Forest Pests and Pathogens in the NCR: A 2025 Summary

By Ava Klink & Megan Nortrup

Figure 1. Canopy thinning caused by beech leaf disease at Prince William Forest Park in 2025.

NPS / Andrejs Brolis

Overview

Pests and pathogens are an ongoing threat to forests in the National Capital Region (NCR). They range from newly emerging invasive species to long-established pests. Since 2006, the NCR’s Inventory & Monitoring Network (NCRN I&M) has tracked the distribution and impact of priority forest pests through its long-term forest vegetation monitoring. This summary highlights trends through 2025 and examines how the region’s most significant pests and pathogens are reshaping eastern deciduous forests.

Beech Leaf Disease

Since its first NCR detection at Prince William Forest Park in 2021, beech leaf disease (BLD) has spread rapidly. It is now present in all NCR parks with high densities of American beech trees (Fagus grandifolia). Forest monitoring data collected through 2025 show that BLD has not been detected at Manassas National Battlefield Park or Wolf Trap National Park for the Performing Arts, both of which support only sparse beech populations.

Figure 2. Beech tree density per hectare from 2010 to 2025 across all National Capital Region parks. Symbols show healthy beech trees (green circles), beech trees infected with beech leaf disease (yellow triangles), and dead beech trees (black squares).

NPS

Figure 2. shows the average densities of healthy, infected, and dead beech trees per hectare across NCR parks. Each point represents the most recent visit to each plot, which typically spans a four-year period, averaged over the 4-year monitoring cycle. Of the 90+ plots monitored only in 2025, approximately 80% of beech trees showed symptoms of BLD. Despite the widespread distribution of BLD in NCR parks, monitoring data has not yet shown a corresponding increase in beech mortality.

BLD threatens both native American beech (Fagus grandifolia)—currently the most common tree species in the region—and the non-native European beech (Fagus sylvatica). Maps showing areas of beech abundance in each NCR park can help park managers identify the areas most likely to see changes.

We do not yet fully understand how climate, soil conditions, competition, and other environmental stressors will influence BLD progression and tree survival. It is anticipated that BLD will likely kill smaller beech trees and saplings sooner than it kills larger ones. Estimates of survival time for infected beech are around 4 to 5 years for smaller trees and around 10 years for larger, mature trees.

Characteristic BLD symptoms include dark banding between leaf veins, thickened and leathery leaves, leaf curling or puckering, and reduced leaf size. As the disease progresses it causes canopy thinning, branch dieback, and eventually, mortality. Canopy loss allows more sunlight to reach the forest floor, creating conditions that favor invasive plant growth while altering the forest's natural appearance and the visitor experience.

Beech loss may alter soil nutrient cycling through increased leaf litter and change forest microclimates by increasing light penetration and creating warmer, drier conditions. These effects in turn may alter plant communities, insects, amphibians, reptiles, and other forest-dependent species.

As with any pest or pathogen, natural resistance is possible as the pest spreads. A wide range of treatment options for BLD is also evolving rapidly. If you have questions about treatment options, please contact the NCR’s Integrated Pest Management Coordinator.

Emerald Ash Borer

Figure 3. Density of ash trees per hectare across all NCR parks from 2009 to 2025. Symbols show live ash (green circles) and standing dead ash trees (black squares). Vertical lines represent 95% confidence intervals.

NPS

Emerald ash borer (EAB) virtually wiped out mature ash trees (Fraxinus spp.) from NCR parks between 2014 and 2024 (Figure 3). The number of standing dead ash trees peaked in 2023.

Despite this widespread mortality, regeneration of young trees is occurring. Ash seedlings and saplings have increased across the region (Figure 4). Pumpkin ash (F. profunda) and green ash (F. pennsylvanica) have been resprouting from stumps in wetland areas of several parks. Particularly dense ash regeneration has also been documented at Catoctin Mountain Park.

Figure 4. Ash seedling, sapling, and tree densities between the 2009-12 and 2022-25 monitoring periods in the National Capital Region.

NPS

However, as young ash trees grow large enough to provide suitable habitat for EAB, they are also likely to become infested and eventually die. For this reason, ash is excluded from NCRN I&M assessments of forest regeneration in NCR parks.

Some NCR parks are working to restore habitats where large numbers of ash trees died. For example, a reforestation project at Poplar Grove in Catoctin Mountain Park removed standing dead ash trees and invasive vegetation and planted a diverse mix of native tree saplings.

Spotted Lanternfly

The invasive planthopper spotted lanternfly (SLF), has spread quickly throughout the NCR since 2021. Public messaging to report and squash SLF has been vigorous. Yet to date, NCRN forest monitoring data has not shown significant impacts on forest health. Unlike the emerald ash borer which feeds only on a single genus, SLF feeds on more than 100 species of woody and herbaceous plants. It does have a strong preference for the invasive tree-of-heaven, Ailanthus altissima.

Figure 5. An invasive tree-of-heaven with sooty mold growth resulting from spotted lanternfly.

NPS / Ashley

While feeding, SLF excretes a sticky “honeydew” that can promote the growth of sooty mold on leaves and other surfaces (Figure 5). However, in most forest settings, SLF contributes only minor additional stress to trees and other vegetation. It is not known to cause mortality on its own. The greatest documented impacts have occurred in commercial vineyards, orchards, nurseries, and ornamental landscapes. Effects on natural forest ecosystems have been limited. NCRN I&M continues to monitor trees in the region for any unexpected signs of decline that may indicate longer-term impacts from SLF.

Elm Zigzag Sawfly and Other Pests

Figure 6. Elm zigzag sawfly damage on elm leaf.

Matt Bertone / NC State University

First detected in Maryland in 2022, the elm zigzag sawfly (Aproceros leucopoda) was reported in Frederick County, MD in July 2026. This invasive insect feeds on native American elm (Ulmus americana), slippery elm (U. rubra), and winged elm (U. alata). It also feeds on non-native species, including Siberian elm (U. pumila), English elm (U. parvifolia), Chinese elm (U. procera), and even Japanese zelkova (Zelkova serrata).

Young larvae produce distinctive zigzag feeding patterns along leaf margins (Figure 6). Older larvae can consume entire leaves. Large infestations may completely defoliate trees. Repeated defoliation weakens trees over time and can contribute to tree mortality. Learn more about the elm zigzag sawfly.

Newer pests like BLD, SLF, and elm zigzag sawfly get considerable attention. Yet many now-resident pests continue to cause tree decline, mortality, and changes in forest composition, including:

• hemlock woolly adelgid (Adelges tsugae)
• dogwood anthracnose (Discula destructiva)
• Dutch elm disease (Ophiostoma ulmi and O. novo-ulmi)
• chestnut blight (Cryphonectria parasitica)

Spongy moth (Lymantria dispar) has periodic outbreaks, however its frequency and severity is reduced by established biological control agents, broad-scale treatments, and naturally occurring pathogens.

Everyone Can Help

Anyone can take steps to reduce the spread of pests and pathogens. Here are a few important things to remember:

• Don’t transport firewood. Buy wood 10 miles or less from where you burn it.
• Clean off your shoes after visiting a park or between visits to different parks.
• Avoid introducing new pests and pathogens by planting local, native species.
• Stay on marked trails and always keep your dog on leash.
• Report sightings of any new pests to your local county authorities or department of agriculture and report them on iNaturalist.
• Learn more about caring for trees from the International Society of Arboriculture.

Further Reading

• National Park Service. 2025. Treating Beech Leaf Disease
• North Carolina State Extension. 2022. Elm Zigzag Sawfly
• Penn State Extension. 2024. Beech Leaf Disease
• Shepherd, Brianna L., David J. Burke, and Katharine L. Stuble. 2025. Fagus grandifolia growth and mortality a decade after the emergence of Beech leaf disease. Trees, Forests and People, Volume 20, https://doi.org/10.1016/j.tfp.2025.100836
• USDA Animal and Plant Health Inspection Service. 2026. Emerald Ash Borer
• USDA Animal and Plant Health Inspection Service. 2025. Spotted Lanternfly

Literature & Data Cited

Brolis AJ, Schmit JP, Ashley LM. 2026. National Capital Region Network Forest Vegetation Monitoring Data Package - cumulative through 2025. National Park Service. Fort Collins CO. https://doi.org/10.57830/2318583

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