Mimosa webworm (Homadaula anisocentra, family Galacticidae) nests on honeylocusts (Gleditsia triacanthos, family Fabaceae) are becoming evident in southwest Ohio. Populations of this non-native moth appear highly localized and hit-or-miss thus far this season. However, where populations are high, the damage will progress until honeylocusts become brown.
The webworm moth’s common name references another member of the Fabaceae family, the mimosa tree (a.k.a. silk tree) (Albizia julibrissin). Although mimosa trees are considered the primary host, they are range-limited in Ohio. Thus, the webworms keep it all in the family and are more commonly found on honeylocusts.
Background: Traveling the Silk (Tree) Road
Mimosa was first brought to the U.S. from Asia as an ornamental in the mid-1700s. Mimosa webworm was accidentally introduced into the U.S. from China in the early 1940s. Contrary to some online references that claim the webworms were first found on honeylocust trees, a 1943 scientific paper described the webworm as a new pest of mimosa in the Washington, D.C. region.
Mimosa webworm continues to be found on its namesake host. However, a paper published in 1947 reported that the non-native moth had developed a taste for honeylocust. This paper also provided a hint that all honeylocust trees are not equal in the compound eyes of the mimosa webworm moth.
Once mimosa webworms jumped ship to utilize honeylocusts, the moths used their newfound host to spread across much of the eastern and Midwestern U.S. Their spread was aided by honeylocusts becoming a go-to tree to replace American elms (Ulmus americana) killed by Dutch elm disease.
Digging Deeper (Into Silk Nests)
Mimosa webworm caterpillars consume the upper or lower leaf epidermis along with the mesophyll. The remaining epidermis dehydrates and turns from brown to reddish-brown.
The caterpillars feed gregariously within sticky webs spun over the foliage. Dense nests are filled with small pellets of dark brown frass (= insect excrement). The caterpillars only feed on leaflets enveloped by their silk nests.
Attention is usually drawn to an infestation by clusters of "torched" leaves and leaflets. The leaves may be so tightly encased in webbing that the foliage looks like it’s melting.
The webworm moths have at least three generations per season in Ohio, with populations typically increasing with each generation. However, the caterpillars of each generation don’t wander forth to establish new nests. They stay at home to build new additions.
Research published in 1993 revealed that the caterpillars spread a water-soluble chemical on the webbing that stimulates female moths to lay eggs. Thus, females typically lay their eggs in the nests in which they developed. New eggs are silver-white and turn coral-red as they age.
First-generation nests are expanded by second- and third-generation caterpillars as a result of the females laying eggs on previous nests. This partially explains why the moths commonly fly below our radar until nests are fully expanded by the third generation and leaves turn brown.
However, webworm development is not always synchronized. The generations may slightly overlap, meaning that it's common to find relatively large caterpillars in nests containing small caterpillars. This is particularly true between the second and third generations.
First- and second-generation caterpillars pupate in the nests. Third-generation caterpillars vacate the nests by making controlled descents on silk threads so they can pupate in the soil. However, if the caterpillars of any generation deplete their food supply, they will also rappel from their nests to search for “greener pastures.”
Impacts
Mimosa webworms are generally considered an aesthetic as well as a nuisance pest on healthy, established trees. Torched leaves cemented together with sticky silk mar the appearance of heavily infested trees.
Rappelling caterpillars become repelling if they drop onto unsuspecting picnickers or into associated food and beverages (e.g., mimosa cocktails?). They can become a serious nuisance pest around backyard swimming pools where honeylocusts have long been a favored tree owing to their filtered shade, good branch structure, and small leaflets that minimize fall pool maintenance.
However, the vast majority of the damage occurs in mid-to-late summer after established trees have produced and stored enough carbohydrates through photosynthesis to support next season’s new growth. Despite the tree’s appearance, the caterpillars cause no significant harm to the overall health of healthy, established trees.
The impact may be different for newly planted trees as well as older trees planted in confined spaces, such as in "tree wells" or between streets and sidewalks, the so-called "devil's strip." The chronic stress associated with such locations places trees at high risk of failing to recover from heavy defoliation.
The added stress of a heavy mimosa webworm infestation may push the trees over the edge or make them susceptible to opportunistic borers such as the honeylocust borer (Agrilus difficilis). This is particularly true if webworm outbreaks occur during a drought year.
Management
Hosts with the Most
Research has revealed that there are distinct differences in host suitability among the thornless honeylocusts (G. triacanthos var. inermis). A paper published in 1990 showed females reared on 'Moraine' produced significantly fewer eggs compared to females reared on 'Imperial', 'Shademaster', 'Sunburst', and 'Skyline'.
‘Moraine’ honeylocust has been around since 1949; it was the first shade tree to be issued a patent (Plant Patent 836). It remains available in the nursery trade. Indeed, quoting Michael Dirr and Keith Warren in “The Tree Book” (2019, Timber Press): “Vase-shaped, with upward stretching and arching branches, it provides good clearance below, to 50’ tall, 35’ wide. The authors favor it, and nurseries should keep it in production.”
The Nature of Nature
According to a paper published in 1986, the environment also plays a key role in mimosa webworm population dynamics. Like their namesake host tree, overwintering mimosa webworm pupae have a low-temperature Achilles' heel. The 2014-15 winter polar vortex had a serious impact on the winter survival of mimosa webworm pupae in Ohio.
In fact, 2020 was the first season since the calamitous Polar Express that we saw a return of noticeable webworm damage in Ohio. However, Ohioans continue to enjoy only highly localized webworm populations, so the recovery is far from complete.
The 3-Ps
Although the mimosa webworm moth is a non-native, this exotic pest has been with us long enough to become targeted by predators, parasitoids, and pathogens (the 3-Ps). A paper published in the Great Lakes Entomologist in 1987 reported recovering nine parasitoids, including both flies and wasps, from overwintering pupae. A study conducted in Ames, IA, and published in 1990 found parasitism rates by the wasp, Elasmus albizziae, on first-generation mimosa webworm pre-pupae to range from 44% to 47% over three consecutive years.
Indeed, the pictures below shows a parasitoid wasp I found cavorting among early instar mimosa webworms. Its antlered antennae indicate this wasp belongs to the Family Eulophidae. Wasps in this family are ectoparasites, meaning they lay their eggs on the surface of their victims. The resulting wasp larvae bore a hole through the integument to zip in and out as they consume the victim's innards.
I took the following picture of a potter wasp (Parancistrocerus leionotus, family Vespidae) grabbing webworm caterpillars to provision their young. Potter wasps are so named for creating pot-like mud structures; however, this species only uses mud to fashion chambers in rock crevices.
The Insecticide Option
Insecticide applications may be required to protect vulnerable trees. Of course, as with using any pesticide, the label is the law. It’s mandatory to read and follow label directions.
Keep in mind that “general” insecticides such as pyrethroids (e.g., bifenthrin, permethrin, beta-cyflurin, etc.) are effective against mimosa webworms. However, they can kill bio-allies such as the aforementioned parasitoid wasps, which provide natural control of mimosa webworms. Also, pyrethroids are known to produce “secondary outbreaks” of spider mites by killing predacious mites.
Several biorational insecticides are effective against mimosa webworms and present minimal risk to non-target arthropods. Effective active ingredients include spinosad (e.g., Conserve), azadirachtin, chlorantraniliprole (e.g., Acelepryn), and indoxacarb (e.g., Provaunt WDG), which is specific to caterpillars.
Products based on the caterpillar-killing forms of the naturally occurring bacterium, Bacillus thuringiensis var. kurstaki (Btk) can also be effective, but thorough coverage is required because Bt is a stomach poison with no contact activity. Also, Btk is most effective against early instars.
Keep in mind that the dense webworm nests present a significant barrier to insecticide penetration. This is particularly true for second and third-generation nests.
Systemic insecticides circumvent this problem by delivering the active ingredient to the leaves. The systemic insecticide, acephate (e.g., Lepitect or Lepitect Infusible), is effective against mimosa webworms and also presents a lower risk to beneficials compared to topically applied standard insecticides. It is applied as soil drenches or trunk injections.
Selected References
Bastian, R. A., & Hart, E. R. (1990a). Honeylocust clonal effects on developmental biology of mimosa webworm (Lepidoptera: Plutellidae). J. Econ. Entomol, 83(2), 533-538.
https://academic.oup.com/jee/article-abstract/83/2/533/2215361?redirectedFrom=fulltext
Bastian, R. A., & Hart, E. R. (1990b). First-generation parasitism of the mimosa webworm (Lepidoptera: Plutellidae) by Elasmus albizziae (Hymenoptera: Eulophidae) in an urban forest. Environmental Entomology, 19(2), 409-414.
https://doi.org/10.1093/ee/19.2.409
Clarke, J. F. G. (1943). A new pest of Albizzia in the District of Columbia (Lepidoptera: Glyphterygidae). Proc. U.S. Nat. Mus. 93: 205-208.
https://repository.si.edu/handle/10088/16413
Hart, E. R., Miller, F. D., & Bastian, R. A. (1986). Tree Location And Winter Temperature Influence On Mimosa Webworm Populations In A Northern Urban Environment1. Arboriculture & Urban Forestry (AUF), 12(10), 237-240.
https://www.cabidigitallibrary.org/doi/full/10.5555/19860613722
North, R. C., Hart, E. R., & MingXian, L. (1993). Solvent deactivation of mimosa webworm larval webbing (Lepidoptera: Plutellidae). The Great Lakes Entomologist, 26(2), 4.
https://scholar.valpo.edu/cgi/viewcontent.cgi?article=1814&context=tgle
Miller, F. D., Cheetham, T., Bastian, R. A., & Hart, E. R. (1987). Parasites recovered from overwintering mimosa webworm, Homadaula anisocentra (Lepidoptera: Plutellidae). The Great Lakes Entomologist, 20(3), 7.
https://scholar.valpo.edu/tgle/vol20/iss3/7
Wester, H. V., & St. George, R. A. (1947). Life History and Control of the Web-worm, Homadaula albizziae. J. Econ. Entomol. 40:546-552.
https://academic.oup.com/jee/article-abstract/40/4/546/912973?redirectedFrom=PDF
