Illinois Tree Pests: What Is Killing the Trees

Q: What is killing the trees in Illinois?

Oak wilt in every county, and a spotted lanternfly foothold in Cook County that is still growing. That comes from the The Morton Arboretum, for the Illinois Forest Health programme, 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.

Q: When would I actually notice oak wilt in Illinois?

June 1 to August 31 is the main window, and what you would see is a single oak browning from the top down and shedding half green leaves while its neighbours stay green. The calendar on this page lays out every agent Illinois 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 Illinois, 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 Illinois?

Neither. It is a count of photographs that two other people confirmed, so it is a record of where people looked. Illinois has 735 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 Illinois 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.

Oak wilt

Leads the list

Red oak group fast, white oak group slowly

4

Agents documented

named in the 2025 state report

1

On watch this month

Lanternfly

735

Photographed here

research grade iNaturalist records, all time

The list

What Illinois 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. Illinois’s most recent edition is the 2025 one, published by the The Morton Arboretum, for the Illinois Forest Health programme.

The 2025 Illinois forest health report puts oak wilt in every county in the state and calls it a major urban and forest tree disease, and it records several new spotted lanternfly infestations in Cook County adding to the one found in September 2023. The Illinois team also looked for sudden oak death and laurel wilt during the field season and detected neither. Established by The Morton Arboretum, Illinois Forest Health Highlights 2025. Restated here in our own words.

Oak wilt

Bretziella fagacearum

Not federally quarantined

A fungus that plugs an oak's water vessels. A red oak can go from green to bare in a few weeks in midsummer.

Kills: Red oak group fast, white oak group slowly
When you would notice it: Jun 1 to Aug 31: a single oak browning from the top down and shedding half green leaves while its neighbours stay green.

Commonly confused with Oak wilt, bur oak blight, anthracnose and plain drought all brown an oak. Only a laboratory can tell you which, and this is the single most common case where a homeowner needs a diagnosis before a decision.

Spotted lanternfly

Lycorma delicatula

State quarantines only

The one people actually see. A planthopper that swarms trunks by the hundred and rains sticky honeydew on everything under it.

Kills: Rarely kills mature trees outright. Kills tree of heaven and grapevine, and stresses maple, walnut, birch and willow
When you would notice it: Jul 15 to Nov 15: adults, in numbers, on trunks and fences. Then again Oct 1 to May 15: egg masses on any hard surface, which is the stage that hitches a ride on your car.
Photographed in Illinois: 70 research grade iNaturalist records, all time

Commonly confused with A lanternfly on your maple is not what is killing your maple. It is a real quarantine pest and a genuine nuisance, and it is also the thing people blame for damage that turns out to be drought, girdling roots or construction.

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 Illinois: 175 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.

Dutch elm disease

Ophiostoma novo-ulmi

Not federally quarantined

The one that already happened. It removed the American elm from the American street between the 1930s and the 1970s and it is still here.

Kills: American elm, and other elms to varying degrees
When you would notice it: Jun 1 to Aug 31: flagging: one branch high in the crown yellowing and wilting while the rest of the tree is green.

Commonly confused with Elm yellows, elm leaf beetle and simple drought all yellow an elm. Flagging, one branch at a time from the top, is the classic Dutch elm pattern.

What Illinois says it does NOT have

This is the rarest thing in the whole subject and it is worth more than a map with a blank space on it. The The Morton Arboretum, for the Illinois Forest Health programme states that sudden oak death and laurel wilt have not been confirmed in the state. If you are looking at a sick tree and that was your first guess, the state has already looked and not found it. Read the 2025 report.

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 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
Oak wilt	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	not typically visible
Lanternfly	second window	second window	second window	second window	second window	not typically visible	main window	main window	main window	main window	main window	second 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
Dutch elm	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	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 Illinois, and a warm or late year moves them. The current month is outlined.

Live

Has anybody seen it in Illinois lately

The panel below reads iNaturalist as you load the page: research grade records in Illinois 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 Illinois

Reading iNaturalist…

The monthly curve, from a stored snapshot

Below is when these records were actually made in Illinois, 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.

Spotted lanternfly records in Illinois, by month

70 research grade iNaturalist records of Lycorma delicatula in Illinois, 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 Illinois comes from the state agency, not from this chart. Curves are drawn only where a state has at least 25 records.

Emerald ash borer records in Illinois, by month

175 research grade iNaturalist records of Agrilus planipennis in Illinois, 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 Illinois 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
Spotted lanternfly	91,063	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.
Oak wilt	30	Diagnosed in a laboratory, not from a photograph.
Dutch elm disease	1	Diagnosed in a laboratory, not from a photograph.

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

The rules

What is regulated in Illinois

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 Illinois appears in the sources behind this page. State plant regulatory orders change without much publicity, and the Illinois 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 Illinois

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.
The Morton Arboretum, for the Illinois Forest Health programme, Forest Health Highlights 2025The Illinois 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 Illinois and when it shows itself; the decision about your tree is an arborist’s.

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

Tree pests in other states

Illinois fall foliageA dying tree is not autumn colour. Here is when the real colour arrives, so you can tell them apart.Illinois ant seasonCarpenter ants colonise wood that is already decaying. Finding them is a symptom, not the cause.Illinois termite swarm seasonThe other insect people find in dead wood, and the one that actually threatens the house.Illinois 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.

Illinois tree pest questions

What is killing the trees in Illinois?

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 oak wilt in Illinois?

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 Illinois?

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 Illinois, 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?

Split firewood logs with the bark peeled back to show winding beetle galleries in the wood beneath — Emerald ash borer found in wood intercepted at a California agricultural inspection station. This is the whole argument for the firewood rule in one photograph: the beetle was in the wood, and the wood was in a vehicle. “Emerald Ash Borer in firewood (5815090311)” — USDA Forest Service Pacific Southwest Region 5 — Public domain

Everything on this page about what is present in Illinois comes from the The Morton Arboretum, for the Illinois Forest Health programme 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.