Various pests and pathogens affect tree health. Some cause superficial damage that can make a tree look “sick”, through discolored leaves or a thinning canopy, but without affecting the overall health of a tree. Others can cause significant internal stress to a tree that may lead to mortality in just a few years. In Cuyahoga County, the decline of ash trees (Fraxinus spp.) due to the exotic emerald ash borer (EAB) (Agrilus planipennis) has greatly impacted canopy cover.

EAB, a wood-boring beetle, completes its lifecycle on ash trees. Adult beetles lay their eggs on the bark of ash trees and once larvae hatch, they burrow into the cambium (area between the bark and wood) of the tree, boring “S”-shaped patterns or galleries into the tree as they feed. This feeding process damages trees by destroying the tissues that transport water and nutrients, effectively girdling or “choking” the tree. As adult beetles emerge, they bore distinctive “D”-shaped holes in the trunk. Once an ash tree is infested, EAB causes mortality within 5-6 years.

Native to Asia, this beetle was first identified as the killer of ash trees in the Detroit Michigan area in 2002. The beetle likely arrived in the United States 10 or more years prior as a result of an accidental introduction of infested wood shipping containers originating from Asia. Since then, EAB has spread to 35 U.S. states and 5 Canadian provinces, killing hundreds of millions of ash trees.

The first record of EAB in Cuyahoga County was in 2006. Since then, EAB has contributed to tree loss in communities where ash was a significant component of street trees and the urban landscape. Ash trees generally make up between 6-10% of native forests and occur with greater prevalence in habitats found in the western portion of the county compared to the east. The outbreak of EAB has caused mortality levels that reduced overall Urban Tree Canopy Cover within the county.

The impact of EAB and ash loss reflects the importance of understanding tree health and the condition of our urban forests. Proper survey, inventory, and management prescriptions will help maintain the current canopy cover. Tree health assessments should identify other possible pests and pathogens based on their potential host tree. Early detection and rapid response may provide the best outcome for managing new afflictions that would compromise trees and forests in the future. A few of the other known pests and pathogens on the watchlist include: Hemlock Wooly Adelgid, Asian Longhorned Beetle, Spotted Lanternfly, Beech Leaf Disease and Thousand Cankers Disease. Each of these can cause mortality in their host trees and should be reported when suspected.

For reporting and more information on tree health, visit the tree health page from ODNR’s Division of Forestry.

Prepared by: Constance E. Hausman, Ph.D., Plant and Restoration Ecologist, Cleveland Metroparks