The nests of first-generation Mimosa Webworm (Homadaula anisocentra, family Galacticidae) are now becoming evident on honeylocust (Gleditsia triacanthos) in southwest Ohio. Honeylocust is considered the 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 rare.
NOTE: mimosa webworms will not infest black locust trees (Robinia pseudoacacia). “Flaming” black locusts are produced by a native beetle called the Locust Leafminer (Odontota dorsalis). Conversely, locust leafminer beetles will not infest honeylocust trees.
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. However, that perception may change as successive generations of webworms expand their nests.
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 three generations per season in Ohio with moth populations typically increasing with each generation. Currently, the webworms in the southwest part of the state are transitioning between the first and second generations with most nests containing pupal cocoons.
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 dispense a water-soluble chemical onto the webbing that stimulates female moths to lay eggs. So, females lay their silverish-white eggs on the nests from which they developed. New eggs are silverish-white and turn coral-red as they age.
Consequently, 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 nests are enlarged by the second and third-generation caterpillars.
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.
Mimosa Webworm Impact
Mimosa webworms are generally considered an aesthetic as well as a nuisance pest problem on healthy, established trees. Torched leaves cemented together with sticky silk mares 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.
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.
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: 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 widespread webworm damage. It appears the relatively mild winters recently enjoyed by Ohioans are allowing localized webworm populations to continue to build in some parts of the state.
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 in Wyoming, OH. 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. However, topical applications are not generally recommended for two reasons. First, they will kill the bio-allies such as the aforementioned parasitoid wasps that provide natural control of mimosa webworms. Second, dense webworm nests present a significant barrier to insecticide penetration. This is particularly true for second and third-generation nests.
Systemic insecticides 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 drench or trunk sprays. Acephate (e.g., Lepitect or Lepitect Infusible) applied as soil drenches or trunk injections is also effective.
It appears we are beginning to transition from first to second-generation webworms in southern Ohio. This means if the overarching goal is to protect vulnerable trees by preventing leaf loss, the battle is rapidly becoming lost. Arguably, using an insecticide this late in the game would mostly be a "feel good" application. Tree care professionals should note the infestations in the records they keep on their clients so trees can be closely monitored next year to target first-generation webworms. After all, a localized outbreak this season does not mean a repeat next season.