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Tree Pests and Pathogens of Most Concern

Emerald Ash Borer
Hemlock Woolly Adelgid
Oak Wilt 
Beech Bark Disease
Thousand Cankers Disease 
Balsam Wooly Adelgid

Armillaria Root Rot

Heterobasison Root Disease (HRD)

Spotted Lanternfly (SLF)

Asian Longhorned Beetle (ALB)

Sugar Maple Borer


What is it? - Emerald Ash Borer is an exotic beetle that was discovered in southeastern Michigan near Detroit in 2002. The adult beetles nibble on the foliage of ash, but cause little damage. The larvae feed on the inner bark of ash trees, damaging the tree's ability to transport water and nutrients.

How does it spread? - The adult Emerald Ash Borer beetle can fly short distances. Occasionally it can fly long distances in the right conditions, but typically, EAB is spread by transporting firewood. It is also through transporting ash materials that harbor the live emerald ash borers.

What can you do about it? -
It is key to know the symptoms of EAB:

  1. thinning or dying of ash tree crowns

  2. suckers at the base of the tree

  3. splitting bark

  4. tunneling under the bark

  5. D-shaped exit holes and woodpecker activity.

The United States Department of Agriculture (USDA) recommends the following to help manage this pest:

  • Call the USDA Emerald Ash Borer Hotline at 1-866-322-4512 or your local USDA Animal and Plant Health Inspection Service (APHIS) office if you think you’ve found an EAB infestation. Find contact information for your local APHIS office at the USDA’s website.

  • Record the area where you found the insect and take photos of the insect along with any damage.

  • Don’t move firewood from your property or carry it across state lines.

  • Buy firewood from local sources and burn it where you buy it.

  • Buy kiln-dried firewood.

  • Before spring, burn your remaining firewood supply to eliminate the chance of EAB spreading to live trees.



What is it? - Hemlock woolly adelgid are tiny sap-sucking insects that secrete white wax as they feed on hemlock shoots and branches. Over time as these bugs feed on the needles, shoots, and branches; tree growth slows down and the trees may appear to be grayish-green. This happens because their saliva is toxic to the tree and causes it to eventually drop its needles and causes its twigs to die back. 

How does it spread? - Hemlock woolly adelgid is spread by wind, birds, animals, and accidental movement by people (such as moving firewood). These insects are also parthenogenic, which means that they are all female and produce offspring without mating.

What can you do about it? - Do not remove potentially infested material from the site. Instead, take pictures and report any concerns. Heavy infestations can kill the tree within four to ten years, and trees are also weakened and made vulnerable to attack by other insects and diseases. Some trees recover, although the reasons are not well understood.

  1.  If a single tree becomes infected with Hemlock Woolly Adelgid - In infested areas, the insect can be managed on individual trees through the use of insecticides, horticultural oils and insecticidal soaps. Keeping ornamental hemlocks well-watered and healthy can help them to withstand an infestation.                                                                                                                                                                                       

  2.  If a forest becomes infected - Widespread insecticide treatment in forests is not practical and salvaging dead or dying trees is the most common management technique.                                                                                                                                                                                    

  3. Another approach to managing this disease is to introduce other insects that are native to where this disease originated from. One predatory beetle (Sasajiscymnus tsugae) has been released in the eastern U.S. to help control populations of hemlock woolly adelgid. Its adults and larvae prey on hemlock woolly adelgid and help to reduce its numbers. In North America there are a few native predators but they do not eat enough of the adelgids to prevent damage to hemlocks. Laricobius negrinus, a predatory beetle found in the western U.S., also preys on hemlock woolly adelgids and is currently being released in the eastern states.


What is it? - Oak wilt is a fungus that kills oak trees during the spring and summer months. The fungus always kills red oaks, but white oaks can survive infection. 

Red oaks infected with oak wilt will die in less than a month. Leaves of infected oaks will brown from the tips toward the veins. Infected trees will shed brown and green leaves during the summer time.

How does it spread? - Fungal mats form under the bark of trees that died the previous year. Fungal mats, also called “pressure pads,” can cause obvious cracks in the bark of dead trees. Finding a fungal mat on a dead oak is the only way to diagnose oak wilt in the field. The fungal spores produced by pressure pads can infect fresh wounds on other oaks between April 15th and July 15th. 

​The best way to prevent oak wilt is to avoid wounding oak trees between April 15th and July 15th. Another good way to prevent oak wilt is by not moving firewood. Not moving firewood is also a great way to prevent the overland spread of many forest pests.

​Once the fungus has killed a red oak, it will spread underground to neighboring red oaks through root connections called “root grafts.” Red oaks with trunks greater than 20 inches across can be more than 100 feet apart in the forest and still be connected underground.

What can you do about it?  

  1. If a single tree becomes infected with oak wilt - Cut it down and pull the stump with an excavator the year it is infected. The stump can be left in the hole, but the wood from the infected tree must be sawn into lumber, burned, or covered with a tarp by the following April. Tarped wood must stay covered until July 15th. After July 15th it can no longer infect other oak trees.​                                                                                                             

  2. If the fungus has already killed trees and is spreading - In this case, the fungus is moving underground. Contact your Conservation District Forester to evaluate the site. The forester can figure out the size of the infected area, and a vibratory plow or excavator can be used to dig a root graft barrier (a 5' deep narrow trench) around the infected area. After the root graft barrier is in place, all red oak trees within the infected area must be harvested.

Healthy trees can be injected with a substance that protects the trees from oak wilt infection. These substances, called 'fungistats,' are not available to the general public. Contact local tree care professionals for assistance. ​​

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What is it? - Thousand Cankers Disease is a disease that affects black walnut trees and is caused by a native twig beetle along with a newly identified pathogen. The pathogen will kill small areas of tissue which will result in the formation of cankers. As more cankers form, branches will die and eventually the entire tree will die as the cankers interrupt the flow of fluids.

How does it spread? - Walnut twig beetles will feed on tree branches and introduce the fungal pathogen to the tree. This beetle and fungus can be transported into new areas in walnut logs, firewood, and staves used for woodworking. The beetles and fungus are restricted to the tree bark so consider the movement of any bark materials from dead or dying trees as the highest risk for additional disease spread.

What can you do about it? - ​If you notice that black walnut trees have wilting leaves or dying branches during the summer, carefully inspect the tree. If there is no obvious cause of the problem, such as a broken branch, take notes and report the tree. Suspect twigs will have slightly sunken spots, often darker than the surrounding area. If the bark is scraped away, dark ovals on light-colored healthy tissue suggest Thousand Cankers Disease. In the center of the canker, a small hole or dark spot might be present. Identifying and confirming this disease requires specialized expertise.

Beetles can emerge by the thousands from any dead and dying wood pieces. Chipping does not kill the beetles, but does interfere with their breeding, and often results in wood pieces still large enough to allow beetles to emerge and spread Thousand Cankers Disease. Prompt destruction by burning or burying is probably the only sure way to prevent spread from infested walnut. Be sure to check for local air quality and fire restrictions prior to burning. Covering infested wood with plastic or other material is not sufficient.

Beech Bark Disease

What is it? - Beech bark disease (BBD) is caused by both a sap-feeding scale insect and a fungus. American beech trees are first infested with beech scale. Scale feeding allows infection by the Neonectria fungus. The fungus kills the wood, blocking the flow of sap. Affected trees decline in health and eventually die. Some infected trees break off in heavy winds before dying – a condition called "beech snap" (see photo). The scales are covered with white wool, turning infested portions of the tree white.

How Does it Spread? - The scale insect is dispersed via wind, and first attacks the host tree, creating entry ways for the Neonectria fungus to enter. This fungus is dispersed via wind and rain splash, and enters the host tree via the pathways created by the scale insect.

What Can You do About it? -  Don’t move beech firewood or logs from infested areas to un-infested areas. Controlling the natural spread of BBD is not feasible because both the scale and fungus are moved by animals and the wind. Once scale infests trees in your area, watch for resistant trees. Report new finds.

For more information, click HERE.


Balsam Wooly Adelgid

What is it? - Balsam Wooly Adelgid is an invasive, non-native species of sap-feeding insect that attacks true fir trees, including balsam, concolor (white) and fraser fir. Repeated attacks weaken trees, cause twig gouting, kills branches and, over the course of several years, cause trees to die. There are nearly 1.9 billion balsam fir trees in Michigan's forests. As the third largest Christmas tree-growing state in the country, Michigan produces nearly 13.5 million fir trees each year, grown on over 11,500 acres. True fir trees, including forest, landscape and Christmas trees, are susceptible. It was first introduced to North America from Europe around 1900, and was first noted in the Pacific Northwest around 1930.

How Does it Spread? - Balsam woolly adelgid can be introduced on infested nursery stock, Christmas trees, firewood or tree products. Though the insects don't move far on their own, they can be carried by wind, wildlife or vehicles to new locations.

What Can You do About it? - If you notice white, waxy material on twigs, branches or stems, or twig gouting on fir trees, do not move them! Take photos, note the location and report it to:

  • Michigan Department of Agriculture and Rural Development, or phone the MDARD Customer Service Center, 800-292-3939.

  • Or - use the Midwest Invasive Species Information Network (MISIN) reporting tool.

  • Or - download the MISIN smartphone app and report from your phone -

For more information click HERE.

Armillaria Root Rot

What is it? - Armillaria root rot is a pathogen that attacks deciduous and coniferous trees, as well as shrubs. It is caused by the Desarmillaria fungus (formerly known as Armillaria). fungus attacks and kills the vascular cambium (the tissue that generates bark and wood) in woody roots, then spreads laterally to the main stem, which can girdle the base of the trunk and kill the entire tree. Armillaria is also a white rot wood decay fungus which destroys the strength of wood in roots and at the base of infected tree trunks, thereby increasing the likelihood of tree failure. This dual nature of Armillaria, both as a pathogen (killing the living tissues in a tree) and a saprobe (living on dead or non-functional wood after the infected host dies), presents a challenge to management because its inoculum (infective tissue or propagules) can persist for decades below ground as mycelium (vegetative fungal tissue) living in partially-decayed woody roots (residual roots) long after the infected host plants have died or were removed.

How Does it Spread? - Armillaria root rot has a complex life history that involves vegetative spread and sexual reproduction via spore dispersal. Vegetative spread of the fungus below ground, as it grows from root-to-root contact between adjacent hosts, often results in a large clonal population of the fungus, creating a widening circle of dead trees, especially noticeable in forests, vineyards, and orchards as a disease center.

The pathogen reproduces by release of basidiospores produced by its mushrooms. These basidiospores are involved in dispersal but are thought to seldom play an active role in infection of new hosts, instead possibly colonizing dead stumps, downed trees, and other woody debris near the parent mycelium. Two spores, each of which have half the genetic equivalent of their parent, must germinate and fuse together in the same wood to form a new fertile mycelium, capable of producing its own mushrooms and, thus, spores. There are no other spore-bearing phases in the Armillaria life history. Infection is thought to proceed primarily by direct Armillaria-to-host contact, either when healthy roots grow into contact with residual roots or when rhizomorphs grow out from infected roots and contact susceptible roots.

What Can You Do? - Reduce stress on trees: 

  • Mulch the soil around the base of the tree.

  • Water trees during drought.


Heterobasidion Root Disease

What is It? - Heterobasidion root disease (HRD), formerly known as annosum root rot, is considered one of the most destructive diseases of conifers in the northern parts of the world. Many woody species have been reported as HRD hosts in the world. HRD is known to infect and kill red, white, and jack pines, white and Norway spruces, balsam fir and red cedar. Infection has been discovered on some hardwood species, but impacts appear to be minimal.

How Does it Spread? - Infection by the HRD fungus (Heterobasidion irregulare; formerly H. annosum) most often occurs when spores, produced by the fruit body, land and germinate on the surface of a wound, or freshly cut stump. The HRD fungus colonizes the stump, moves into the root tissue and progresses from tree to tree via root contact at the rate of approximately 3.2- 6.5 ft/yr. Infection through root and lower stem wounds can also occur. The fungus degrades both the lignin and the cellulose and causes a stringy yellow decay in the roots and lower stem. 

What Can You Do? - Once this disease is in a stand, it is very difficult to control. Prevention is the best approach.

If you are planning a thinning, consider treating freshly cut stumps of susceptible tree species with fungicides. Stumps should be treated as soon as possible after cutting and no later than one day after cutting. 

Many factors, such as tree species, distance from HRD-infected stands, and time of year influence the risk of infection and impact by HRD.

For more information, click HERE!

Spotted Lanternfly (SLF)

What is It? - Spotted lanternfly (SLF) is an invasive pest from Asia that primarily feeds on tree of heaven (Ailanthus altissima) but can also feed on a wide variety of plants such as grapevine, hops, maple, walnut, fruit trees and others. While deviously beautiful pest has not yet been confirmed in Michigan, it is high up on the list of potential invasive pests, and is capable of causing great harm.

How Does it Spread? - SLF can jump and fly short distances, they spread primarily through human activity. They lay their eggs on vehicles, firewood, outdoor furniture, stone, etc. which are inadvertently transported to new areas, causing the insect to spread.

What Can You Do? 

  • Learn how to identify SLF.

  • Inspect outdoor items such as firewood, vehicles, and furniture for egg masses.

  • If you visit other states with SLF, be sure to check all equipment and gear before leaving. Scrape off any egg masses.

  • Destroy egg masses by scraping them into a bucket of hot, soapy water or a baggie/jar of hand sanitizer.


For more information on SLF, click HERE!

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Asian Longhorned Beetle (ALB)

What is It? - Asian longhorned beetle (Anoplophora glabripennis), or ALB, is a destructive wood-boring pest that feeds on maple and other hardwoods, eventually killing them. It most likely came to the United States inside wood packaging material from Asia.

How Does it Spread? - Since beetle larvae live deep inside trees the majority of the year, they can easily and unknowingly be moved in firewood, live trees, or fallen timber. ALB more commonly spread by natural means; under their own power they can fly distances greater than 400 yards.

What Can You Do? - There is no known "cure" for ALB. Therefore, prevention, and early detection is key in eradication. For more information on steps top take for prevention, and early detection, click HERE!

In quarantined or regulated areas, the USDA recommends:

  • Do NOT move any regulated material such as firewood, nursery stock, wood debris or lumber from host trees.

  • Do NOT move firewood. Get your firewood where you plan to burn it.

  • Move brush, leaves and twigs of regulated materials that are less than ½ inch in diameter to approved disposal sites. Call your State ALB Program for a disposal site near you.

  • Allow officials access to your property for inspection and, if necessary, eradication work.

  • ALB is difficult to control in forest situations. Individual trees can be protected with the systemic insecticide imidacloprid, applied to the soil. Follow label instructions carefully.

For more information, click HERE!

Sugar Maple Borer

What Is It? - Sugar maple borer is a native species of long-horned, wood boring pest found throughout the range of sugar maple (Acer saccharum) . While it does not generally cause mortality to trees, it is one of the most destructive insects to sugar maple, causing severe decline in the value of infected trees. As well as be highly destructive to sugarbush areas. The open wounds left by the sugar maple borer create entry ways for Eutypella canker, staining fungus, and other pathogens.

How Does it Spread? - The beetle flies from host tree to host tree, laying eggs within crevices in the bark. The eggs hatch, and the larva begin their two year life-cycle, before pupating and transforming into a beetle.

What Can You Do? - Silvicultural management of maple stands is the most effective approach to managing sugar maple borer infestations. Other steps include:

  • Do not harvest too many trees and do not permit livestock grazing in maple stands;

  • Cut and promptly burn infested trees or tree parts to prevent the adult insects from emerging;

  • For ornamental trees, insert a wire deep into the galleries in order to kill the larvae;

  • During summer, look for dust piles on the trunk and destroy the young larvae by cutting through the bark into the galleries with a knife.

For more information on sugar maple borer, click HERE, and HERE!!


Russula emtica; Photo Credit: Joshua Isaac

For more information regarding Michigan's forest pests and pathogens of concern, click HERE, and HERE!


Strobilomyces strobilaceus; Photo Credit: Joshua Isaac

Tree Pests and Pathogens of Less Concern

For information on black knot fungus, click HERE, and HERE!

For information on anthracnose, click HERE, and HERE!

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