Wyoming Tree Pests: What Is Killing the Trees

Q: What is killing the trees in Wyoming?

Mountain pine beetle in the Black Hills, and an unusual urban problem: thousand cankers killing planted black walnut in Cheyenne. That comes from the Wyoming State Forestry Division, 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 mountain pine beetle in Wyoming?

June 1 to September 30 is the main window, and what you would see is pitch tubes and boring dust in bark crevices this year, red needles next year. The calendar on this page lays out every agent Wyoming 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 Wyoming, 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 Wyoming?

Neither. It is a count of photographs that two other people confirmed, so it is a record of where people looked. Wyoming has 1 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 Wyoming 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.

Mountain pine beetle

Leads the list

Lodgepole, ponderosa, limber and whitebark pine

2

Agents documented

named in the 2025 state report

0

On watch this month

nothing in its window in October

1

Photographed here

research grade iNaturalist records, all time

The list

What Wyoming 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. Wyoming’s most recent edition is the 2025 one, published by the Wyoming State Forestry Division.

Thousand Cankers Disease (TCD) continues to cause black walnut tree mortality in Cheyenne.

Wyoming State Forestry Division, Forest Health Highlights 2025

Mountain pine beetle

Dendroctonus ponderosae

Not federally quarantined

The native beetle behind the red hillsides of the Interior West. Drought and warm winters are what turn it from background noise into an outbreak.

Kills: Lodgepole, ponderosa, limber and whitebark pine
When you would notice it: Jun 1 to Sep 30: pitch tubes and boring dust in bark crevices this year, red needles next year.

Commonly confused with Every red conifer in the Rockies gets called beetle kill. Drought, root disease, dwarf mistletoe and simple old age all do it too.

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.

What Wyoming 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 Wyoming State Forestry Division states that emerald ash borer has 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 wide valley view of conifer forest in which most of the trees are rust red and dead, with a lake beyond — Beetle killed forest seen from Mount Fraser in British Columbia. The rust red trees are dead or dying lodgepole pine; the green ones are what is left. This is a mountain pine beetle landscape rather than a US one, and it is here because nothing shows the scale of a bark beetle outbreak better. “Mt Fraser - Pine Beetle Damage” by Themightyquill, CC BY-SA 3.0

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
Mountain 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	not typically visible	not typically visible	not typically visible
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

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 Wyoming, and a warm or late year moves them. The current month is outlined.

Live

Has anybody seen it in Wyoming lately

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

The Wyoming agents are found by survey, not by camera

None of the agents Wyoming documents is one a person can reliably photograph and have identified from a photograph. Bark beetles work under bark and the wilts are diagnosed on a culture plate. The state agency is the only detection system that exists for these, which is why the report linked above matters more here than anywhere.

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
Mountain pine beetle	31	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.

Dead grey tanoak trees standing among green vegetation on a California hillside — Tanoak killed by Phytophthora ramorum, photographed for the USDA Forest Service. Sudden oak death is the West Coast answer to the question this page is about, it is federally quarantined, and APHIS states plainly that there is no cure. “Lithocarpus densiflorus dead” by Joseph O'Brien, USDA Forest Service, CC BY 3.0 US

The rules

What is regulated in Wyoming

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

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.
Wyoming State Forestry Division, Forest Health Highlights 2025The Wyoming 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 Wyoming and when it shows itself; the decision about your tree is an arborist’s.

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

Tree pests in other states

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

Wyoming tree pest questions

What is killing the trees in Wyoming?

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 mountain pine beetle in Wyoming?

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 Wyoming?

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 Wyoming, 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?

A certified arborist working in the crown of a large elm tree that is being taken down — A certified arborist removing an infected elm in Saint Paul, Minnesota. This is the end of the road for Dutch elm disease and it is also the point of this whole page: the decision about a specific tree belongs to somebody standing under it with a credential. “Dutch Elm-3” — Susan G. Lesch — Public domain

Everything on this page about what is present in Wyoming comes from the Wyoming State Forestry Division 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.