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FIGURE 1.—Hemlock woolly adelgid ovisacs. |
Hemlock Woolly Adelgid
Native to Asia, the hemlock woolly adelgid (Adelges
tsugae) is a small, aphidlike insect that threatens the health
and sustainability of eastern hemlock (Tsuga canadensis)
and Carolina hemlock (Tsuga caroliniana) in the Eastern
United States. Hemlock woolly adelgid was fi rst reported in
the Eastern United States in 1951 near Richmond, Virginia.
By 2005, it was established in portions of 16 States from
Maine to Georgia, where infestations covered about half
of the range of hemlock. Areas of extensive tree mortality
and decline are found throughout the infested region, but
the impact has been most severe in some areas of Virginia,
New Jersey, Pennsylvania, and Connecticut.
Hemlock decline and mortality typically occur within 4
to 10 years of infestation in the insect’s northern range,
but can occur in as little as 3 to 6 years in its southern
range. Other hemlock stressors, including drought, poor
site conditions, and insect and disease pests such as
elongate hemlock scale (Fiorinia externa), hemlock looper
(Lambdina fiscellaria fiscellaria), spruce spider mite
(Oligonychus ununguis), hemlock borer (Melanophila
fulvogutta), root rot disease (Armillaria mellea), and
needlerust (Melampsora parlowii), accelerate the rate and
extent of hemlock mortality.
Hosts
The hemlock woolly adelgid develops and reproduces
on all species of hemlock, but only eastern and Carolina
hemlock are vulnerable when attacked. The range of
eastern hemlock stretches from Nova Scotia to northern
Alabama and west to northeastern Minnesota and eastern
Kentucky. Carolina hemlock occurs on dry mountain
slopes in the southern Appalachians of western Virginia,
North and South Carolina, Georgia, and Tennessee. Eastern
hemlock is also commonly planted as a tree, shrub, or
hedge in ornamental landscapes. At least 274 cultivars of
eastern hemlock are known to exist.
Description
The hemlock woolly adelgid is tiny, less than 1/16-inch
(1.5-mm) long, and varies from dark reddish-brown
to purplish-black in color. As it matures, it produces a
covering of wool-like wax fi laments to protect itself and its
eggs from natural enemies and prevent them from drying
out. This “wool” (ovisac) is most conspicuous when the
adelgid is mature and laying eggs. Ovisacs can be readily observed from late fall to early summer on the underside of
the outermost branch tips of hemlock trees (figure 1).
Life History
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FIGURE 2.—Hemlock woolly adelgid nymphs in dormancy. |
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| FIGURE 3.—Chemical treatment using the soil injection method. |
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| FIGURE 4.—Predators introduced for control in the Eastern United States,
left to right (origin): Sasajiscymnus tsugae (Japan), Scymnus
sinuanodulus (China), and Laricobius nigrinus (Western North America). |
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The hemlock woolly adelgid is parthenogenetic (all
individuals are female with asexual reproduction) and has
six stages of development: the egg, four nymphal instars,
and the adult. The adelgid completes two generations a year
on hemlock. The winter generation, the sistens, develops
from early summer to midspring of the following year
(June–March). The spring generation, the progrediens,
develops from spring to early summer (March–June). The
generations overlap in mid to late spring.
The hemlock woolly adelgid is unusual in that it enters a
period of dormancy during the hot summer months. The
nymphs during this time period have a tiny halo of woolly
wax surrounding their bodies (figure 2). The adelgids
begin to feed once cooler temperatures prevail, usually in
October, and continue throughout the winter months.
The ovisacs of the winter generation contain up to 300
eggs, while the spring generation ovisacs contain between
20 and 75 eggs. When hatched, the fi rst instar nymphs,
called crawlers, search for suitable feeding sites on the
twigs at the base of hemlock needles. Once settled, the
nymphs begin feeding on the young twig tissue and
remain at that location throughout the remainder of their
development. Unlike closely related insects that feed on
nutrients in sap, the hemlock woolly adelgid feeds on stored starches. These starch reserves are critical to the
tree’s growth and long-term survival.
Dispersal and movement of hemlock woolly adelgid occur
primarily during the fi rst instar crawler stage as a result
of wind and by birds, deer, and other forest-dwelling
mammals that come in contact with the sticky ovisacs and
crawlers. Isolated infestations and long-distance movement
of hemlock woolly adelgid, though, most often occur as the
result of people transporting infested nursery stock.
Control
Cultural, regulatory, chemical, and biological controls
can reduce the hemlock woolly adelgid’s rate of spread
and protect individual trees. Actions such as moving
bird feeders away from hemlocks and removing isolated
infested trees from a woodlot can help prevent further
infestations. State quarantines help prevent the movement
of infested materials into noninfested areas.
Chemical control options, such as foliar sprays using
horticultural oils and insecticidal soaps, are effective when
trees can be saturated to ensure that the insecticide comes in
contact with the adelgid. Several systemic insecticides have
also proven effective on large trees when applied to the
soil around the base of the tree or injected directly into the
stem (figure 3). Chemical control is limited to individual
tree treatments in readily accessible, nonenvironmentally
sensitive areas; it is not feasible in forests, particularly
when large numbers of trees are infested. Chemical
treatments offer a short-term solution, and applications may
need to be repeated in subsequent years.
The best option for managing hemlock woolly adelgid in
forests is biological control. Although there are natural
enemies native to Eastern North America that feed on
hemlock woolly adelgid, they are not effective at reducing
populations enough to prevent tree mortality. Therefore,
biological control opportunities using natural enemies
(predators and pathogens) from the adelgid’s native
environment are currently being investigated. Several
predators known to feed exclusively on adelgids have
been imported from China, Japan, and Western North
America and are slowly becoming established throughout
the infested region (fi gure 4). It will likely take a complex
of natural enemies to maintain hemlock woolly adelgid
populations below damaging levels. Efforts to locate,
evaluate, and establish other natural enemies continue.
Pesticide Precautionary Statement
Pesticides used improperly can be injurious to humans, animals, and plants. Follow the directions and heed all precautions on the labels.
Note: Some States have restrictions on the use of certain pesticides. Check your State and local regulations. Also, because
registrations of pesticides are under constant review by the Federal Environmental Protection Agency, consult your county agricultural
agent or State extension specialist to be sure the intended use is still registered. |
For additional information or copies of this publication, visit http://www.na.fs.fed.us/fhp/hwa.
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USDA Forest Service
Northeastern Area
State and Private Forestry
11 Campus Boulevard, Suite 200
Newtown Square, PA 19073
www.na.fs.fed.us
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USDA is an equal opportunity provider and employer.
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