Forest Pests & Diseases
How to identify and report hemlock woolly adelgid, beech leaf disease, and other common forest pests of the Great Lakes region.
Invasive insects and diseases threaten a number of critical native species in the Great Lakes Region, including ash, elm, beech, and hemlock.
Learn to identify these trees and their common pests so you can keep your trees healthy and help forest professionals stop the spread.
On this page:
Trees:
American Elm | American Beech | Eastern Hemlock | Ash species
Pests:
Dutch Elm Disease | Elm Yellows | Beech Leaf Disease | Beech Bark Disease | Hemlock Woolly Adelgid | Elongate Hemlock Scale | Emerald Ash Borer | Spotted Lanternfly
American Elm (Ulmus americana)
American elms have simple, oval to elliptical, alternate leaves with doubly serrated edges, meaning the teeth on the leaves each have small teeth of their own. The base of each leaf is asymmetrical. Elm’s dark gray-brown bark has deep furrows. In March or early April, their flowers appear as a dangling bundle. Read more about American elm ▶
Dutch Elm Disease (Ophiostoma ulmi & O. novo-ulmi)
Dutch elm disease is caused by a pathogenic fungus that is spread by elm bark beetles and through elm roots. Symptoms include yellowing, browning, or leaf loss in the tree crown. To confirm, peeling back the bark on branches and stems should reveal brown streaking in the sapwood. Dead trees may reveal brood galleries from where the elm bark beetle hatched and larvae fed.
Elm Yellows (Phytoplasma ulmi)
Another disease impacting American and other native elms is called elm yellows. This is caused by a phytoplasma, a type of bacteria transmitted by leafhopper insects that attacks plant tissues. Symptoms are most common in mid- to late summer, and include drooping, yellow foliage that progresses to dropped leaves and dead branches. It can also cause a “witches’ broom” to appear, a tight cluster of stems and leaves growing on a branch.
How to help: Report a surviving American elm
Researchers at the U.S. Forest Service Northern Research Station are expanding efforts to screen American elm trees that may be tolerant or resistant to Dutch elm disease. They are asking interested professionals and members of the public to identify large, healthy American elm trees on their landscapes and report them for possible study.
Trees must be in good health, at least 22 inches in diameter at breast height, not treated with fungicides to prevent Dutch elm disease, and located in an area where dead or dying American elms are present. Report a “survivor” American elm tree ▶
American Beech (Fagus grandifolia)
Like elms, American beech leaves are simple, alternate, and elliptical. Teeth around the leaf margins come to a distinct point, and unlike elms, the base of the leaf is symmetrical. They have characteristic light gray, smooth, thin bark. In fall, beech nuts are pyramid-shaped encased in spiny husk. Read more about American beech ▶
Beech Leaf Disease (Litylenchus crenatae mccannii)
Beech leaf disease, first spotted in northeastern Ohio in 2012, is caused by a non-native nematode that lives and feeds inside beech buds, damaging the leaf tissues. Symptoms include dark banding between the leaf veins (called interveinal banding), and thick, leathery, or stunted leaves — eventually leading to leaf drop and canopy thinning. Read more about beech leaf disease ▶
Learn more about the beech leaf disease research at Holden Forests & Gardens:
- 30% of Beech Trees Killed by Beech Leaf Disease, New Study Shows
- New Study Confirms Beech Leaf Disease Threatens European Beech Trees, Too
- Beech Leaf Disease-Causing Nematode More Widespread Than Visible Symptoms Suggest, New Study Finds
Beech Bark Disease (Neonectria spp.)
Beech bark disease is a fungal pathogen spread by a fuzzy white insect called woolly beech scale (Cryptococcus fagisuga) which damages the bark while feeding, causing susceptibility to the fungus. Beech bark disease can be visible as white spots on the tree bark (wax deposited by the insects), or as callouses — pock marks across the trunk — that form from the damage.
How to help: Report disease symptoms
Report sightings of symptoms of beech leaf disease and beech bark disease at EDDmapS.org — especially if your county is near the edge or outside of the documented range.
View the current spread of each disease here:
Beech leaf disease on EDDmapS | Beech bark disease on EDDmapS
For the latest info on beech leaf disease treatment, visit our BLD page. And to give your beech trees — healthy or infected — a boost, consider pruning them to help the canopy stay dry during the rainy fall season.
Eastern hemlock (Tsuga canadensis)
Hemlocks are conifers with short, flat, dark green needles that have two parallel white stripes on the underside. Each needle has a tiny stem connecting it to the branch. Stubby cones hanging down from trees can remain year-round. Branches can appear drooping or feathery. Read more about eastern hemlock ▶
Learn more about the hemlock research at Holden Forests & Gardens:
- Protecting Eastern Hemlocks from Tiny Tree-Killing Invaders
- Monitoring Hemlock Woolly Adelgid Colonization
- New Eastern Hemlock Conservation Work Kicks Off at Holden
- A short tale about hemlock trees, bacteria and a nasty insect
Hemlock woolly adelgid (Adelges tsugae)
Hemlock woolly adelgid is an invasive, non-native, aphid-like insect that attaches to the undersides of the base of hemlock needles and feeds on nutrients and sap. In all but the earliest life stages, a white woolly mass up to ~⅛” in diameter covers the dark insect — it’s most prominently visible from November to May. Additional symptoms of an HWA infestation include premature dieback, needle loss, and grey-green discoloration.
Elongate hemlock scale (Fiorinia externa)
Elongate hemlock scale is a small non-native insect that feeds on conifer needles (primarily hemlocks, firs, and spruces). Juvenile “crawlers” and winged male adults can move freely, while female adults don’t move once they’ve begun feeding. The latter are the oblong, 1.5-2 mm long, light or dark brown (sometimes white) scales attached to the underside of needles. Additional symptoms of EHS infestation include yellow banding on the tops of infested needles, premature needle drop, and limb dieback.
How to help: Report sightings of hemlock pests!
Look for elongate hemlock scale on the underside of needles year-round, and hemlock woolly adelgid from late fall through early spring. Report your sightings to help natural resources professionals monitor their spread, study their impacts, and investigate treatment options.
Anywhere: Log an observation of hemlock woolly adelgid or other hemlock pests on EDDMapS.org.
Ohio: You can also contact the Ohio Division of Forestry directly at 614-265-6694 or OhioHWA@dnr.ohio.gov. Please gather a quality photo and as much detail as you can about the location.
Ash (Fraxinus spp.)
Ash trees have compound leaves, meaning each leaf is composed of multiple (from 5 to 11, depending on species) leaflets. The leaflets, as well as the actual leaves and branches, grow opposite one another. On mature trees, the bark can form a characteristic diamond-shaped pattern. Read more about ash trees ▶
Learn more about our ash research at Holden Forests & Gardens:
- Holden’s Clone Orchard Vital Research Spot for Ash Scientists
- Why and How We Collect and Store Tree Seeds at the Holden Arboretum
Emerald ash borer (Agrilus planipennis)
The invasive emerald ash borer has all but decimated ash populations across its native range. The shiny green beetle feeds on all of our native ash species. The female lays eggs in crevices in the tree’s bark, and after hatching, larvae burrow into the tree — feeding on the tissues responsible for water and nutrient transport, which eventually kills the tree. Spot EAB damage from the D-shaped hole left where new adults have chewed their way out of the bark. Dead trees that have lost their bark will reveal squiggly galleries where the larvae lived and fed.
How to help: Report healthy ash trees
The emerald ash borer is already widespread in our region. To help forest health professionals, keep an eye out for “lingering” ash — in areas where the other ash have died, these are surviving mature trees that have not been treated with pesticides. These trees may be resistant to EAB! To report a lingering ash, please use the free app, TreeSnap.
Spotted Lanternfly (Lycorma delicatula)
The spotted lanternfly was first spotted in the U.S. in 2014 in Pennsylvania. It has quickly spread to neighboring states and has shown up as far west as Iowa. Older nymph (juvenile) and adult lanternflies feed on tree tissues, often congregating in large feeding swarms that deplete the tree of nutrients. It’s still unclear what the effects of this insect will be on forest trees and agricultural crops; so far, the most serious damage has occurred on grapevines. The insects also secrete a sticky honeydew that attracts sooty mold fungi. Learn more about the spotted lanternfly from Cornell CALS (external link) ▶
About the GLBFHC
The Great Lakes Basin Forest Health Collaborative is a partnership between Holden Forests & Gardens and the U.S. Forest Service. The GLBFHC connects efforts across federal agencies, conservation groups, researchers, and members of the public who are working together to save our trees from forest pests and diseases. The collaborative works with ash, American elm, American beech, and eastern hemlock, supporting monitoring, research, and breeding of resistant trees. Read more about the GLBFHC or visit the GLBFHC homepage ▶
Forest professionals: The GLBFHC is looking for organizations and individuals who are interested in assisting with: monitoring forests for pest and disease progression, collecting scion and seed using best practices, grafting and propagation, providing greenhouse space, and/or providing properties for planting trials (open space that can be fenced and mowed is ideal). Contact Rachel Kappler, rkappler@holdenfg.org.
About the Long Center for Plant & Environmental Science
At the Holden Arboretum, the scientists at the Ellen Corning Long and T. Dixon Long Center for Plant and Environmental Science take pride in pioneering the science of plants and trees, conducting nationally recognized research while fostering the next generation of leaders in this space. Learn more about the Long Science Center ▶
About Holden Forests & Gardens
Holden Forests & Gardens is made up of two of Northeast Ohio’s most important environmental and cultural institutions — the Holden Arboretum and Cleveland Botanical Garden — and is the 12th largest public garden in the country. HF&G is a leader in plant science, advancing groundbreaking research and sharing discoveries that protect forests and natural areas. Learn more about Holden Forests & Gardens ▶
