Larvae (caterpillars) of various moth species are called webworms because of the profuse silk they form on foliage where they feed.
Webworms typically feed on silk-covered leaves near the terminals (tips) of branches. This helps to distinguish them from tent caterpillars, which usually web inner foliage around the juncture (crotch) of branches.
Cotoneaster webworm. Athrips rancidella (Gelechiidae) larvae feed during spring and summer, mostly on cotoneaster foliage in at least the San Francisco Bay Area. Larvae are dark brown and grow up to 1/2 inch long. They web together groups of leaves and skeletonize their underside while feeding within silken tubes on branches and foliage. Larvae overwinter within silk on plants and in the spring pupate and emerge as adults. The adult is a brown to dark-gray, night-active moth with a wingspan of less than 1/2 inch.
Cypress leaftier. Epinotia subviridis (Tortricidae) is a significant pest of ornamental arborvitae and cypress; it also feeds on junipers and often occurs with the cypress tipminer. The cypress webber on arborvitae, cedar, and cypress and the juniper webworm on junipers cause similar damage.
Cypress leaftier larvae are blackish pink and grow up to 1/2 inch long. They feed singly on foliage inside a cocoonlike shelter that includes gnawed leaves and twigs. Adults are mostly brown to gray moths with black and orangish markings.
Cypress webber. Hypsopygia (=Herculia) phoezalis (Pyralidae) feeds on various broadleaved shrubs and trees. It is a pest mostly on ornamental arborvitae, cedar, and cypress in southern California. The larvae are blackish, grow up to 3/4 inch long, and feed in a group in loosely webbed, silk tunnels. The adult is a mostly dark brown moth with a wingspan of about 1 inch.
Fall webworm. Hyphantria cunea feeds on more than 100 deciduous species of broadleaf ornamentals and fruit and nut trees. The hairy larvae in California populations are commonly gray, orangish, or yellowish brown and feed in a group on shoot terminals. They form increasingly larger, silken tents that become apparent relatively late in the growing season.
Juniper webworm. Dichomeris marginella (Gelechiidae) feeds on junipers. Larvae are mostly yellowish brown with a black head, prothoracic shield, and legs. Older instars have three, dark brown, lengthwise stripes. Young larvae tunnel inside needles and older instars web and chew externally on foliage and grow up to 3/5 inch long. Adults are 1/4 inch long with a wingspan of 2/3 inch. They are coppery-brown moths with a white, longitudinal band on the inner and outer margin of each forewing.
Mimosa webworm. Homadaula anisocentra (Galacticidae) is an occasional or sporadic pest in the Sacramento Valley. It feeds on mimosa (Albizia spp.), and less-often on honeylocust (Gleditsia triacanthos). Young larvae feed in groups covered with silk, causing foliage to turn brown and die. Larvae vary from blackish brown to gray to olive-green and have five, longitudinal, white stripes on the body. Larvae grow up to 2/5 inch long, feed singly, and pupate on hosts in a silken cocoon. The adults are gray to whitish moths with numerous black spots on the forewings and a wingspan of 1/2 inch.
Palo Verde webworm. Faculta inaequalis (Gelechiidae) occurs in Southern California and the Southwestern United States. It feeds on Parkinsonia spp., most commonly on foothills Palo Verde, P. microphylla. Larvae grow up to 1/2 inch long and feed in a group in or on silk tubes or rolled webbing. Adults are about 1/4 inch long and brown to gray moths with markings of black, orange, or both.
Webworms develop through four stages—egg, larva (caterpillar), pupa, and adult (moth). The time of year each life stage is present and the number of generations can vary by the location and species of host plant.
Cypress leaftier. Female moths lay pale, disklike eggs singly on the scalelike leaves of hosts. The emerging larvae feed singly in a cocoonlike nest and can be present most any time of the year. Larvae may periodically abandon their feeding nest and form a new nest on undamaged shoot tips. Most of the larval feeding and foliage browning occur during spring and early summer as larvae mature. Overwintering is as larvae, which pupate during late winter to spring. Adult females can be present and laying eggs anytime from March through September, mostly during April through June. There appears to be two, overlapping generations per year.
Cypress webber. Overwintering is as larvae, which pupate in the spring. By late spring, the adults emerge and mate and females lay eggs in a group on the terminal leaves of hosts. There is one generation per year.
Juniper webworm. Early instars tunnel and feed inside foliage. Older instars feed externally and overwinter in webbing on foliage. Larvae resume feeding in late winter and pupate by late spring. There is one generation per year.
Mimosa webworm. Larvae overwinter and pupate in a whitish cocoon on bark or in litter beneath trees. The adults emerge and mate and females lay eggs in spring. The mimosa webworm usually has two generations per year in California.
The caterpillars chew leaves and form silken tents or webbing on host trees. Their feeding causes foliage to turn brown, die, and drop prematurely. When abundant, larvae can consume all the leaves on a tree. Vigorous and otherwise-healthy plants generally tolerate extensive leaf chewing and moderate defoliation. Most webworm species are not abundant during most years and commonly their presence is innocuous.
Mature larvae of mimosa webworm drop from trees on a silk strand before pupating, When abundant, the strands annoy people and foul surfaces and vehicles under infested trees. Larvae move after dropping from hosts and pupate in white cocoons on buildings or other structures. Sometimes larvae enter homes to pupate.
Webworm abundance is sporadic in part because they are fed on by many parasites and predators. To preserve the effectiveness of these natural enemies, rely on pruning and selective insecticides if feasible when control action is warranted.
Regularly inspect host plants for silken webbing; prune out caterpillar-infested foliage if limited to small portions of the plant. If webbing containing live caterpillars is 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 the silk by the insecticide enhances control.
Bacillus thuringiensis kills only caterpillars (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.
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.
High populations of the cypress leaftier and juniper webworm may not be well-controlled by Bt or spinosad; these insecticides are not persistent and the webworms emerge and feed over an extended period. A broad-spectrum, residual (persistent) insecticide such as carbaryl can be applied in the spring when larvae and the egg-laying adults are present. Do not apply carbaryl if bees are present; carbaryl is very toxic to bees and most natural enemies. Because it poisons parasites and predators, applying carbaryl may increase the frequency or likelihood of webworms becoming abundant again later or cause a secondary outbreak of spider mites.
See Insects Affecting Ornamental Conifers in Southern California for more information. More photos and resources are available at BugGuide and Moth Photographers Group; enter the insect's name in the search box at the top of these webpages.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).