Hemlock Wooly Adelgid
The hemlock woolly adelgid, a small aphid-like insect, has been in the United States since 1924. This introduced insect, believed to be a native of Asia, is a serious pest of eastern hemlock and Carolina hemlock. In the eastern United States, it is present from northeastern Georgia to southeastern Maine and west to eastern Tennessee.
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.
Hemlock woolly adelgid completes two generations per year on hemlock. During March and April, adults of the overwintering generation lay eggs each in a cottony mass on the young twigs. Nymphs, called crawlers, hatch in April and May and within a few days they settle on the twigs near the base of the needles. Here they insert their piercing and sucking mouthparts and feed. They remain in this position 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 are wingless and are able to reproduce. In the middle of June these wingless adults eggs 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.
Hemlock woolly adelgid injures 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.
Dispersal and Monitoring
Hemlock woolly adelgid has been spreading relatively rapidly in North America. 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.
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" roduced 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. 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 attered "wool" of a previous adelgid generation is also present on the branches.
Hemlock woolly adelgid is difficult to control because the fluffy white secretion protects the eggs from pesticides. A good time to attempt control it is in October when the second generation begins to develop. Insecticidal soaps and the horticultural oils seem to be very effective for adelgid control with minimal harm to natural predators and parasites of this pest. Trees that are heavily infested and are showing symptoms of decline should probably be sprayed. Horticultural spray oil can be applied during the winter and before new growth emerges in spring. Oil sprays may damage hemlock during the growing season, especially in dry weather. Registered pesticides containing imidacloprid may be useful for specimen trees located away from water sources.
Cultural: A number of cultural practices may be effective in reducing the risk of hemlocks becoming infested. 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 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). 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.
Mechanical: Eggs and crawlers 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. 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 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 (Tsuga 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. The long-term success of these exotic hemlocks in the forests of eastern North America has not been evaluated.
Adapted from: University of Maryland Cooperative Extension; Connecticut Agricultural Experiment Station, 1999; North Carolina State University Cooperative Extension, 2007