More than 55% of the world population now lives in cities, according to an October 2022 report by the World Bank. This percentage is expected to increase to nearly 70% by the year 2050. As once-natural areas become developed and forests and grasslands are turned into the homes, businesses, institutions and attendant infrastructure that comprise cities, dramatic changes occur in the abundance and diversity of plants and animals. At the innermost parts of cities, tree density and diversity are often dramatically lower than nearby suburban areas or forested parklands within city limits.
Moving up the food web from plants to animals, it is not surprising to find the abundance and diversity of many insects, birds and mammals (excepting humans) dramatically reduced in cities compared to natural areas nearby. While iconic taxa such as butterflies, bees and birds may be missing from cities, some urban adapters thrive in concrete jungles and reach densities far exceeding those in surrounding suburban and rural landscapes. This is the case for several kinds of scale insects.
In a previous article in Tree Care Industry Magazine (“Sap-Sucking Insects: How They Feed, and the Damage They Cause,” October 2022), we explored the biology and damage caused by a dirty-dozen sucking insects that are key pests of trees in landscapes. In this article, we will visit three species of scale insects that often outbreak in cities and learn what factors contribute to these eruptions.
While touring the streets of Washington, D.C., I was impressed by the vast number of oaks lining streets and planted in parks. Pin oaks, Quercus palustris, and willow oaks, Quercus phellos, are two grand native trees often planted in cities throughout much of the eastern United States. These beauties deliver a bonanza of ecosystem services, including sequestering carbon, mitigating pollution, enhancing water infiltration by slowing rainfall, cooling (shading streets, sidewalks, buildings, parks, yards), increasing property values, providing food for wildlife in the form of caterpillars for birds and nuts for squirrels and just looking marvelous with summer greens and autumn russets.
As I walked about, I observed several city blocks lined with oaks, creating near monocultures of Quercus often comprising more than 75% of the urban-forest canopy. But along several streets, oaks young and old had moderate to severe branch dieback, misshapen anemic leaves often cloaked with black sooty mold and branches festooned with the small round bumps.
These symptoms of oak decline are the handiwork of the native soft scale known as oak lecanium scale, Parthenolecanium quercifex. Rascals belonging to a large group of insects called Hemiptera, scale insects and other Hemiptera have sucking mouthparts used to remove fluids from vascular elements of plants. Nymphs and adults stick soda-straw-like mouth parts into the vascular tissue in leaves and stems called phloem. A tiny pump in their head allows them to suck nutrient-rich phloem sap into their bodies, where it is used for growth, development and production of eggs.
Their feeding robs the oaks of nutrients, thereby reducing growth of branches and normal development and function of leaves. The additional insult comes when excess fluids are discharged from the rear end of the scale insect in the form of a waste product called honeydew. Sugar-rich honeydew provides a substrate for the growth of the ugly sooty mold fungus that disfigures leaves, bark and objects below the scale-infested tree. Unfortunately, honeydew also attracts stinging insects such as honeybees, yellow jackets and paper wasps, creating a nuisance and health risk for people and pets.
Let’s circle back to the question of why oak lecanium scales are more abundant in cities than in surrounding suburban and rural locations. Many of these outbreaks can be linked to elevated temperatures found in cities, the phenomenon known as the urban-heat-island effect. Due to the predominance of buildings and hardscape that collect and hold thermal energy, cities can be anywhere from a few degrees to more than 10 degrees warmer than surrounding landscapes. Higher temperatures in the built environment may directly benefit scales and at the same time disadvantage beneficial insects, such as tiny parasitic wasps that help to limit scale outbreaks.
In a series of fascinating studies, entomologists at North Carolina State University found oak lecanium scales to be eight to 12 times more abundant on oak trees in hot spots in cities. The abundance of females with egg-filled sacs was about five times greater on these warmer oak trees compared to those in cooler locations. Each female oak lecanium has the potential to produce more than 1,000 eggs during her lifetime. At warmer temperatures, survival of tiny scale nymphs was 20% greater than at cooler temperatures (Meineke et al. 2013).
In addition to these direct benefits to scales, warm city temperatures also may hamper beneficial insects. Because insects develop more rapidly as temperatures warm, elevated temperatures in the city enable female scales to develop quickly and begin laying eggs sooner than scales in cooler zones. Tiny parasitic wasps attack and kill female scales. However, scales that develop rapidly in the warmth of the city are able to deposit many more eggs before tiny parasitic wasps terminate their reproductive shenanigans (Meineke et al. 2014).
Originally native to Asia, crepemyrtles traveled with botanical explorers from their aboriginal homes to gardens in Europe and the Americas more than 250 years ago. As North America warms, the range of crepemyrtles has expanded from subtropical southern states to mid-Atlantic locations including Maryland and Washington, D.C., where milder winters and hot summers enable them to thrive. Lining the streets in several cities are long allées of crepemyrtles, often comprising more than half of the tree canopy.
Excepting crepemyrtle aphid, Tinocallis kahawaluokalani, which causes messy problems with honeydew and sooty mold but has been called innocuous, crepemyrtles were relatively free of insect pests in the U.S. until the discovery of crepemyrtle bark scale, a.k.a. crepemyrtle felt scale, Acanthococcus (=Eriococcus) lagerstroemiae, in Texas in 2004.
Since the first detection in 2004, it has marched steadily northward – to North Carolina in 2016, Virginia in 2017 and Maryland in 2020.
Range expansion of this and many other scale insects happens in a variety of ways. Infested nursery stock shipped from southern states may convey scales to new locations. Some scale insects climb onto the feet of birds, hair of mammals and bodies of flying insects and become unwittingly transported from infested to uninfested trees. In an even stranger twist, some scale insects actually utilize “take-off” behavior to help them become airborne and then ride the wind from one plant to another.
Like the aforementioned oak lecanium scale, crepemyrtle bark scale feeds on nutrient-rich phloem. To gain sufficient nutrients for growth, development and reproduction, vast quantities of plant sap are imbibed. Once processed, the remaining liquid is excreted in the form of honeydew, resulting in the same issues as previously described for the oak lecanium scale. Removal of vital plant sap and damage caused by thousands of piercing mouthparts can result in branch dieback and death of crepemyrtles.
For arborists and landscape managers who have not yet experienced crepemyrtle bark scale, diagnosis is relatively easy. Both eggs and tiny crawlers are vivid pink. Upon hatching, crawlers move to branches and twigs of the crepemyrtle and settle down to suck sap. After molting to adulthood, females produce
brilliant-white egg sacs (also called ovisacs) in which they deposit from 100 to more than 300 eggs. In warm regions, there may be four generations annually, and it is easy to see how populations can explode seemingly overnight.
While crepemyrtles are the primary target for this rascal here in the U.S., crepemyrtle bark scale also has been found on more than a dozen other plant species, including beauty berry, pomegranate, boxwood, persimmon, privet, brambles and others (Skvarla 2021, Scalenet 2023).
As with the oak lecanium scale, we wonder, why has crepemyrtle bark scale become such a problem, particularly in cities? At least two factors contribute to outbreaks of this scale and other insect pests in cities. Low diversity of forest canopy in cities, as the result of overplanting a single species or genus of tree, can result in catastrophic canopy loss when lethal pests arrive in our country. This is evidenced by the demise of elms to Dutch elm disease and extirpation of ashes due to emerald ash borer (Raupp et al. 2006). In the eastern U.S., extensive plantings of crepemyrtles in cities provide a rich and bountiful resource for this exotic scale insect.
As is often the case with non-native plants, some time after their arrival in a new land, their pests from afar follow them to the invaded realm. Other classic examples of sucking insect pests of woody landscape plants that followed their plant hosts to new lands include euonymus scale, attacking euonymus, and azalea lace bugs, attacking azaleas. This was the case for crepemyrtles when, sometime prior to 2004, the crepemyrtle bark scale found its way to Texas, a case of a non-native scale reunited with its non-native host in a new land.
Feeding the outbreaks
But why do these scales reach outbreaking levels? First and foremost is the fact that crepemyrtle is a susceptible host for this scale, one it has a long history with back in Asia. Second, the full complement of natural enemies that keep the scale at bay in other parts of the world simply is not present here in the U.S. Throughout its worldwide range, crepemyrtle bark scale is attacked and killed by more than a dozen tiny parasitic wasps and eaten by another dozen predatory beetles and lacewing larvae (Scalenet 2023). However, when the scale arrived at its new home in Texas, the full complement of natural enemies did not accompany it on its journey.
Without the full complement of natural enemies from their native regions, non-native pests such as crepemyrtle bark scale, euonymus scale, azalea lace bug and several others reach outbreaking levels in the invaded range (Martinson et al. 2020). Moreover, homegrown predators and parasites that are present in the invaded range simply may not occur in numbers sufficient to prevent scales from outbreaking in cities.
Several studies have linked reduced plant diversity and vegetational complexity found in the built environment with reductions of natural enemies, particularly generalist predators that can play an important role in suppressing pest populations and outbreaks (Tooker and Hanks 2000, Shrewsbury and Raupp 2006, Raupp et al. 2010).
Beneficial insects and predatory spiders wage war on insect pests, but many require nectar and pollen sources, a diversity of prey items found on a diversity of plant species and refuges from their own enemies. These resources are often abundant in diverse and complex natural ecosystems but scarce or missing entirely from inner cities, where impervious surfaces dominate the landscape, nectar and pollen sources are rare and only a relatively low number of plant species dominate the landscape.
Beautiful in spring with blooms and summer and autumn with gorgeous foliage, and durable in a variety of soil types and a wide variety of climatic conditions, members of the genus Acer are among the most widely planted of all deciduous trees in urban forests. Unfortunately, they also are one of the favored hosts for a large number of native and non-native insect pests, including the dreaded Asian longhorned beetle.
The combination of direct mortality caused by the beetle and attempts to eradicate it in established infestations has resulted in the death of thousands of maple trees in cities and forested areas of the United States. However, it is a native, armored-scale insect, gloomy scale, Melanaspis tenebricosa, that experts say, “without exception was the most important pest of shade trees in North Carolina,” and perhaps in cities in 24 other states where it is found (Just et al. 2020).
As we learned in the previous TCI Magazine article mentioned earlier, armored scales do not feed on vascular tissues, but feed on the contents of cells found in leaves and beneath bark. They do not produce honeydew, and the scale resides beneath a waxy cover, which is often diagnostic. Covers of female gloomy scales are circular and grey with a shiny black circle in the center. Male scales have an oblong cover.
There is but one generation annually, and, unlike oak lecanium scales that can lay more than a thousand eggs, gloomy scales lay fewer than 100. In addition to Acer, gloomy scale has been reported on more than two-dozen genera of woody plants. However, populations of these small insects often go unnoticed and build up slowly over several years before they achieve astounding densities of 70 per square centimeter on the bark of trees (Just et al. 2020). Heavy infestations cause decline and dieback in the tree canopy as thousands of scales damage plant tissues. Canopy failure in combination with other biotic and abiotic stressors sometimes leads to tree death.
Researchers have long recognized gloomy scale as an innocuous pest of native trees in forests, but one that becomes an epic pest when it colonizes maple trees in cities. Researchers at North Carolina State University discovered that body size, reproductive output and survival of gloomy scales is positively correlated with warmer temperatures and the amount of impervious surface surrounding maple trees in cities.
Impervious surfaces hold heat, raising ambient temperatures that may benefit scales in many ways. Impervious surfaces also reduce infiltration of rain, thereby denying trees water, the critical resource that drives photosynthesis. Water deprivation creates plant stress that benefits many insects and diseases that exploit and attack trees, leading to cascading effects lethal to stressed trees. Impervious surfaces are likely the number-one enemy of trees in cities (Just et al. 2020).
How to reduce scales
What steps might help reduce scales in cities? Here are a few suggestions. Choose plants adapted for your city’s region – consider soils, rainfall patterns, humidity and temperature regimes. Avoid monocultures and monolithic plantings of trees of the same species or genus. Whenever possible, try to diversify the urban canopy.
Inspect material carefully before purchase and acceptance at a site. Many scales arrive with plants infested at the nursery. At installation, provide sufficient space and proper drainage and soils. Plant at the proper depth and correct potential girdling roots and anything else that harms normal root development. Ensure adequate water, including supplemental water in times of drought, and provide proper drainage. Provide adequate impervious-surface-free space to support normal development and growth for the trees you select.
Avoid high nitrogen fertilizers. They have been linked to increased survival and reproduction of scales. If possible, add flowering trees and perennial and annual herbaceous plants that provide nectar, pollen and prey other than scale insects (alternative prey) to attract and support predators and parasites that put a beat-down on scales and other pests of trees in urban forest.
When insecticidal control is necessary, whenever possible, select insecticides that are least disruptive to natural enemies and beneficial insects, including pollinators. These will include insecticides listed as reduced risk by the EPA and those listed by the Organic Materials Review Institute (OMRI).
References and literature cited
Just, M.G., Dale, A.G., and Frank, S.D. (2020) Gloomy scale (Hemiptera: Diaspididae) ecology and management on landscape trees. Journal of Integrated Pest Management, 11(1): 24; 1-9. doi: 10.1093/jipm/pmaa028.
Martinson, H.M., Raupp, M.J., and Frank, S.D. (2020) How urban forest composition shapes the structure and function of arthropod communities. In P. Barbosa (Ed.), Urban ecology: Its nature and challenges (pp.15-36). CAB International.
Meineke, E.K., Dunn, R.R., Sexton, J., and Frank, S.D. (2013) Urban warming drives insect pest abundance on street trees. PLoS ONE, 8(3): e59687.
Meineke, E. K., Dunn, R. R., Frank, S. D. (2014) Early pest development and loss of biological control are associated with urban warming. Biology Letters. doi: 10.1098/rsbl.2014.0586.
Raupp, M.J., Cumming, A. Buckelew, and Raupp, E. C. 2006. Street tree diversity in Eastern North America and its potential for tree loss to exotic pests. Journal of Arboriculture. 32:297-304.
Raupp, M.J., Shrewsbury, P.M., and Herms, D.A. (2010) Ecology of herbivorous arthropods in urban landscapes. Annual Review of Entomology 55, 19–38. DOI: 10.1146/annurev-ento-112408-085351.
Scalenet scalenet.info/catalogue/Acanthococcus%20lagerstroemiae Accessed January 5, 2023.
Shrewsbury, P. M., and Raupp, M. J. 2006. Do top-down or bottom-up forces determine Stephanitis pyrioides abundance in urban landscapes? Ecological Applications. 16(1):262–72.
Skvarla, M. J. 2021. Crapemyrtle Bark Scale. Penn State Extension. extension.psu.edu/crapemyrtle-bark-scale
Tooker, J.F. B, and Hanks, L. M. 2000. Influence of plant community structure on natural enemies of pine needle scale (Homoptera: Diaspididae) in urban landscapes. Environmental Entomology. 29:1305–11.
Michael J. Raupp, Ph.D., is professor emeritus at the University of Maryland in College Park, Maryland. 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 subject during TCI EXPO ’22 in Charlotte, North Carolina. To listen to an audio recording created for that presentation, go to this page in the digital version of this issue of TCI Magazine online at tcimag.tcia.org and, under the Resources tab, click Audio. Or, under the Current Issue tab, click View Digimag, then go to this page and click here.