I reported on mimosa webworm (Homadaula anisocentra, family Galacticidae) in early August. However, that was before the moth caterpillars started dropping from trees to spoil outdoor activities.

You can read my August 7, 2020, BYGL Alert titled, "Mimosa Webworm on Honeylocusts" by clicking this hotlink:

https://bygl.osu.edu/index.php/node/1664

Damage and Seasonal History

Mimosa webworm 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 so tightly encased in webbing the foliage looks like its melting.

Past research conducted in the upper Midwest and Great Plains found there were two generations per season. A paper published in 1947 noted there was a partial third generation in the Washington, D.C., region. As noted in my earlier BYGL Alert, Dave Shetlar (Professor Emeritus, OSU Entomology) observed three distinct moth flights in his blacklight traps in central Ohio indicating the development of a third generation. My own observations on caterpillar development and nest expansions support the occurrence of a third generation in the southwest part of the state.

Had a third-generation been observed late in the season, it would be reasonable to conclude the occurrence is an occasional anomaly or perhaps an indication of a partial third generation. However, the appearance of a third generation in early August indicates it may be an established annual event perhaps due to climate change (global worming?).

Why is this important? The more generations, the greater the seasonal damage because populations expand with each successive generation. Also, nests become larger and more obvious with each generation. 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 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.

Repelling Rappelling Caterpillars

Mimosa webworm caterpillars make controlled descents on silk threads if they run out of food during the summer or if they're about to pupate. If the caterpillars deplete their food supply, they will go in search of greener pastures. This can occur anytime during the season if caterpillar populations are high.

They also rappel from trees in the fall in preparation for pupation. Caterpillars pupate within their nests during the summer season. However, as fall approaches, the final generation caterpillars rappel from trees on silk threads to seek protected sites where they pupate prior to winter. Their cocoons may be found in the bark crevices of their host tree, under structural siding, beneath door jambs or window frames, etc. The annual mimosa webworm drop is beginning to get underway in central and southern Ohio.

The rappelling caterpillars become repelling if they drop onto unsuspecting picnickers (practicing social distancing!) 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.

It's not a synchronized drop because the current nests contain a range of caterpillar instars. This means the repelling rappelling will continue for some time. Unfortunately, there isn't much that can be done to alleviate the problem. Insecticidal sprays won't penetrate the nests and the caterpillars that are killed become a rain of dead caterpillars rather than dangling live caterpillars.

Hosts with the Most

Mimosa webworm was accidentally introduced into the U.S. from China in the early 1940s. Some online resources report that the webworms were first found on honeylocust trees (Gleditsia triacanthos) in Washington, D.C., landscapes. However, a paper published in 1943 describesH. anisocentraas "A new pest ofAlbizzia[sic] in the District of Columbia." Thus, the first host found in the U.S. to be infested by the non-native mimosa webworm was the non-native mimosa,Albizia julibrissin.

Mimosa webworm continues to be found on its namesake host. However, a paper published in 1947 reported that the non-native moth also has a taste for honeylocust. This paper also provided a hint that webworm host preferences are not equal among all honeylocusts.

The first thornless honeylocust (G.triacanthos var. inermis) wasn't introduced until 1949 with the Siebenthaler Company of Dayton, OH, receiving Plant Patent 836 forG. triacanthos var. inermis'Moraine'. The varietal name "inermis" means "toothless." It was the first shade tree to ever receive a plant patent.

The research published in 1947 references "native honeylocust" and concludes that mimosa webworms have a greater preference for their namesake host compared to honeylocust. Indeed, I continue to look for mimosa webworm nests on native "thorned" honeylocust but have yet to take a picture.

Subsequent research revealed that mimosa webworm moths prefer to lay eggs on thornless honeylocusts compared to mimosa trees. I recently observed this in Trenton, OH, where a thornless honeylocust canopy overlaps the canopy of a large mimosa. While both trees were infested, it was obvious that the webworm nests were larger and greater in number on the honeylocust compared to the mimosa.

Research has also revealed that there are distinct differences in terms of host suitability among the thornless honeylocusts introduced since 'Moraine'. A paper published in 1990 showed females reared on 'Moraine' produced significantly fewer eggs compared to females reared on 'Imperial', 'Shademaster', 'Sunburst', and 'Skyline.'

This leads to two questions drawn from rampant speculation. First, is the highly localized nature of mimosa webworm outbreaks based on chance or is it associated with a localized abundance of more susceptible cultivars? Second, has the genetic resistance against mimosa webworm been reduced or lost as honeylocusts were selected for preferred horticultural traits?

With a Little Help from My Friends

It's too late this season to do much about mimosa webworm other than to plan for next season. The caterpillars are beginning to drop from their nests placing them out of reach of both topical and systemic insecticides. Although some damage will continue to develop from late bloomer larvae, the vast majority of this season's damage has already occurred.

Planning for next season includes planning to closely monitor trees that were heavily infested this season. This is particularly important for recently planted young trees. Healthy established honeylocusts are capable of handling heavy infestations unless the foliage is seriously damaged year-after-year.

Thankfully, repeat performances seldom occur. While it's true that localized webworm populations may slowly rise year-after-year until they peak in an "outbreak;" it's also true that populations will eventually collapse due to natural enemies, environmental calamities, or a combination of both.

Research has shown that overwintering mimosa webworm pupae have a low-temperature Achilles' heel. For example, the polar vortex that flowed across Ohio during the 2014-15 winter had a serious impact on mimosa webworm. In fact, this is the first season since the calamitous polar express that we've seen widespread webworm damage.

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. The picture below shows a parasitoid wasp I found earlier this season cavorting among early instar mimosa webworms in Wyoming, OH. Its antlered antennae indicate this wasp belongs to the Family Eulophidae. Wasps in this family areectoparasitoids 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.

The pictures below shows the potter wasp,Parancistrocerus leionotus, (family Vespidae, subfamily Eumeninae) searching a mimosa webworm nest for caterpillar meat to provision its jug-like larval pots. The females collect water and then mix it with dry soil using their mandibles. They transport the mud mixture to their nesting site where they fashion individual pots ranging in size from 1/4" - 6/16" in diameter. The entire construction process may require hundreds of trips over several hours.

As the pot nears completion, the female lays a single egg attached by a thread to the inside surface of the nest. They then provision the pot with a caterpillar or beetle larva that has been paralyzed by a sting. Once provisioned, the pot is sealed with a clay plug leaving the hapless paralyzed prey to await the wasp's hatchling larva. On a side note, potter wasps provide a beneficial twofer service because they are also important plant pollinators.

Ron Wilson

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