CAES: Hemlock Woolly Adelgid

Hemlock Woolly Adelgid
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Hemlock Woolly Adelgid, Adelges tsugae (Annand)

Mark S. McClure
Valley Laboratory
The Connecticut Agricultural Experiment Station
153 Cook Hill Road
P.O. Box 248
Windsor, CT 06095

Telephone: (860) 683-4979 Fax: (860) 683-4987

What it is and How to Stop It

Hemlock woolly adelgid (HWA) is a small aphid-like insect from Japan that has become a serious pest of eastern hemlock, Tsuga canadensis, in the eastern United States. Even though an adelgid is smaller than a period at the end of this sentence, it is easily recognized on the young hemlock twigs by the presence of a dry, white woolly substance that covers its body and its egg masses. HWA injures hemlock by sucking sap and by injecting a toxic saliva while feeding. This causes the needles on infested branches to desiccate, turn a grayish-green color, and then drop from the tree usually within a few months. Most buds are also killed, so little if any new growth is produced on infested branches. Death of major limbs usually occurs within two years and progresses from the bottom of the tree upwards, even though the infestation may be evenly distributed throughout the tree. Trees often die within four years, but some survive longer in a severely weakened condition with only a sparse amount of foliage at the very top of the crown. These weakened trees are unsightly and have little chance for recovery. They often fall victim to wood-boring insects and diseases and are readily broken and thrown by wind.

HWA can not be managed in the forest at this time. However, hemlocks growing in nurseries and ornamental landscapes can be saved by carefully monitoring for the presence of the adelgid, by implementing various cultural practices to enhance tree vigor and to discourage pest invasion, and by using mechanical and chemical measures as needed to control adelgids.


HWA is parthenogenetic (all individuals are females) and completes two generations of development each year. During March and April, adults of the overwintering generation lay about 100 eggs each in a cottony mass on the young twigs. Nymphs (called crawlers) hatch from these eggs during a period of several weeks during April and May. Within a few days, they either disperse from the tree or settle on the twigs near the base of the needles where they insert their piercing and sucking mouth parts. There they feed and remain throughout their development. This spring generation matures by the middle of June. Some of the adults produced at this time are winged individuals that are unable to reproduce on hemlock. Therefore, they leave the hemlock tree is search of spruce, and because no suitable spruce host is available in the eastern United States, they soon die. Other adults produced at this same time are wingless and are able to reproduce on hemlock. In the middle of June these wingless adults lay about 100 eggs each in a cottony mass on the twigs. Crawlers which hatch in early July settle on the new growth and soon thereafter become dormant until the middle of October when feeding resumes. Nymphs feed and develop during the winter and mature by spring.

How to Monitor the Pest

Because HWA can damage trees so quickly, it is important to detect infestations early. Frequent visual inspection is the most effective means of determining whether or not a hemlock is infested. For most of the year the dry, white "wool" produced by the adelgid on the twigs is quite conspicuous contrasted against the dark green needles. It is particularly noticeable in spring on the undersides of the young twigs. Further evidence of HWA infestation is the thinning or grayish-green (not red or yellow) color of the needles on some branches. Usually by the time these symptoms appear, the tattered "wool" of a previous adelgid generation is also present on the branches.

Cultural Control Methods

Reducing invasion by adelgids: Because birds, squirrels and deer are important dispersal agents, any effort to discourage these animals from visiting hemlocks will reduce the risk of those trees becoming infested. Care should also be taken when moving plants, logs, firewood, or bark chips from infested areas onto an non-infested property, especially from March through June when adelgid eggs and crawlers are abundant. Cleaning vehicles, clothing, etc. after visiting forests, recreational areas, parks or other properties with infested hemlocks is also advisable during this period. Infestations of HWA often start on large outstanding hemlocks that intercept the prevailing wind or that are especially attractive to birds and other wildlife. When such a tree becomes heavily infested, it can serve as an effective "launch pad" for adelgid eggs and crawlers. Selective removal of these heavily infested reservoir trees from the immediate vicinity will retard the establishment of new infestations

Improving tree health: HWA infests and kills hemlocks of all sizes and ages, even in habitats with seemingly optimal growing conditions. However, trees that are growing in poor sites or experiencing stress from drought and other agents succumb to adelgid attack more quickly. Therefore, maintaining good growing conditions can play an important role in the survival of hemlock. Because hemlock is a shallow rooted tree, it is particularly prone to stress when precipitation is abnormally low. Therefore, during periods of drought, trees should be watered as often as needed to ensure that they receive 1 inch of water per week (including rainfall) over the area beneath the drip line of the crown. Water should be applied slowly so that the roots will be soaked thoroughly. Pruning may also be of some value in improving the health of hemlock. Removing dead and dying branches and limbs from hemlock will promote new growth by allowing more light to reach the foliage, and will reduce the likelihood of attack by insect pests and diseases. Although applying fertilizer may improve the growth and vigor of non-infested trees, fertilizing infested hemlocks with nitrogen also enhances adelgid survival and reproduction. As a result, a fertilized hemlock becomes more heavily infested and more severely injured than an unfertilized one. Although nitrogen fertilizer should not be applied to an infested hemlock, fertilizing a tree after adelgids have been controlled may encourage growth and stimulate recovery. The potential risks and benefits of applying fertilizers which do not contain nitrogen to adelgid-infested hemlocks are unknown.

Mechanically removing adelgids: Eggs and crawlers of HWA are readily dislodged from the young hemlock twigs by wind and rain. Most of these individuals are unable to find their way back onto the tree and die. Therefore, intentionally dislodging eggs and crawlers by directing a strong stream of water at infested branches periodically during April through June may be of some value in reducing HWA numbers. Clipping the more heavily-infested twigs from hemlock branches will also reduce adelgid density on a given tree. However, extensive clipping may have undesirable effects on the general appearance and health of the tree.

Planting resistant hemlock species: Two Japanese hemlock species, Tsuga diversifolia and T. sieboldii, and two western North American hemlock species, T. heterophylla and T. mertensiana, are resistant to HWA. Although HWA infests these resistant species, it rarely reaches high enough densities to cause injury. Therefore, planting these resistant hemlocks should reduce the impact of HWA in the ornamental landscape. Of the four species, T. heterophylla is most similar to eastern hemlock in appearance, growth form, and utility. However, the likelihood for long term success of these hemlocks in the eastern United States in unknown.

Chemical Control Methods

Deciding whether or not to use pesticides: The use of chemical pesticides is an essential component of any control program for HWA. Even though cultural control measures can significantly reduce adelgid numbers on hemlock, infested trees are usually unable to survive for more than a few years unless chemical pesticides are applied. It is important to understand at the outset that hemlocks will need to be protected from the woolly adelgid as often as necessary until the danger has passed. This may be for a period of several years until all the unprotected hemlocks in the vicinity have died and can no longer serve as a source for re-infestation. Therefore, the initial decision on whether or not to use chemical control measures should consider the value of the trees relative to the anticipated cost of protecting them over the long term. It may be advisable to identify individual trees or groups of trees that have special value or significance on the property and to concentrate control efforts on those trees. This may be more successful than the overly ambitious approach of trying to save everything at first, only to lose it all when the resources have been depleted a few years hence.

What you need to know about pesticides: Several pesticides are registered for HWA. Some are available for homeowner use, while others are available for commercial use only by a licensed arborist. Because each of these pesticides has a relatively short life in the environment, treating an non-infested tree with pesticide offers little or no protection from invasion by hemlock woolly adelgid. Therefore, hemlocks should be treated only when an adelgid infestation is known to be present. Before applying any pesticide, read the product label very carefully. It will provide important information on safety, toxicity, and methods and rates of application.

Applying pesticide sprays: The most common and effective method for controlling HWA on ornamental hemlocks is to thoroughly drench infested trees with horticultural oil, insecticidal soap, or any one of numerous petrochemical insecticides that are specifically labeled for this use. Oil and soap are used most often because they are highly effective in killing adelgids, and yet they are relatively safe to the applicator, to beneficial insects, and to the environment. Unlike the petrochemical insecticides which kill by contact or ingestion, the oil and soap selectively kill soft-bodied insects, such as adelgids, by "suffocation" rather than by poisoning. It is essential that all parts of the infested hemlock be drenched thoroughly with insecticide. This precludes control on very large trees (usually those greater than about 80 feet tall) and those in forest settings. A backpack or garden hose sprayer may be sufficient to drench trees less than 30 feet tall, but taller trees may require the services of a professional arborist using a hydraulic sprayer. Fortunately, it is unnecessary to target a particular life stage of the adelgid for control; all are equally susceptible. Therefore, pesticide sprays can be applied any time during the year, weather permitting. One thorough application each year may be enough, if there are no other infested hemlocks within 100 yards from which adelgids could readily disperse. However, two spray treatments each year are usually necessary for most situations. If two applications each year are needed, an effective strategy is to spray in early April and again during the first half of June. Another option is to substitute a spray during the last half of September for the April treatment. Either of these schedules will target both adelgid generations and minimize the impact of immigration. Because hemlock adelgid propagates and injures hemlocks so quickly, it is advisable to spray as soon as a new infestation is detected, and then to adopt one of the maintenance schedules described above if necessary.

Applying pesticides by stem injection and implantation: Introducing concentrated chemical pesticides into the stem of infested hemlocks by injection or implantation can control HWA on trees that are very tall or growing in areas inaccessible to spray equipment, or where spraying is undesirable such as near waterways and in recreation areas. The injection technique involves drilling small shallow holes into the root flares at the base of the tree and inserting into these holes pressurized plastic capsules containing liquid pesticide. The pesticide moves into and up the tree to where it is intercepted by the feeding adelgids. The implantation technique involves drilling larger, deeper holes in a spiral around the trunk of the tree and inserting in these holes a plastic cartridge containing a powdered pesticide within a gelatin capsule. The sap flow dissolves the capsule and the pesticide and carries it throughout the tree. These techniques, when applied in mid-May, have controlled adelgids for up to six months. Although stem injection and implantation provide alternatives to spraying for control of HWA, several considerations may limit their use. Both of these procedures require the pesticide to move in the sap flow. Therefore, they may be effective only on newly-infested, uninjured trees, because feeding by HWA quickly restricts the tree’s ability to uptake and distribute water. The degree of wounding of the tree involved in the ongoing use of these drilling techniques is also of concern. Furthermore, because the injection method involves the use of highly concentrated liquid pesticides, its availability to the homeowner may be restricted.

Applying pesticides by soil injection and drenching: Introducing systemic insecticides into the roots of infested hemlocks in May is another alternative to protecting trees that can not be sprayed thoroughly. The soil beneath the tree’s crown can either be drenched or injected with a hydraulic injection needle. The pesticide is then taken up by the roots and distributed throughout the tree where it can control HWA for five months or more. Unlike stem injection and implantation, these soil treatments do not wound the tree. However, as was the case with the stem treatments, trees must have a healthy sap flow for these soil techniques to be effective.

Evaluating the effectiveness of chemical controls: One of the most difficult tasks confronting the homeowner is to evaluate the effectiveness of efforts to control hemlock woolly adelgid. Unfortunately, the "wool" can persist on the twigs for several months after the adelgid has been killed. Therefore, the presence of "wool" is not necessarily indicative of living adelgid and an unsuccessful control effort. The simplest way to determine if further control measures are needed is to disregard the tattered, off-color "wool" on the older twigs, and to look for the production of fluffy, white "wool" only on the very youngest twigs.

Biological Control Methods

Hope on the horizon: Several native insects, including beetles, flies, and lacewings, are occasional predators of HWA in North America. Unfortunately, none of these has had a significant impact on adelgid populations or has shown much potential for biological control. In Japan, however, there are several effective natural enemies. Two species in particular, an oribatid mite (Diapterobates humeralis) and a ladybird beetle (Pseudoscymnus tsugae), are especially effective at locating and destroying infestations of HWA in Japan. We are now evaluating the potential of these arthropods as biological control agents in the eastern United States in the hope that someday they can be part of an integrated program for managing HWA in our forests, nurseries and ornamental landscapes.


Hemlock woolly adelgid, Adelges tsugae, is a small aphid-like insect from Japan that has become a serious pest of eastern hemlock, Tsuga canadensis, in the eastern United States. It can not be managed in the forest at this time. However, hemlocks growing in nurseries and ornamental landscapes can be saved by carefully monitoring for the presence of the adelgid, by implementing various cultural practices to enhance tree vigor and to discourage pest invasion, and by using mechanical and chemical measures as needed to control adelgids.


Content Last Modified on 6/28/2012 9:59:22 AM