Tree Diseases and Invasives


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



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? - 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? - 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. ​​

Thousand Cankers Disease.jpg


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.