When Forests Come Down with a Bug: Forest Pests in the Greater DC Area

By Hannah Harkness, NCRN I&M Science Communications Intern

Forest Pests: A Quick Look

Forests of the National Capital Region (NCR) face persistent threats from a variety of pests and pathogens. As part of the eastern deciduous forest, our forests are rich in biodiversity. This includes native species and naturalized forest pests, while other newly introduced pests are only now arriving and spreading. Forest monitoring by NCR’s Inventory & Monitoring Network (NCRN I&M) team has documented the extent and effects of some of these pests and pathogens in our parks.

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Pests that are Here Now: Beech Leaf Disease | Emerald Ash Borer | Spotted Lanternfly | Spongy Moth | Oak Decline & Oak Wilt
Pests on the Horizon: Elm Zigzag Sawfly | Beech Bark Disease

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Pests that are Here Now

Characteristic symptoms of beech leaf disease on beech leaves.

University of Rhode Island

Beech Leaf Disease

An emerging threat to NCR’s most common tree species, the American beech (Fagus grandifolia), beech leaf disease (BLD) was first confirmed in Prince William Forest Park in 2021. While BLD is relatively new to NCR parks, it has been in the U.S. for over a decade and can be found in 12 states, from Michigan to Rhode Island and Maine down to Virginia. It is of particular concern in NCR parks where beech trees are a dominant or codominant species in the forest including:

• Prince William Forest Park (PRWI)

• Rock Creek Park (ROCR)

• George Washington Memorial Parkway (GWMP)

• Chesapeake & Ohio Canal National Historical Park (CHOH) at the Potomac Gorge

• Monocacy National Battlefield (MONO)

• National Capital Parks- East (NACE)

So far, I&M forest monitoring has confirmed BLD outside of a monitoring plot in C&O Canal NHP near Fort Duncan (in September 2023) and in 16 monitoring plots in Prince William (Figure 1). The disease is spreading rapidly enough that it is likely to be found throughout Prince William in 2024.

Figure 1: Beech Leaf Disease distribution in Prince William Forest Park as of 2023. Yellow squares and red triangles show BLD confirmed in 2022 and 2023, respectively. Green tree icons show where BLD was absent. Grey clouds show where BLD status is unknown. Empty circles show plots without beech.

NPS/Brolis

Narrative Description of Figure 1

This map shows the distribution of Beach Leaf Disease in Prince William Forest Park. Confirmed BLD, indicated by yellow squares and red triangles for 2022 and 2023 respectively, is relatively evenly distributed across PRWI, totaling 16 instances of confirmed BLD in monitoring plots across the park. The park is shaded in green in the center of the map. The map legend sits in the upper right corner of the map, along with an arrow indicating north pointing to the top of the map. The scale in kilometers sits at the bottom right. 

BLD first attacks the buds of beech trees, leading to leaf damage that is identified by dark banding between leaf veins. Starting low on the tree and then moving into the canopy, the disease causes branch die back and can eventually lead to tree mortality over several years, especially in younger trees.

Treatment options for BLD are still in development.

Emerald Ash Borer

The emerald ash borer (EAB) (Agrilus planipennis) is a pervasive forest pest that has killed most of NCR’s ash trees since its emergence in the region in 2014. NCRN I&M data shows that from a high of 300,000 trees in 2009, only 42,000 living adult ash trees remain in NCR parks as of 2023 (Figure 2). Seedling and sapling ash trees on the other hand have surged (Figure 3).

Figure 2: Ash tree density in NCR parks. Green dots show living tree density. Black squares show standing dead tree density. Both circles and squares represent rolling 4-year monitoring periods labelled by ending year (e.g., 2016 point shows data for 2013-2016) except for 2021, 2022 and 2023 which show 5-year rolling periods. Vertical lines show estimated error.

NPS

Narrative Description of Figure 2

This line graph shows the density of ash trees in NCR parks. The x-axis shows time in years, from 2009 to 2023. The y-axis shows ash tree density in trees per hectare. One line on the graph, denoted with data points represented by green circles, shows living ash tree density. This line shows a dramatic decline in living ash starting in 2015. The second line on the graph, denoted by black squares, represents dead ash tree density. This line shows a consistent increase in dead ash each year starting in 2016. 

EAB larvae attack mature trees underneath their bark, eating through the phloem layer that carries vital water and nutrients. This invasive beetle almost exclusively targets ash trees, but has been known to also attack fringe trees (Chionanthus virginicus) and cultivated olive trees (Olea europaea). EAB causes nearly 100% mortality in all native ash trees (white, green, and pumpkin ash in NCR and blue and black ash to the north and west).

While the number of living ash trees continues to decline, the number of ash seedlings in the region has been increasing (Figure 3). Ash trees can produce a large number of seeds that germinate quickly, and stressed canopy trees sometimes produce a larger bumper crop of seeds before they die, likely contributing to this increase in ash seedlings. Deer management is also assisting seedling survival, along with the increased sunlight reaching the forest floor from the loss of canopy trees. Ash saplings have also increased in wetlands, due to resprouting from stumps of green ash (Fraxinus pennsylvanica) and pumpkin ash (Fraxinus profunda) trees that have lost their main trunk.

Figure 3: Density of ash trees (individuals/hectare; shown by tree icons) and saplings (leaf icons) on left axis, and ash seedlings (inverted V-shaped samara icons) on the right axis, at four time intervals. Each icon represents a full four-year monitoring period. Monitoring was paused in 2020 due to COVID-19, so the last time interval spans five years.  Seedlings are greater than 15 cm tall, but less than 1 cm diameter at breast height (DBH).

NPS

Narrative Description of Figure 3

This line graph shows the density of ash trees, saplings and seedlings at four time intervals. On the x-axis, time is represented in four intervals: 2006-09, 2010-13, 2014-17, and 2019-23. The y-axis shows the number of ash trees per hectare; the left y-axis represents trees and saplings, ranging from zero to 30, and the right y-axis represents seedlings, ranging from zero to over 1500. The line representing ash trees is represented by tree icons, showing a decline in ash trees over time. The line representing ash saplings is represented by leaf icons, and shows a small increase in ash saplings in the last time interval. The line representing ash seedlings is represented by inverted V-shaped icons, and shows an increase in density at each time interval. 

Biological control may forestall the complete loss of ash trees in the eastern U.S. The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) has released millions of parasitoid wasps that are natural predators of EAB. The ideal effect of these stingless wasps would be to bring this devastating forest pest into balance within the ecosystem.

Spotted Lanternfly

USDA APHIS

Spotted Lanternfly

Spotted Lanternfly (SLF) (Lycorma delicatula) was first identified in the U.S. in 2014, in Berks County, Pennsylvania. Since then, it has spread throughout the northeastern U.S. and was found in the NCR in 2021. NCRN I&M has documented SLF throughout NCR parks. Because SLF adults are highly mobile, their presence in a forest plot does not guarantee that they are attacking the plants they rest on. For this reason, it is difficult to determine how SLF is influencing tree mortality. Current research suggests SLF stresses but does not kill trees, and is more of a threat to trees already stressed by other infections or pests.

This invasive insect has a diverse diet and has the capacity to damage a variety of tree species. It is known to prefer the non-native tree of heaven (Ailanthus altissima) as a food source, and anecdotal observations in NCR forests so far show it clustering on grape vines (Vitis spp.). Continued monitoring will be required to determine how SLF is spreading and the damage it is causing in the region.

Spongy Moth

NCR parks last experienced a large outbreak of spongy moth (Lymantria dispar dispar) around 2007 - 2009. In 2022 and 2023, this now-naturalized forest pest surged in some eastern states, leading to alerts of possible recurrences in the NCR. However, recent sightings in forest plots monitored by NCRN I&M have so far been incidental.

While the spongy moth attacks a variety of deciduous trees (trees that lose their leaves annually), they prefer to target oak species, especially those with loose bark. The adult moths do not feed, but their larvae are voracious feeders that can defoliate entire trees. This weakens the tree, leaving them vulnerable to other pests, pathogens, and stressors.

Management and treatment for spongy moth is varied and includes pheromone lures, Bacillus thuringiensis kurstaki (Btk), and a spongy moth-specific virus, nucleopolyhedrosis virus (NPV) commonly known as Gypchek. These plus the naturally occurring fungal pathogen Entomophaga maimaiga can all help diminish spongy moth outbreaks.

Oak Decline & Oak Wilt

Oaks are a dominant part of eastern forests and a common genus of trees found in the NCR. In response to declines in oak populations across the country in recent years, NPS will be working with researchers at Virginia Tech starting in 2024 to determine if oak mortality in NCR is unusually elevated and if it is related to certain habitat conditions or if it varies over time. Results of this project will help inform parks about threats to oaks.

Oak Wilt

Rare in the NCR, oak wilt is caused by a fungus (Bretziella fagacearum) carried from tree to tree by insects. Various oak species react differently to the disease, but the fungus can colonize the vascular system of the tree and cause dramatic leaf wilting, sometimes leading to tree mortality.

First discovered in the U.S. almost a century ago, oak wilt is scarce in the region, but could pose significant concern if it begins to spread. In parts of the country where oak wilt is more prevalent, some parks try to prevent and treat the disease with fungicides, tree growth regulators, and fertilizers that are used in stages to help trees recover from oak wilt symptoms and prevent disease transmission.

Oak Decline

Oak decline is caused by multiple stressors. Climate change, changes in forest dynamics, tree age, and exposure to invasive pests heavily influence tree health. These combined factors can wear down the trees’ ability to recover and increase oak susceptibility to secondary factors such as infectious diseases.

Pests on the Horizon

Elm Zigzag Sawfly

An emerging pest in the NCR is the elm zigzag sawfly (Aproceros leucopoda). I&M has yet to document these invasive insects in NCR parks, but they were first recorded in Virginia in 2021, followed by seven nearby eastern states by 2023. They are most common in the Shenandoah Valley.

Elm zigzag sawflies damage native and non-native elms alike by eating through the fleshy parts of leaves and significantly defoliating trees. This damage can weaken trees and make them susceptible to other stressors. Continued monitoring will reveal how elm zigzag sawflies are affecting elms in the region.

Beech Bark Disease

Beech bark disease has not currently been identified in the National Capital Region. However, the disease is present in nearby areas and could eventually spread. Learn more about beech bark disease.

Native Insects are Beneficial

Despite some of the unchecked bad apples mentioned here, insects are an important part of all our natural ecosystems. They make sure detritus is broken down, flowers are pollinated, birds and reptiles are fed, and so much more! Among them are other tree-feeding critters that pose little threat to eastern trees including the following:

• Cicadas (various species) are common forest insects that, as larvae, feed on tree roots and as adults feed on tree sap. However, they do not cause significant damage to trees (maybe only to eardrums!).

• Webworm moth (Atteva aurea) caterpillars make web-like nets around tree branches and feed off of the foliage, but don’t cause significant impacts to trees.

Parks Taking Action

Dealing with newly introduced pests is not easy work. But park managers do have some tools to try and reduce infestations and the spread of different pests and pathogens. The targeted and cautionary use of systemic insecticides has been successful in reducing pests like hemlock woolly adelgid. Fungicides have also been valuable in reducing oak wilt in northeastern parks. Biological controls are also a potential factor to curb EAB and other pests.

The NPS is also taking action through the Resilient Forest Initiative which supports collaboration between national parks and offers resources on multiple aspects of forest management. NPS staff can learn more at: Resilient Forests Initiative Resources and Support (sharepoint.com)

Everyone Can Help

There are actions you can take to help reduce the number of pests and pathogens in our parks! Some things to remember:

• Don’t transport firewood. Buy wood where you burn it.

• Clean off your shoes after visiting a park or between visits to different parks.

• Report any pest sightings to your local county authorities or department of agriculture, and report them on iNaturalist.

• Consult with local forestry experts and ISA Certified Arborists for advice and assistance on tree-related issues.

Further Reading

• Borowy, Dorothy. 2020. Oak Decline
• Duan J. J., Gould J. R., Quinn N. F., Petrice T. R., Slager B. H., Poland T. M., Bauer L. S., Rutledge C. E., Elkinton J. S., Van Driesche R. G. 2023. Protection of North American ash against emerald ash borer with biological control: ecological premises and progress toward success. BioControl.
• Faubert, Heather. 2023. University of Rhode Island Cooperative Extension. Beech Leaf Disease in RI 2023 update
• Juzwik J., Appel D. N., MacDonald W. L.,Burks, S. 2011. Challenges and Successes Managing Oak Wilt in the United States. Plant Disease.
• National Park Service. 2022. Beech Trees in the National Capital Area
• Oten K., Bertone M., and D. Serpan. 2022. NC State Extension. Elm Zigzag Sawfly
• Tait, Nicholas. 2023. Ash Tree Update 2022.
• US Forest Service. 2023. Beech Leaf Disease: An Emerging Forest Threat in Eastern U.S.
• US Forest Service. 2023. Spongy moth a growing forest health threat, keeping managers on their toes

Learn More about the National Park Service's Inventory & Monitoring Efforts

To help protect natural resources ranging from bird populations to forest health to water quality, National Park Service scientists perform ecological Inventory & Monitoring (I&M) work in parks across the country. The National Capital Region Network, Inventory & Monitoring program (NCRN I&M) serves national parks in the greater Washington, DC area. To learn more about NCRN I&M forest monitoring, you can visit the NCRN forest monitoring webpage.