Maryland Tree Pests: What Is Killing the Trees

Q: When would I actually notice beech leaf disease in Maryland?

May 1 to July 31 is the main window, and what you would see is dark interveinal banding on newly expanded leaves, seen from below looking up. The calendar on this page lays out every agent Maryland documents the same way.

Q: My tree is dying. Who decides what to do about it?

A certified arborist standing under the tree, and for several of the materials involved somebody with a pesticide licence. Several of the diseases here can only be confirmed in a laboratory, so a diagnosis on a particular tree is a job for someone standing in front of it. What this page tells you is what is documented in Maryland, when it shows itself, and exactly which three doors to knock on.

Q: Does the iNaturalist number on this page mean the pest is or is not in Maryland?

Neither. It is a count of photographs that two other people confirmed, so it is a record of where people looked. Maryland has 13,348 research grade records across the agents we track. A zero means nobody photographed it, not that it is absent, and for something that spends its larval life under bark those are very different statements. Presence in Maryland comes from the state agency.

Q: Are the carpenter ants in my tree what killed it?

Almost never. Carpenter ants excavate wood that is already soft and damp, so finding them is evidence the decay came first. They are a symptom worth taking seriously as a sign of internal decay, especially in a tree near a house, which is exactly the case where an arborist should look at it. The ant season page for your state covers what carpenter ants are actually doing.

Q: What can I actually do to stop this?

One thing, and it is the same thing every federal agency says. USDA APHIS puts it in seven words on its spongy moth page: "Buy local firewood and burn it on site. Do not move firewood." Long distance firewood movement is how most of the agents on this page reached most of the places they are, and it is the only part of the problem an ordinary person controls.

Beech leaf

Leads the list

American beech (Fagus grandifolia), and ornamental European beech

7

Agents documented

named in the 2025 state report

3

On watch this month

Beech bark, Spongy moth, Pine beetle

13,348

Photographed here

research grade iNaturalist records, all time

The list

What Maryland documents

Every state forestry agency publishes a Forest Health Highlights report each year in cooperation with USDA Forest Service Forest Health Protection. It is the only source that answers this question uniformly for all fifty states, and it is what this page is built from. Maryland’s most recent edition is the 2025 one, published by the Maryland Department of Agriculture and Maryland Forest Service.

The walnut twig beetle was first detected in Maryland in 2013 and by October 2014 thousand cankers disease was confirmed.

Maryland Department of Agriculture and Maryland Forest Service, Forest Health Highlights 2025

Beech leaf disease

Litylenchus crenatae

Not federally quarantined

Dark bands between the veins of a beech leaf, visible from underneath against the sky. First recorded in Ohio in 2012 and moving fast.

Kills: American beech (Fagus grandifolia), and ornamental European beech
When you would notice it: May 1 to Jul 31: dark interveinal banding on newly expanded leaves, seen from below looking up.
Photographed in Maryland: 264 research grade iNaturalist records, all time

Commonly confused with Banded beech leaves look enough like drought scorch or anthracnose that people wave them off. The difference is geometry: beech leaf disease bands follow the veins in stripes, and they are darkest looking up through the leaf.

Hemlock woolly adelgid

Adelges tsugae

Not federally quarantined

White woolly dots on the underside of hemlock needles. The tree starves over several years and does not come back on its own.

Kills: Eastern and Carolina hemlock (Tsuga)
When you would notice it: Nov 1 to Apr 30: the white woolly egg sacs, which stand out most on bare winter branches.
Photographed in Maryland: 257 research grade iNaturalist records, all time

Commonly confused with The woolly dots are easy to mistake for spider egg sacs or scale. Position is the tell: adelgid sits at the base of a needle, on the underside of the twig, in rows.

Emerald ash borer

Agrilus planipennis

Federal quarantine repealed

The reason the ash trees died. A half inch metallic green beetle whose larvae girdle the tree from the inside.

Kills: Ash (Fraxinus), and white fringetree
When you would notice it: May 15 to Sep 1: adult beetles, and thinning in the top of the crown. Then again Nov 1 to Mar 31: blonde bark where woodpeckers have stripped it, plus D shaped exit holes, both far easier to see with the leaves off.
Photographed in Maryland: 241 research grade iNaturalist records, all time

Commonly confused with Every metallic green beetle in the Midwest gets reported as emerald ash borer, and most of them are six spotted tiger beetles or Japanese beetles. The tell is the tree, not the beetle: emerald ash borer is only ever on ash.

Thousand cankers disease

Geosmithia morbida

Not federally quarantined

Thousands of tiny cankers, each around one beetle tunnel, that merge until the branch dies. Named for the arithmetic.

Kills: Black walnut and other walnuts (Juglans)
When you would notice it: Jul 1 to Sep 30: yellowing and thinning in the upper crown of a walnut, then branch by branch dieback over several years.

Commonly confused with Walnut anthracnose yellows and drops leaves every August in humid country and alarms people annually. Thousand cankers kills from the top down and does not recover.

Beech bark disease

Cryptococcus fagisuga with Neonectria

Not federally quarantined

A scale insect first, then a fungus through the wounds it makes. Not the same thing as beech leaf disease, and the two now overlap.

Kills: American beech
When you would notice it: Sep 1 to Apr 30: a white waxy crust on the trunk, then sunken cankers, then a snapped stem in a wind event.

Commonly confused with Beech bark disease is on the TRUNK and beech leaf disease is on the LEAF. They are unrelated organisms with similar names attacking the same tree in the same forests, and confusing them is the most common mistake in this whole subject.

Spongy moth

Lymantria dispar

Federally quarantined

The caterpillar that eats a whole oak ridge bare in June, formerly called the gypsy moth. Still federally quarantined, and the boundary still moving.

Kills: Oak above all, plus aspen, birch, apple and many others. Repeated defoliation kills
When you would notice it: May 15 to Jul 15: caterpillars, frass falling like rain, and bare crowns in high summer. Then again Aug 1 to Apr 30: tan felt egg masses on trunks, walls, grills and wheel wells.
Photographed in Maryland: 713 research grade iNaturalist records, all time

Commonly confused with A defoliated oak in June looks dead and usually is not. Healthy oaks refoliate. It is the second and third consecutive year, especially with drought on top, that kills them.

Southern pine beetle

Dendroctonus frontalis

Not federally quarantined

A native beetle the size of a rice grain that in outbreak years kills pine by the stand, in expanding spots visible from the air.

Kills: Loblolly, shortleaf, Virginia and pitch pine
When you would notice it: Jun 1 to Oct 31: popcorn sized pitch tubes on the trunk, then a crown that fades yellow and then red.

Commonly confused with Ips engraver beetles and black turpentine beetle also attack stressed pines and are far more common in a drought year. Southern pine beetle is the one that kills healthy trees in expanding spots.

A bare, dead white ash tree standing beside a suburban road with a second thinning ash behind it — A dead white ash on Windybush Way in Ewing Township, Mercer County, New Jersey, photographed in June 2023 with a second one going the same way behind it. This is what emerald ash borer looks like from a car: a bare tree in full summer. “A dead White Ash killed by Emerald Ash Borer, Ewing Township, Mercer County, New Jersey” by Famartin, CC BY-SA 4.0

The timing

When you would actually notice it

This is the part almost nobody publishes. A pest has a season in which it shows itself to a person walking past, and it is frequently not the season you would guess. Hemlock woolly adelgid is easiest to find in the middle of winter, when the white egg sacs stand out on a bare branch and the insect is not moving. Emerald ash borer signs are clearest with the leaves off, because that is when you can see where the woodpeckers have been.

When in the year you would actually notice each one. A second, lighter band is a second window: several of these are easiest to spot in winter, when the leaves are off and the sign is on the bark.
Agent	J	F	M	A	M	J	J	A	S	O	N	D
Beech leaf	not typically visible	not typically visible	not typically visible	not typically visible	main window	main window	main window	not typically visible	not typically visible	not typically visible	not typically visible	not typically visible
Adelgid	main window	main window	main window	main window	not typically visible	not typically visible	not typically visible	not typically visible	not typically visible	not typically visible	main window	main window
Ash borer	second window	second window	second window	not typically visible	main window	main window	main window	main window	main window	not typically visible	second window	second window
Thousand cankers	not typically visible	not typically visible	not typically visible	not typically visible	not typically visible	not typically visible	main window	main window	main window	not typically visible	not typically visible	not typically visible
Beech bark	main window	main window	main window	main window	not typically visible	not typically visible	not typically visible	not typically visible	main window	main window	main window	main window
Spongy moth	second window	second window	second window	second window	main window	main window	main window	second window	second window	second window	second window	second window
Pine beetle	not typically visible	not typically visible	not typically visible	not typically visible	not typically visible	main window	main window	main window	main window	main window	not typically visible	not typically visible

Windows are ours, built from what the federal agencies publish about each organism’s life stages and about what is visible when. They are typical seasons for the country, not a forecast for Maryland, and a warm or late year moves them. The current month is outlined.

Live

Has anybody seen it in Maryland lately

The panel below reads iNaturalist as you load the page: research grade records in Maryland in the last twelve months, for the agents that a person can actually photograph. Research grade means the observer posted a photograph with a date and a location and at least two other people agreed on the identification.

Recent confirmed sightings in Maryland

Reading iNaturalist…

The monthly curve, from a stored snapshot

Below is when these records were actually made in Maryland, by month, across the whole history of the platform. Switch to the adjusted series to divide out observer effort: the raw counts partly track how many people were outdoors with a phone, and the adjusted index removes that.

Beech leaf disease records in Maryland, by month

264 research grade iNaturalist records of Litylenchus crenatae in Maryland, grouped by the month they were observed. Snapshot taken 2026-08-29.This is when people PHOTOGRAPHED it, which is a good proxy for when it is visible and a poor one for how much of it there is. Presence in Maryland comes from the state agency, not from this chart. Curves are drawn only where a state has at least 25 records.

Hemlock woolly adelgid records in Maryland, by month

257 research grade iNaturalist records of Adelges tsugae in Maryland, grouped by the month they were observed. Snapshot taken 2026-08-29.This is when people PHOTOGRAPHED it, which is a good proxy for when it is visible and a poor one for how much of it there is. Presence in Maryland comes from the state agency, not from this chart. Curves are drawn only where a state has at least 25 records.

What iNaturalist cannot see, and why that matters

Two of the agents on this page can be photographed on a leaf. The rest cannot, and the national record counts make the gap impossible to argue with. Sudden oak death has killed millions of trees and has a few hundred research grade records in the entire history of the platform, because it is confirmed on a culture plate. The gap in this table is a gap in visibility, not in damage.

Research grade iNaturalist records in the United States, whole platform history. The gap between the top group and the bottom group is not a measure of how much damage each one does.
Agent	US records	Why
Spongy moth	28,732	Sits on a leaf, a needle or a trunk where anybody can photograph it.
Beech leaf disease	8,003	Sits on a leaf, a needle or a trunk where anybody can photograph it.
Hemlock woolly adelgid	7,880	Sits on a leaf, a needle or a trunk where anybody can photograph it.
Emerald ash borer	4,362	Sits on a leaf, a needle or a trunk where anybody can photograph it.
Beech bark disease	803	Lives under bark or in the crown, where a phone camera cannot reach it.
Southern pine beetle	92	Lives under bark or in the crown, where a phone camera cannot reach it.
Thousand cankers disease	0	Diagnosed in a laboratory, not from a photograph.

A fallen ash log with the bark gone, covered in winding S shaped larval tunnels and small dark exit holes — The record of an infestation, on a dead ash in southeastern Wisconsin that fell and lost its bark. Every winding channel is one larva that ate its way around the trunk under the bark. The small dark holes are where the adults left. “Bore remnants” by TheLostPariah, CC BY 4.0

The rules

What is regulated in Maryland

The most common wrong belief about this whole subject is that emerald ash borer is still federally quarantined. It is not, and has not been since the start of 2021. Four other agents are, and the boundaries move by federal order several times a year, so the issuing agency, linked below, is the source for the current boundary.

APHIS removed its domestic EAB quarantine regulations in 2020. The final rule was effective January 14, 2021.

USDA APHIS, Emerald Ash Borer as of April 23, 2026

No current state level quarantine specific to Maryland appears in the sources behind this page. State plant regulatory orders change without much publicity, and the Maryland plant health contacts at the reporting link below are the authority on what is in force.

The one thing you can actually do

Almost every agent on this page reached almost every place it is by riding in a piece of wood somebody put in a car. USDA APHIS says it in three words on both its emerald ash borer page and its spongy moth page:

Buy local firewood and burn it on site. Do not move firewood.

USDA APHIS, Spongy Moth

APHIS links dontmovefirewood.org from its own pest pages, so that is where this page points too. The campaign is run by The Nature Conservancy and its text is copyrighted, so nothing on this site quotes it; the wording above is APHIS’s own and is in the public domain.

What to do

You have a dying tree in Maryland

You have a dying tree. Here is where to go.

Whether to treat a tree or take it down is a decision for somebody standing under it with a credential and a licence. These are the three doors.

Find a certified arboristA diagnosis on a specific tree, and any decision about what to do with it, belongs to somebody standing under it with a credential. That is not us. International Society of Arboriculture keeps the directory.
Maryland Department of Agriculture and Maryland Forest Service, Forest Health Highlights 2025The Maryland forest health programme is the body that documents what is actually here, and its annual report is what this page is built from.
Report a suspected pestAPHIS keeps the current contact for every state plant health director and state plant regulatory official. For a regulated pest, that call is the one that matters.

Several of the diseases on this page can only be confirmed in a laboratory, and several of the materials used against these organisms are restricted use and need a licensed applicator. This page tells you what is documented in Maryland and when it shows itself; the decision about your tree is an arborist’s.

A metallic green emerald ash borer beetle held on a piece of bark between finger and thumb — An emerald ash borer held on a piece of bark in Oregon in August 2022, photographed by the US Fish and Wildlife Service. Oregon was the first West Coast detection. The beetle is about half an inch long, which is most of the reason nobody notices it. “Emerald ash borer (52485411408)” — USFWS Pacific — Public domain

Tree pests in other states

Maryland fall foliageA dying tree is not autumn colour. Here is when the real colour arrives, so you can tell them apart.Maryland ant seasonCarpenter ants colonise wood that is already decaying. Finding them is a symptom, not the cause.Maryland termite swarm seasonThe other insect people find in dead wood, and the one that actually threatens the house.Maryland droughtDrought stress is what turns a background beetle into an outbreak. This is the other half of the story.The national tree pest guideWhich quarantines are real, which were repealed, and every state side by side.

Maryland tree pest questions

What is killing the trees in Maryland?

Beech and hemlock in the west, ash everywhere, and one of the few confirmed thousand cankers footholds east of the Mississippi. That comes from the Maryland Department of Agriculture and Maryland Forest Service, in its Forest Health Highlights report for 2025, which is the annual document every state publishes in cooperation with USDA Forest Service Forest Health Protection. It is linked from this page.

Is there still a federal emerald ash borer quarantine?

No, and this is the single most common wrong belief about this subject. USDA APHIS states on its own emerald ash borer page: "APHIS removed its domestic EAB quarantine regulations in 2020. The final rule was effective January 14, 2021." Federal effort moved to biological control. Some states kept quarantines of their own, and those are the rules that still bind. Four other agents on this site are still federally quarantined: Asian longhorned beetle, spongy moth, sudden oak death, and none of them is the one people expect.

When would I actually notice beech leaf disease in Maryland?

My tree is dying. Who decides what to do about it?

Does the iNaturalist number on this page mean the pest is or is not in Maryland?

Is a bare or brown tree in autumn a dying tree?

Usually not, and telling the two apart is a real skill. Autumn colour arrives across a whole neighbourhood at once, moves from the outside of the crown inward, and the leaves drop cleanly. A dying tree browns out of step with everything around it, often on one side or from the top down, and the dead leaves frequently stay hanging on the branch. If you want the normal colour dates for Maryland, they are on the fall foliage page for the state.

Are the carpenter ants in my tree what killed it?

What can I actually do to stop this?

Rows of small white woolly balls along the underside of hemlock twigs against a blue sky — Hemlock woolly adelgid ovisacs on the underside of eastern hemlock branches, photographed for the USDA Forest Service. This is the thing to look for, and the time to look is winter: the white stands out and the insect is not moving. “Adelges tsugae 1276001” by Michael Montgomery, USDA Forest Service, CC BY 3.0 US

Everything on this page about what is present in Maryland comes from the Maryland Department of Agriculture and Maryland Forest Service Forest Health Highlights report for 2025 and from USDA APHIS. Sentences in quotation marks are verbatim from those sources with the issuer named; everything else is ours. iNaturalist counts are a record of where people looked and are not a survey. MyWild states no diagnosis, recommends no treatment and names no product. The national tree pest guide.