- 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.
Applying chemical insecticides can control populations of Hemlock Woolly Adelgid on ornamental hemlocks (individual tree treatments). However, these treatments are only possible in accessible areas. In the forest, the best and most practical means of managing adelgid populations is biological control using natural enemies (parasitoids, predators, and pathogens). At this time, there are no known parasitoids of HWA and native predators and pathogens are not capable of maintaining HWA populations at non-damaging levels. As an alternative, scientists are encouraging biological control by rearing and releasing a complex of non-native predators. At present, several adelgid predator beetles are at various stages of evaluation and development.
One of the most promising predators currently being evaluated and established throughout the range of HWA is a tiny but voracious ladybird beetle Pseudoscymnus tsugae. Native to Japan, this beetle has proven to be an effective predator of HWA on individual trees and is highly selective and synchronized with its prey.
In 1999-2000, the Connecticut Agricultural Experiment Station reared and released more than 120,000 P. tsugae beetles in Connecticut, while the New Jersey Department of Agriculture reared 270,000 P. tsugae beetles, releasing 135,000 at various state locations. The remaining beetles were released in Massachusetts, Rhode Island, New York, Pennsylvania, Maryland, West Virginia, North Carolina, and the George Washington and Jefferson National Forests in Virginia.
Three additional species of predaceous beetles from China are being investigated and evaluated. These three beetles, all in the genus Scymnus, are strong candidates for biological control agents, but need further testing. Also, Laricobius nigrinus beetles were collected from British Columbia and successfully established on HWA infested hemlock branches at the Virginia Polytechnic Institute and State University quarantine facility. This promising predator has recently been released from quarantine and is currently being used in controlled field studies.
Regulatory Outlook for Ohio
The Ohio Department of Agriculture (ODA) and Ohio nursery owners and growers have been discussing the adoption of an exterior quarantine against the hemlock woolly adelgid. ODA wants to prevent hemlock woolly adelgid from entering Ohio because Ohio’s hemlocks are important components of specialized ecological niches such as those found in the Hocking Hills and Mohican regions. Ohio’s hemlocks are also a valuable nursery sales commodity and an infestation of HWA would restrict the sale of hemlocks to other states.
Four states, Maine, Michigan, New Hampshire and Vermont, have enacted state exterior quarantines, which limit or ban the importation of hemlocks from infested areas or counties in other states. Tennessee is also considering such action.
Unfortunately, some of the areas where HWA has now been found are among the finest hemlock growing regions in the country. The mountains of western North Carolina are renowned for the quality of hemlocks grown there. Many Ohio retailers buy trees from this area and hemlock woolly adelgid was found there this past year. Reportedly, HWA has been found in the native forest, not in nurseries. In at least one North Carolina case, a single find of hemlock woolly adelgid was enough to add the county to the federal list of infested counties.
In general, the hemlock woolly adelgid problem is largely confined to the forest. Nurseries that maintain an insecticide spray schedule against this pest usually remain free of it. Dormant oil applications can be very effective if 100% coverage is achieved. Other insecticides are also quite effective during the growing season. The movement of infested nursery stock, however, is still an important potential means of long-distance spread of the pest.
Inspections for hemlock woolly adelgid can be difficult. At some times of the year, the adelgid makes a white cottony egg sac that is easily spotted. At other times of the year or in newly infested trees, the adelgid may exist as a juvenile and inspecting for it can be like looking for specks of pepper near the base of needles. This is why the above states preferred to completely ban hemlocks originating from counties where the pest has been found.
At this time the Ohio Department of Agriculture is investigating the possibility of taking a slightly different stance in its exterior quarantine. The ODA is working on an exterior quarantine that would require an insecticide spray program combined with an inspection before stock could be shipped from a nursery in a county where HWA has been found. A state phytosanitary certificate would be required to accompany shipments from a state that has infested counties. The certificate would have to attest that the hemlock shipment originates from either a county where hemlock woolly adelgid has not been found or a nursery that maintains an insecticide spray program for the control of this insect. The certificate would also have to attest that the hemlock trees have been inspected and found free of hemlock woolly adelgid.
The Ohio Department of Agriculture is still trying to decide how specific to describe the required insecticide spray program in the quarantine. The program may require a dormant season horticultural oil application in combination with a growing season application of an insecticide effective against hemlock woolly adelgid. The shipping states department of agriculture will be responsible for monitoring the required insecticide spray programs. This would most likely be accomplished through compliance agreements.
The Ohio Department of Agriculture will continue to research issues surrounding adoption of this quarantine. The final conditions of the quarantine will reflect the best science available while taking into consideration the monetary and ecological importance of Ohio’s hemlocks. Dialogue will continue with the Ohio Nursery and Landscape Association and individual nursery owners to obtain input on how a quarantine would affect the industry. If discussions do lead to implementation of a quarantine, it could begin as soon as 2002.
In the meantime, if you are buying hemlock trees from eastern states, 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. The department can take action against shipments infested with harmful pests, even prior to adoption of a formal quarantine.