Mimosa Webworm

Classification

Common name: mimosa webworm

Scientific name: Homadaula anisocentra Meyrick, 1922

Family: Galacticidae (no common name)

Order: Lepidoptera (moths and butterflies)

Description

Mature caterpillars are up to 1 inch (25 mm) long, grayish to dark brown and have five white stripes running the length of the body (Figure 1). The head is brown and the body is slender. They are very active and, if disturbed, will wriggle and thrash and lower themselves on silk strands. The pupae are 6 mm long, yellowish-brown, and are found within whitish silken cocoons.

Figure 1. Mimosa webworm caterpillar. Photograph by Joe Boggs via iNaturalist, used under a CC BY-NC 4.0 license.

Adult mimosa webworms are steel-gray moths with small black dots on their forewings (Figure 2). Adults have a 13 mm wingspan. They may be confused for other gray, spotted moths but careful examination of the spot patterns and wing shape can distinguish them (Figure 3).

Figure 2. Adult mimosa webworm moth. Note the steel-grey color and pattern of the black spots on the wings. Photograph by stevewalternature via iNaturalist, used under a CC BY-NC 4.0 license.

Figure 3. Adult American ermine moth, which might be confused for a mimosa webworm moth. However, note the white (instead of steel grey) coloration, different pattern of black spots on the wings, and different shape of the wings. Photograph by Nikolett Tóth via iNaturalist, used under a CC BY-NC 4.0 license.

The eggs are very small, oval, and white but turn a rose color just before hatching (Figure 4).

Figure 4. Eggs (the round, pink structures) and young caterpillars. Note the webbing and scrapped appearance of the leaves where the caterpillars have fed. Photograph by Joe Boggs via iNaturalist, used under a CC BY-NC 4.0 license.

Range

Mimosa webworms are native to China and Japan. They were introduced into North America in the 1940s and first detected in Washington, D.C. They have since spread throughout the eastern United States and Canada as far west as Texas and Nebraska and can be found wherever host trees are present.

Hosts

Mimosa webworms feed on the leaves of Persian silk trees (Albizia julibrissin) and honey locust trees (Gleditsia triacanthos).

Persian silk trees are also called mimosa trees, although this name should be avoided as it is applied to other species of Albizia, as well as plants in the genus Mimosa, which are not closely related to Albizia. Persian silk trees are frequently planted as ornamental trees due to their fine leaves, attractive summer flowers, and spreading crown. However, Persian silk trees are an aggressive invasive species in the United States, so their continued planting as ornamentals is discouraged.

Honey locusts are a native species that range throughout the central United States as far north and east as central Pennsylvania. Wild-type plants are generally covered in long thorns that are thought to be a defense against large, extinct ice age herbivores (Figure 5). However, rare wild trees are thornless, and these were developed into commercial varieties that were first released publically in 1949. Since then, thornless honey locusts have been popular street and urban trees in many areas of the country (Figure 6). Unfortunately, thornless varieties are often extremely susceptible to mimosa webworm feeding.

Figure 5. Wild type honey locusts are usually covered in long thorns. Photograph by lanechaffin via iNaturalist, in the public domain.

Figure 6. Thornless honey locusts planted as ornamental trees in an urban setting. Photograph by Shirley Zundell via iNaturalist, used under a CC BY 4.0 license.

Life History

Mimosa webworms have two generations per year in Pennsylvania. First generation adults emerge in early to mid-June and they may live for several weeks. Females lay eggs on the terminal leaves and small twigs as well as on webs from the previous year (Figure 4). Larvae hatch in mid- to late June and immediately begin to feed on the lower surface of leaflets. Mature larvae from the first generation pupate in mid-July and the second generation of adults appear from late July through mid-August. Second generation larvae are active during August and early September. Larvae from this generation may be so abundant that they web leaves throughout the entire crown of the tree causing it to look completely brown. Pupae from this second generation overwinter inside cocoons within the web or in to bark crevices, house siding, mortar joints, or other protected areas.

Damage

As larvae feed, they spin a web around leaflets and continue to feed within this protected area. The foliage appears skeletonized, turns brown, and then may die. If left alone, an infestation may progress and completely defoliate a tree by early September. Often an entire plant may be webbed by September. Mature larvae are sometimes a nuisance as they silk down and get into homes and other dwellings in search of a place to overwinter.

Management

While it may be aesthetically unpleasing, otherwise healthy trees can withstand one or even a few years of total defoliation so one option is to do nothing. However, care must be taken to make sure a tree is healthy as in combination of other stressors, such as drought stress, caterpillar feeding can damage long-term health and even kill trees. Urban trees planted in "tree wells" between paved surfaced and recently transplanted trees can be particularly susceptible to feeding damage.

Thornless cultivars of honey locust are very susceptible to attack by this pest. The cultivar 'Sunburst' is especially vulnerable, while 'Moraine', 'Shademaster', and 'Imperial' are less susceptible. When planting new trees, avoid susceptible varieties or at least avoid planting large stands of them.

Keeping leaf debris and webbed foliage cleaned up from beneath and around host trees can help reduce the local webworm population and lessen the need for future interventions.

Extensive plantings of susceptible thornless honey locust may be defoliated to the point that chemical management is warranted. Various parasitoid wasps attack mimosa webworm caterpillars and the extensive webbing protects the caterpillars from topical pesticides, so broad-spectrum spray pesticides should be avoided. Bt and other biorational insecticides may be applied when the caterpillars are small (mid- to late June for the first generation, August for the second generation) and the webbing is not extensive. Systemic insecticides can be applied as a soil drench or as a trunk spray or injection, although the latter options are restricted to use by licensed pesticide applicators.

Additional photos and information

Mimosa Webworm on Honeylocusts

Mimosa Webworm: It's Time for "Torched" Honeylocusts

Written by Barbara Knupp and Gregory A. Hoover, Sr. Extension Associate, 2012; Reviewed and revised by Michael Skvarla, 2023.