Basic biology and human assists collude for the rapid spread of SLF

First detected in Berks County, Pennsylvania, in 2014, near the town of Pikeville, this phloem-feeding sap sucker, the Spotted Lanternfly (SLF), now has breeding populations in more than 17 states. These include Connecticut, Delaware, Illinois, Indiana, Kentucky, Maryland, Massachusetts, Michigan, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, Tennessee, Virginia and West Virginia, as well as the District of Columbia, with isolated detections in several other states.

By its own locomotion, SLF is believed to move only about three to four miles per year by walking, hopping and flying. Last year, with the detection of an established population in Davidson County, Tennessee, these noisome rascals had traveled some 660 miles from their first detection site in Berks County. At the “natural” expected rate of four miles per year, this feat should have taken about 165 years. They spanned the distance in a mere nine years, moving roughly 73 miles per year.

How did SLF spread so far and so quickly?

A constellation of several biological attributes, abetted by humans, contribute to SLF’s remarkable ability to spread in the eastern United States. Two of these factors lie in the core biology of SLF and in our highly developed and efficient transportation systems.

Two elements of biology predispose SLF to rapid dispersal with human support. For those not familiar with the biology of the pest, its life cycle consists of eggs that overwinter and hatch in spring into nymphs that suck phloem sap from more than 100 known host plants. Hatchlings molt three more times as they grow and before the fourth-instar nymph sheds it exoskeleton and becomes an adult.

After several months of sucking phloem sap from many species of forest and ornamental trees, females mate and deposit egg masses. This is when biology becomes a problem. While many plant pests deposit eggs on a single or limited number of host-plant species, SLF lay eggs on a wide variety of trees.

More problematic is the fact that they also deposit eggs on inanimate substrates, including stones, fallen trees and branches and a wide assortment of human-made objects, including lawn furniture, fence posts, masonry and vehicles. This proclivity provides opportunities for human-assisted transport, such as cars, trucks and recreational vehicles laden with eggs, to move around the nation, or for vehicles to transport objects housing lanternfly eggs or nymphs and adults to distant destinations.

Further complicating the human-assisted transport is the fact that egg masses of spotted lanternfly are rather cryptic. After depositing 30 to 60 eggs in parallel rows, the female applies a layer of whitish putty-like material to the eggs. As egg masses age and weather, they turn brown and readily blend in with the color and texture of bark on many species of trees. (Photo 1)

When deposited on stones or masonry products, egg masses can easily be overlooked as mud or debris. This inherent camouflage is thought to be part of the reason spotted lanternfly was not detected as it entered North America with stone products shipped from abroad.

Geography and transportation corridors play a key role in the rapid spread of SLF in the eastern United States. A distribution map of SLF in 2019, five years after its detection in Pennsylvania, showed a fairly uniform cluster centered around Berks County, with an outlying infestation in northern Virginia, site of a second infestation.

Eastern Pennsylvania and western New Jersey are laced with major railway lines and interstate highways running north and south, east and west. These connect large seaboard ports in New Jersey and Pennsylvania to outlying warehouses and distribution centers inland. The current SLF distribution map, from November of 2023, clearly shows an expanding, descending tongue of infested counties in western Virginia that tracks the route of interstate highway 81 from the generally infested area further north. (Map 1)

An additional piece of the puzzle that factors into the movement of SLF is its intimate association with yet another invasive species, Ailanthus altissima, tree of heaven (TOH). TOH is recognized as one of the key hosts for supporting the growth and development of SLF. During the critical period of egg development, female SLF aggregate by the hundreds on TOH to feed and produce eggs. Immortalized by Betty Smith as the “tree that grows in Brooklyn,” TOH is renowned for its ability to thrive in poor soils and highly disturbed locations. These include rights of way that accompany highways and railways, as well as minimally managed landscapes that sometimes accompany warehouses and distribution centers. Proximity of TOH to transportation corridors likely contributes to the rapid spread of SLF.

What are the chances SLF will continue to spread?

Excepting some form of divine intervention, SLF has likely become an unwanted but permanent resident in the U.S. During the early stages of the lanternfly onslaught, more than a million lanternflies had been killed by volunteers in Berks and surrounding counties in Pennsylvania. Despite these herculean efforts, in the intervening years, SLF has spread and established breeding populations in 17 states and has moved more than 600 miles from its point of first detection.

In 2019, scientists in the Agricultural Research Service of USDA published a map of the United States depicting the potential distribution of SLF based on a wide number of environmental factors, including temperature, elevation and rainfall patterns. (To view that map, go to the digital version of this article online and click here.) In comparing that USDA map to Map 1, you can see that in the intervening four years, predictions for the range expansion are quite accurate.

One surprising trend is that several regions rated as medium and low risk in 2019 now support breeding populations of SLF in 2024. Perhaps the most disturbing prediction identifies several areas of high climatic suitability in wine-growing regions of California, Oregon and Washington. To date, the largest economic crop losses to SLF have occurred in vineyards.

What have we learned about damage caused by SLF?

Shortly after the detection of SLF in the U.S., concern arose that this invader would become a major economic pest of fruit crops and trees growing in forests and managed landscapes. An economic report in 2020 projected that if SLF became widespread in Pennsylvania, it could result in crop and tree losses of more than $300 billion dollars annually, with an accompanying loss of nearly 3,000 jobs.

Without a doubt, grape growers have experienced the greatest crop losses to date, where the direct injury of SLF feeding has killed grape vines. In a recent “myth-busting” article, Professor Kelli Hoover reported that SLF rarely kills trees with the exception of TOH. However, a separate report notes that black-walnut saplings may be killed by SLF.

To my way of thinking, killing grapes represents a major problem, but killing invasive TOH might not be such a bad thing – just saying.

Professor Hoover’s recent studies did find that prolonged feeding by SLF confined for several years on silver maples, weeping willows, river birches and TOH resulted in reduced diameter growth, lower below-ground starch reserves and reduced nutrient content in leaves. These effects were density dependent, with reductions greatest at higher densities of SLF.

However, because SLF in the wild readily moves from tree to tree during the growing season and its numbers change greatly from year to year, its negative impact on an individual tree in the forest or landscape would be less than that observed in this study.

My direct observations and discussions with arborists of problems caused by SLF in managed landscapes hinge on three types of indirect injury resulting from an infestation.

First and foremost is the concern clients express when they discover vast numbers of large, conspicuous, mobile insects on their trees, hardscapes and homes. (Photo 2) Most people want them gone.

Second, spotted lanternflies produce prodigious amounts of honeydew as they feed, especially as late instar nymphs and adults. Honeydew rains down on vegetation below and provides the substrate for the growth of black sooty-mold fungus, which wrecks the appearance of underlying plants and fouls patios, lawn furniture and slow-moving people. (Photo 3) Moreover, because sooty mold coats leaves, photosynthetic capacity and growth of fouled plants is reduced.

Third, as the name implies, honeydew excreted by SLF provides a rich source of carbohydrates for many insects. Colonies of social insects that sting, including paper wasps, hornets and yellow jackets, reach their seasonal zenith in late summer and autumn, coincident with vast amounts of honeydew produced by SLF. The resulting visitation of hordes of stinging insects to trees infested with SLF creates a health risk to homeowners, children and pets who might be stung. (Photo 4)

On an SLF-infested property in western Maryland, arborists and this author were attacked and stung by bald-faced hornets and yellow jackets on several occasions while removing trees. To my way of thinking, this is perhaps the greatest risk posed by SLF, so be careful when working on SLF-infested properties.
What advances have been made using insecticides to manage SLF?

Early in the establishment of SLF, mechanical-control tactics such as squashing nymphs and adults and removing and destroying egg masses on trees were recommended for reducing populations in residential areas. Unfortunately, more than 95% of SLF egg masses deposited on trees are beyond the reach of humans standing on the ground, and this tactic is unlikely to reduce SLF abundance from year to year, locally or regionally.

Much has been learned about insecticidal control of SLF in recent years. Many contact insecticides, particularly synthetic pyrethroids such as bifenthrin and cyfluthrin, provide excellent short-term and residual activity against both nymphs and adults. Carbaryl and natural pyrethrins provide excellent immediate control but poor residual activity for feeding stages.

In addition to pyrethrins, which may be labeled for organic food production (OMRI listed), several other OMRI-listed active ingredients, including neem oil, horticultural oils, some essential/botanical oils and insecticidal soaps, can provide good immediate control but lack effective residual activity. Not surprisingly, dinotefuran applied as a soil drench, trunk spray or trunk injection provides excellent control of SLF adults. Imidacloprid trunk injections also provide excellent residual activity.

Several of the aforementioned insecticides, including the synthetic pyrethroids and dinotefuran, are toxic to bees, and cautions should be used when applying these materials to flowering plants.

Early attempts to use contact insecticides to kill eggs using horticultural and botanical oils yielded variable results, with control rates ranging from less than 10% mortality up to 75%. However, studies conducted in 2018 and 2019 by scientists at Penn State found several paraffinic and mineral oils applied to egg masses at a 3% rate provided up to 75% control.

One promising advance in insecticidal control employs the use of a naturally occurring fungus, Beauveria bassiana, which has been formulated as a contact insecticide. Current research underway has generated encouraging results that this biopesticide may be effective in killing nymphs and adults of SLF. (Photo 5)

Is any help from Mother Nature on the way?

Early in the invasion of SLF in the U.S., we read things like “… as with many other insects arriving to the United States from other parts of the world and finding no established natural enemies here, it has become very invasive.” Back then, it seemed there might be no stopping these rascals, but now we know better. In autumn of 2018, in a forested area near Reading, Pa., a massive die-off of SLF was observed. Molecular analyses of SLF taken at the site of the population collapse revealed that two naturally occurring fungi, Batkoa major and Beauveria bassiana, teamed up to put a beatdown on SLF.

A recent publication by scientists at Penn State University summarized more than a thousand observations of acts of predation on various life stages of SLF. Arthropods led the carnage with more than 200 attacks by spiders, 196 attacks by mantids, 177 attacks by wasps, 55 attacks by sucking predators like assassin bugs and another 21 attacks by other arthropods. (Photo 6)

Twenty-some families of birds accounted for more than 500 attacks, with ground dwellers like chickens and pheasants leading the way. Death delivered by members of the cardinal, mocking bird, wren and several other bird clans contributed to the total. (Photo 7)

Mammals, amphibians, fish and non-feathered reptiles also got into the act, accounting for 106 additional observations. In addition to biological control provided gratis by many indigenous predators, a small parasitic wasp imported to kill eggs of gypsy moth is now attacking and killing eggs of SLF. At the time of this writing, scientists at USDA are evaluating parasitic wasps that attack eggs and nymphs of SLF in Asia, with the hope that they can be imported, reared, vetted and safely released in the United States to help reduce SLF populations here.

Moving forward with SLF

There is little doubt that SLF is here to stay and will continue its spread throughout the U.S. However, greater awareness of what it looks like, how it lives, where it is and how it spreads, coupled with local and regional quarantines, will help to slow the spread. Fears that SLF is a widespread killer of established trees in forests and managed landscapes should be put to rest. We now know that a vast cadre of homegrown agents of biological control, ranging from fungi to insects, spiders, reptiles, birds and mammals, is taking its toll on SLF.

Arborists and homeowners have a wide variety of effective contact and systemic insecticides to kill SLF on herbaceous plants, shrubs and trees. Many of these products are listed by OMRI for organic food production and can be incorporated into existing PHC programs. Although SLF presents unique challenges to the tree care industry, science and commercial enterprise have advanced our knowledge and provided the opportunity to deal with this diabolical invader in environmentally responsible and economically beneficial ways.

Special thanks to Brian Eshenaur and the New York State Integrated Pest Management Program of Cornell University for providing the updated maps of
spotted-lanternfly distribution in the United States. Thanks also to Dr. Paula Shrewsbury for providing editorial comments that improved the article.

References

Using community science to identify predators of spotted lanternfly, Lycorma delicatula (Hemiptera: Fulgoridae), in North America. Anne E. Johnson, Alison Cornell, Sara Hermann, Fang Zhu and Kelli Hoover. Bulletin of Entomological Research. 2023. 1–8.

Biology and Management of the Spotted Lanternfly, Lycorma delicatula (Hemiptera: Fulgoridae), in the United States. Julie M. Urban and Heather Leach. 2023. Annual Review of Entomology. 2023. 68:151–67.

Effects of long-term feeding by spotted lanternfly (Hemiptera: Fulgoridae) on ecophysiology of common hardwood host trees. Kelli Hoover, Lidiia Iavorivska, Emily K Lavely, Osariyekemwen Uyi, Brian Walsh, Emelie Swackhamer, Anne Johnson, David M Eissenstat. Environmental Entomology. 2023. 52: 888–899.

Online resources

Spotted lanternfly biology and lifecycle – scan the QR code or go to this article at tcimag.tcia.org and click here.

Spotted lanternfly management guide:
https://extension.psu.edu/spotted-lanternfly-management-guide

Spotted lanternfly lore: Penn State experts clear up falsehoods about pest. By Amy Duke, click here.

Michael J. Raupp, Ph.D., is professor emeritus at the University of Maryland in College Park, Md. His writing, research and scientific outreach have received a dozen national and international awards. He is a regular guest on television and radio. His most recent book, 26 Things that Bug Me, introduces youngsters to the wonders of insects and natural history, while Managing Insects and Mites on Woody Plants, published by the Tree Care Industry Association, is a standard for the arboricultural industry. Visit his websites at www.bugoftheweek.com and https://www.youtube.com/user/BugOfTheWeek.

This article was based on his presentation on the same topic during TCI EXPO ’23 in St. Louis, Missouri. To watch a pre-recorded video created for that presentation, go to TCI Magazine online at tcimag.tcia.org. Under the Resources tab, click Video. For February Digital Edition click here.