- For additional information on the hemlock woolly adelgid, visit
- Keep an eye out for white cottony growths on the needles and twigs or other signs of hemlock woolly adelgid. If you see such signs, call your local nursery inspector or ODA’s Plant Pest Control office at 614-728-6400 and someone will inspect those trees.
- If you have questions about a forest pest call your local forester.
Hemlock Woolly Adelgid has infested and devastated eastern states and without monitoring it threatens the ecological balance that exists in the naturally occurring Eastern Hemlock forest in Ohio. According to the USDA Forest Service, HWA is the single greatest threat to the health and sustainability of hemlock as a forest resource in eastern North America. The potential ecological influence of this exotic insect pest can be compared with that of gypsy moth, Dutch elm disease, and chestnut blight. Hemlock is also one of the most common backyard species planted by homeowners. There are over 274 cultivars of eastern hemlock, making it one of our most cultivated landscape tree species.
HWA is native to Asia, where it is a harmless inhabitant of several hemlock species. It is present in Japan, India, southwestern China, and Taiwan. HWA was first observed in North America in the 1920’s on western hemlock (Tsuga heterophylla) and mountain hemlock (Tsuga mertensiana) in Oregon and British Columbia. HWA has now been found from northern California to southeastern Alaska where it is generally harmless and not considered a pest.
Hemlock woolly adelgid was first observed in the eastern U.S. around 1950 in Virginia. The insect has steadily spread north and west from its point of introduction and currently inhabits many eastern states throughout the native range of eastern hemlock. HWA is a serious pest of eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana) and is responsible for extensive mortality and decline of hemlock trees in the eastern United States. Hemlock is one of the most common backyard species planted by homeowners. There are 274 cultivars of eastern hemlock, making it one of our most cultivated landscape tree species.
The hemlock woolly adelgid, Adelges tsugae Annand (Homoptera), is a small aphid-like insect that feeds on stored nutrients from young twigs of hemlocks. Hemlock is the only known host in North America. Adults over-winter and begin laying eggs in March and April. The brownish-orange eggs hatch and reddish-brown crawlers are present from late March to June.
The crawlers move to a desirable location on a twig and begin feeding. Young adelgids cover themselves with a white woolly substance and continue to develop until maturity. Adult adelgids are black, approximately 2 mm long, and all are females. Each female is capable of laying up to 300 eggs. There are two generations produced each year.
HWA spreads mainly as eggs and crawlers, which are carried by wind, birds, other forest animals, and people. Because HWA survives in central Japan where winter temperatures drop below -35 degrees C and summer temperatures exceed 40 degrees C, it will probably continue to spread and threaten eastern and Carolina hemlocks throughout much of their natural ranges in North America where similar climatic conditions exist.
Adelgid feeding can result in loss of needles and new shoots and can seriously impair tree health, eventually leading to tree mortality. The longer individual hemlock trees and stands are infested with HWA the higher the hemlock mortality. For individual trees, mortality occurs 3-10 years after trees are initially infested with HWA. Mortality may be caused by HWA itself or hastened by one or more other stressors such as scales, mites, loopers, borers, drought, or poor site conditions. However, some individual trees can persist longer. It’s unclear how much HWA damage a tree can sustain before the damage causes mortality.
Reports from forested areas in New Jersey and Connecticut indicate that significant mortality occurred three to five years after the entire stand was infested with HWA. Heavy stand mortality accelerated around 8-9 years in New Jersey. Connecticut stands generally have >90% mortality after 10+ years of HWA infestation.
HWA Management Strategies
Management for HWA can include a variety of methods, including chemical treatments, tree removals, and biological controls. All three are currently being used in Ohio and other regions of the eastern United States.
Chemical treatments for HWA include applications of insecticides through soil drenches, tree injections, and basal bark treatments. Each method is used in different situations depending on soil conditions, tree size and location, and hemlock density in an infested stand.
Tree removals are generally used in areas with small, isolated populations, or where chemical treatments are not practical. Removals can be done in areas with chemical treatments to increase control of HWA.
In the past 15 years, biological control for HWA has greatly expanded. There are several species of predator beetles that have been released throughout the eastern U.S. to help control HWA. Two of the most commonly released beetles are Laricobius nigrinus and Scymnus coniferarum, both native to the Pacific Northwest. A third species, Sasajiscymnus tsugae is native to Japan, but shows promise in assisting with biological control in the United States. Each of these species feeds on HWA during a different season of the year, so releasing a combination allows for year-round control of the adelgid.
HWA management generally includes some combination of all three management strategies. Chemical treatments can be used to keep high-importance hemlocks alive while establishing biological control populations, and removals can be implemented to control small, heavily infested trees that are impractical to treat to reduce the overall HWA populations.
HWA Management at Shade River
In 2012, HWA was discovered at Shade River State Forest, in Meigs County near the Ohio River. This infestation is along the leading edge front as HWA moves across the eastern United States. Management for HWA in this area has included a combination of tree removals and chemical treatments since its discovery. Future plans for the Shade River infestation include continued removals and treatments as well as biological control.
HWA Management in the Hocking Hills
Eastern hemlocks dominate the vegetation in and around the beautiful sandstone gorges of the Hocking Hills. The Hocking Hills Tourism Association estimates Hocking Hills State Park annual visitations to be over 3 million since 2010. In 2011, tourism generated $115 million in business activity, which supported more than 900 jobs in Hocking County (Ohio Department of Development). Loss of hemlocks in the region would seriously impact the tourism industry in the region.
In late winter 2013, a single woolly mass was detected by Ohio Department of Agriculture nursery inspectors on a hemlock tree at Cantwell Cliffs in Hocking Hills State Park. Since that discovery, the Ohio Department of Natural Resources (Divisions of Forestry and Parks) has partnered with OSU Extension, Ohio Department of Agriculture and others to address this potentially devastating threat from HWA to the Hocking Hills.
Current efforts to address HWA in the Hocking Hills and Ohio include:
- Region-wide surveys to determine extent of infestation
- Mapping of infested trees
- Develop educational and training opportunities
- Evaluate soil characteristics and research options for the development of a plan to address HWA at Hocking Hills State Park
- Chemically treat over 500 hemlock trees in the Hocking Hills
- Continued monitoring of treated trees to evaluate effectiveness of treatments
- Annual surveys of hemlock stands to detect new HWA infestations quickly
HWA Management Plans for Ohio
Future management for HWA in Ohio will include all three management strategies. Biological control in the state has already been initiated, with field insectaries being started in urban locations in Washington County. L. nigrinus beetles were collected in spring of 2013 with assistance by OSU Extension and ODA, and were released in three locations with heavy HWA populations. These release sites will be supplemented with additional beetles, both L. nigrinus and S. coniferarum, in future years to increase their populations so that beetles can be collected here for releases to other areas of the state as needed.
Surveys for new HWA populations are conducted every winter in counties with native stands of hemlocks. Homeowners with hemlocks in their landscapes or woodlands are encouraged to regularly inspect their trees for signs of HWA. HWA can be easily identified from late October through early June as white “woolly” masses on the underside of hemlock branches. Early detection of HWA provides the best opportunity to successfully treat the infested trees.