches. This helps to distinguish them from tent caterpillars, which usually web inner foliage around the juncture (crotch) of branches.
Fall webworm larvae grow up to 1 inch long and vary greatly in color. In California, they commonly are gray, orangish, or yellowish brown and have black, orange, or white hairs arising from black or orange projections (tubercles) along the body.
The adult (moth) has a wingspan of about 1-1/2 inches and a hairy body that is white on top. The forewings are all white or white with brown or black spots. The hind wings (hidden when at rest) and abdomen are orangish.
Adult moths emerge in the late spring or early summer. In June or July the females lay globular, white or yellow eggs in large masses on host foliage. About 1 week after they are laid, eggs hatch and the larvae feed in groups as they form and gradually enlarge silken tents during summer through early fall.
Fall webworms overwinter in dark-brown to reddish cocoons on the host bark or the ground. In California there are one or two generations per year.
From July to September, fall webworm caterpillars eat leaves (except not the veins) and form silken tents on host trees. The caterpillars do not feed on fruit or nuts, but these may become sunburned and drop prematurely if defoliation is severe. Yield quantity may be reduced the season after fruit or nut trees are extensively defoliated.
Regularly inspect hosts for silken tents during the summer. On small trees, cut out and destroy infested twigs (e.g., use a pole saw, a blade on the end of a telescoping pole). Alternatively, remove webworm tents and the caterpillars inside by twirling or scrubbing the webbing into a ball using a toilet brush or similar tool attached to a telescoping pole, then dispose of or crush the larvae.
If nests are abundant and cannot be pruned off or tolerated, apply Bacillus thuringiensis (Bt) or spinosad with a high-pressure sprayer to penetrate webbed foliage. The addition of a wetting agent to increase penetration of webbing by the insecticide enhances control.
Bacillus thuringiensis kills only butterfly and moth larvae that feed on sprayed foliage. A second application of Bt about 7 to 10 days after the first is recommended because of its short persistence. Using this selective insecticide avoids adversely affecting the parasitic tachinid flies and wasps that kill webworm larvae and pupae and Trichogramma wasps that parasitize the eggs.
The somewhat-selective spinosad can adversely affect bees and certain natural enemies. Because spinosad is toxic to bees for several hours after the spray has dried, do not apply it to plants that are flowering.
If caterpillar-infested trees are large or tall, hiring a pest control company with the equipment and experience to effectively spray trees can be a good idea. Discuss in advance with the company how they plan to control your pest problem. Request they apply caterpillar-selective Bt, or spinosad if the plants to be sprayed are not blossoming.
See BugGuide from Iowa State University and Moth Photographers Group from Mississippi State University for more information.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).