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The hemlock woolly adelgid is a small aphid-like insect that feeds on several species of hemlock (Tsuga spp.) in Asia, its homeland, and in North America where it was introduced. To date, populations of this pest in eastern North American forests have been unmanageable (although new biological control programs offer promise). However, hemlocks growing in nurseries and landscapes can be managed through an integrated pest management (IPM) approach. This is important because hemlock is a unique and versatile landscape species for which there is no good substitute. An IPM approach for A. tsugae on ornamental hemlocks includes carefully monitoring for the presence of the adelgid, implementing various cultural practices to enhance tree vigor and to discourage pest invasion, using mechanical and chemical control measures as needed to reduce adelgid populations, and promoting biological control by encouraging the activity of natural enemies. Description: A fully grown adult of the hemlock woolly adelgid is only about the size of a period on this printed page. However, this insect is easily recognized during most of the year by the presence of a dry, white woolly substance on the young twigs. This "wool" is associated with all stages of the adelgid, but it is most abundant and conspicuous during spring when egg masses are present. An egg mass resembles the tip of a cotton swab, although somewhat smaller. Adelges tsugae injures eastern and Carolina hemlock by sucking sap and probably also 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 new growth is produced on infested branches. Dieback 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.
Life Cycle: Hemlock woolly adelgid completes two generations per year on hemlock. During March and April, adults of the overwintering generation lay 50 to 300 eggs each in a cottony mass on the young twigs. Nymphs (called crawlers) hatch from these eggs in April and May. Within a few days, they settle on the twigs near the base of the needles where they insert their piercing and sucking mouthparts. 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. Other adults produced at this same time are wingless and are able to reproduce. In the middle of June these wingless adults lay 50 to 300 eggs each in a cottony mass on the twigs. Crawlers hatch in early July and settle on the new growth. They soon become dormant until the middle of October, when feeding resumes. Nymphs feed and develop during the winter and mature by spring. Dispersal: Adelges tsugae has been spreading relatively rapidly in North America even though its life stages are wingless and are firmly attached to hemlock twigs by mouthparts for most of the year. Eggs and crawlers, the only stages that are unattached, are abundant from March through June when they are readily dispersed by wind, birds, deer and other forest dwelling mammals, and humans during logging and recreational activities. Moving of infested nursery plants could also facilitate the spread of this pest. Monitoring: Because hemlock woolly adelgid can damage trees so quickly, it is important to detect infestations early and to implement a management program. 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 in contrast to the dark green needles. It is particularly noticeable on the undersides of the young twigs. A new infestation could be difficult to detect during July through September when adelgids are small dormant nymphs with very little "wool" associated with them. However, periodic examination of hemlocks throughout the year would minimize the chance of a new infestation being overlooked. Further evidence of an adelgid 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. Control Methods: A number of cultural practices may be effective in reducing the risk of hemlocks becoming infested by hemlock woolly adelgid. 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 uninfested 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 Adelges tsugae often start on large 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. 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 dripline 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 other insect pests and diseases. Although applying fertilizer may improve the growth and vigor of uninfested 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. Mechanically removing adelgids: Eggs and crawlers of hemlock woolly adelgid are readily dislodged from the young hemlock twigs by wind and rain. Most of these dislodged 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 an integrated approach to managing hemlock adelgid populations. Clipping heavily-infested twigs from branches will also reduce adelgid populations on a tree. However, extensive clipping may have undesirable effects on the appearance and health of the tree. Planting resistant hemlock species: Two Japanese and two western North American hemlock species are much more resistant to hemlock woolly adelgid than are their eastern North American counterparts. Of these four resistant species, the western hemlock (T. heterophylla) is most similar to eastern hemlock in appearance, growth form, and utility. Although adelgids do infest these resistant species, they seldom reach densities high enough to cause injury. Therefore, planting resistant Japanese and western hemlocks should reduce the impact of the adelgid in the ornamental landscape. However, the long-term success of these exotic hemlocks in the forests of eastern North America has not been evaluated. Biological Control: There are a number of insects that can be found feeding on the hemlock woolly adelgid in eastern North America, but overall they have not been able to keep up with adelgid reproduction. The Halloween lady bug, Harmonia axyridis, a recent arrival in the Northeast, is now found in considerable numbers on infested hemlocks, and may be having a beneficial impact. More promising is the current research on natural enemies from Japan and China. Of the several new predatory beetles that have been discovered in Asia in the past few years, the one that has been researched the most is Pseudoscymnus tsugae. It has been released in several states and appears capable of permanent establishment in the Northeast. These insects are not yet available for homeowner distribution, but they offer hope for the future, and may influence homeowners to maintain their hemlock trees for a few more seasons while the potential for biological control is evaluated. Deciding whether or not to use pesticides: The use of chemical pesticides is an essential component of any integrated approach to managing populations of hemlock woolly adelgid. 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. 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 all the trees at first, only to lose them all when the resources have been depleted a few years hence. What to know about pesticides: Several pesticides are registered for control of hemlock woolly adelgid. 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 uninfested tree with pesticide offers 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. Also be aware that regulations governing the use of pesticides for controlling hemlock woolly adelgid may differ between states and are subject to change. Applying pesticide sprays: The most common and effective method for controlling hemlock woolly adelgid 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 including diazinon, fluvalinate, imidacloprid and malathion. Horticultural oil and insecticidal 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 insects indiscriminately by contact or ingestion, the oil and soap selectively kill soft-bodied insects, such as adelgids, by "suffocation" rather than by poisoning. 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 at any time during the year, weather permitting. Two thorough spray treatments each year are necessary for most situations.
Adapted
from the University of Maryland Cooperative Extension and
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