What comes to mind when someone asks, “What’s happening with the locusts?”. Is the person asking about black locust trees (Robinia pseudoacacia) or honeylocust (Gleditsia triacanthos)? Maybe they’re asking about swarming grasshoppers (family Acrididae), or they’re confusing cicadas (family Cicadidae) with locusts. Of course, their question is clear if they ask about Buckeyes.
Currently, Mimosa Webworm (Homadaula anisocentra, family Galacticidae) nests are causing honeylocusts (Gleditsia triacanthos) to appear burnt orange in central Ohio. Honeylocust is considered an alternate host of this non-native moth in much of the U.S.; however, it’s the primary host in Ohio where mimosa trees (a.k.a. silk trees) (Albizia julibrissin) are somewhat rare.
“Blazing” black locusts are also beginning to appear along highways in various parts of Ohio. However, the culprit is the Locust Leafminer (Odontota dorsalis). The leafminer doesn’t infest honeylocusts and mimosa webworm doesn’t infest black locusts. Locust leafminer populations rise and fall dramatically from year to year, and highly localized pockets have developed this season in Ohio, but that’s another Alert.
Mimosa webworm caterpillars feed gregariously as skeletonizers within sticky webs spun over the foliage; they only feed on leaflets enveloped by their silk nests. Attention is usually drawn to an infestation by clusters of orangish-brown "torched" leaves and leaflets that are so tightly encased in webbing that the foliage looks like it’s melting.
Thus far, mimosa webworm populations appear to be highly localized. For example, Dave Shetlar (a.k.a. “The Bug Doc”) has reported that webworms are common in and around Hilliard, a western suburb of Columbus, OH. Curtis Young (OSU Extension, Van Wert County) is finding high populations in the northwest part of the state and showed participants pictures of nests with eggs and early instar caterpillars during our weekly BYGL Zoom Inservice today.
Traveling the Silk (Tree) Road
Mimosa (a.k.a. silk tree) (Albizia julibrissin) 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 the go-to tree to replace American elms (Ulmus americana) killed by Dutch elm disease.
Digging Deeper (Into Silk Nests)
Mimosa webworms have at least three generations per season in Ohio with moth 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 silverish-white eggs on the nests from which they developed. New eggs are silverish-white and turn coral-red as they age.
The overall result of females laying eggs on previous nests is that first-generation nests are expanded by second and third-generation caterpillars. This partially explains why the moths commonly fly below our radar until leaves turn brown as second and third-generation caterpillars enlarge the nests.
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 and the moths emerge from the nests. Third-generation caterpillars vacate the nests by making controlled descents on silk threads so they can pupate in the soil. However, first, second, and third-generation caterpillars may also rappel from their nests to search for “greener pastures” if they deplete their food supply.
Impact
Mimosa webworms are generally considered an aesthetic as well as a nuisance pest on healthy, established trees. Torched leaves cemented together with sticky silk mars 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 acquired 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." Such locations may subject trees to high heat coupled with inadequate moisture to the root system.
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 terms of 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 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 only enjoy 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 Lake Entomologist in 1987 reported nine parasitoids including both flies and wasps were recovered 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 picture 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 pictures 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. However, topical applications present two challenges. First, general insecticides such as pyrethroids (e.g., bifenthrin, permethrin, etc.) can kill the bio-allies such as the aforementioned parasitoid wasps that provide natural control of mimosa webworms.
The second challenge is the dense webworm nests present a significant barrier to insecticide penetration. This is particularly true for second and third-generation nests.
Topical biorational insecticides that present minimal risk to beneficials are effective; however, they should be applied to early first-generation nests before the webbing becomes too dense. Effective active ingredients include spinosad (e.g., Conserve), azadiractin, and chlorantraniliprole (e.g., Acelepryn). Products based on the caterpillar-killing forms of the naturally occurring bacterium, Bacillus thuringiensis (Bt) can also be effective; however, thorough coverage is required because Bt is a stomach poison with no contact activity.
Systemic insecticides also present a lower risk to beneficial insects. The neonicotinoids clothianidin (e.g., Arena 50WDG), dinotefuran (e.g., Safari, Transect, etc.), and acetamiprid (e.g., TriStar) are effective against these caterpillars. Applications should follow label directions relative to soil drenches or trunk sprays. Acephate (e.g., Lepitect or Lepitect Infusible) applied as soil drenches or trunk injections is also effective.
Selected References (Listed by Publication Date)
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
Webster, H. V., and F. A. St. George. 1947. Life history and control of the webworm, Homadaula albizziae. J. Econ. Entomol. 40:546-552.
https://academic.oup.com/jee/article-abstract/40/4/546/912973?redirectedFrom=PDF
Hart, E.R., Miller, F.J. and Bastian, R.A., 1986. Tree location and winter temperature influence on mimosa webworm populations in a northern urban environment.
https://www.cabidigitallibrary.org/doi/full/10.5555/19860613722
Miller, F. D.; Cheetham, T.; Bastian, R. A.; and Hart, E. R. 1987. "Parasites Recovered From Overwintering Mimosa Webworm, Homadaula Anisocentra (Lepidoptera: Plutellidae)," The Great Lakes Entomologist, vol 20 (3) Available at: https://scholar.valpo.edu/tgle/vol20/iss3/7
Bastian, R.A., and E.R. Hart. 1990. First-Generation Parasitism of the Mimosa Webworm (Lepidoptera: Plutellidae) by Elasmus albizziae (Hymenoptera: Eulophidae) in an Urban Forest, Environmental Entomology, Volume 19, Issue 2, 1 April 1990, Pages 409–414 https://doi.org/10.1093/ee/19.2.409
Bastian, R. A., and E. R. Hart. 1990b. Honeylocust clonal effect on developmental biology of mimosa webworm (Lepidoptera: Plutellidae). J. Econ. Entomol. 83: 533-538.
https://academic.oup.com/jee/article-abstract/83/2/533/2215361?redirectedFrom=fulltext
North, R.C., E.R. Hart, and L. MingXian. 1993. Solvent Deactivation of Mimosa Webworm Larval Webbing (Lepidoptera: Plutellidae). The Great Lakes Entomologists, Vol. 26 (2): 113-119.
https://scholar.valpo.edu/cgi/viewcontent.cgi?article=1814&context=tgle
