Damage by non-native Mimosa Webworms (Homadaula anisocentra, family Galacticidae) was a topic of discussion during this week's BYGLive! Zoom Inservice. Despite their common name, mimosa webworms are most often found in Ohio on honeylocusts (Gleditsia triacanthos).
Dave Shetlar (Professor Emeritus, OSU Entomology) reported that he is picking up a third flight of adult moths in his light trap in central Ohio. These moths will initiate the third generation. He also noted that moths made sudden appearances on two separate occasions earlier this season which clearly defined the first two generations. Curtis Young (OSU Extension, Van Wert County) reported high localized populations in northwest Ohio. Brad Bonham (The Garden Fairy, Wyoming, OH) noted that browned clusters of leaves can be seen on honeylocusts in the city of Wyoming in the southwest part of the state.
This non-native nest-maker was accidentally introduced into the U.S. from China in the early 1940s. The first infestations were found on honeylocust in Washington, D.C. landscapes that had been planted to replace American elms killed by Dutch elm disease. Since that time, the webworm has spread across much of the eastern and Midwestern U.S.
There are three generations per season in Ohio as indicated by Dave's light trap catches and other observations. 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 larvae pupate in the nests with moth emerging from the nests.
The caterpillars vacate nests in the fall to construct cocoons in protected locations where they spend the winter. Pupation occurs in the spring with adults appearing in late spring. This is considered a "summer pest" with first-generation caterpillars appearing sometime in June.
The caterpillars feed gregariously as skeletonizers within 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 tightly encased in webbing.
Female moths lay silverish-white eggs that turn coral-pink. The eggs are often laid on or near the nests from which the females developed. Consequently, first-generation nests are expanded by second and third-generation caterpillars. This partially explains why the moths may fly below our radar until leaves damaged by the first generation caterpillars turn brown and large nests produced by the second and third generation become evident.
It's common for localized population trajectories to slowly rise year-after-year until peaking in an "outbreak" and then collapsing. Infestations may not return for several years. For this reason, the depredations of mimosa webworms alone is not considered sufficient to kill established trees. On healthy honeylocust trees, mimosa webworms are generally considered an aesthetic as well as a nuisance pest problem. The nests make trees unsightly and caterpillars will occasionally drop from infested trees onto unsuspecting backyard gardeners, grill masters, dog walkers, etc.
Although the mimosa webworm moth is a non-native, it appears that this exotic pest has been with us long enough to be discovered by a number of predators and parasites. 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 pictures 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 ectoparasitoids 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.
While healthy honeylocust trees can recover from occasional mimosa webworm outbreaks, 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 added stress 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.
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 webworm. Second, dense webworm nests present a significant barrier to insecticide penetration. This is particularly true for second and third-generation nests.
If insecticides are required, systemic insecticides are the best option because there is a reduced chance they will kill the beneficial insects. Of course, if the goal is to prevent leaf loss on vulnerable trees, the application timing should target first-generation caterpillars. Also, preventing first-generation nests from fully developing will reduce the attraction of trees to second and third-generation female moths.
Dave Shetlar noted that the systemic 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 products applied as soil drenches or trunk injections (e.g. Lepitect or Lepitect Infusible) are also effective.
Third-generation mimosa webworms are well underway in southern Ohio. As Dave's light trap indicated, the third generation will soon be underway in the central part of the state. This means if the overarching goal is to protect vulnerable trees by preventing leaf loss, the battle is largely 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 records kept 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.
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
Rex A. Bastian, Elwood R. Hart, 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, available at https://doi.org/10.1093/ee/19.2.409