![Adelges tsugae closeup](https://upload.wikimedia.org/wikipedia/commons/0/0b/Adelges_tsugae_closeup._TN.jpg)
The hemlock woolly adelgid (; ''Adelges tsugae''), or HWA, is an
insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...
of the order
Hemiptera
Hemiptera (; ) is an order (biology), order of insects, commonly called true bugs, comprising over 80,000 species within groups such as the cicadas, aphids, planthoppers, leafhoppers, Reduviidae, assassin bugs, Cimex, bed bugs, and shield bugs. ...
(true bugs) native to
East Asia
East Asia is the eastern region of Asia, which is defined in both geographical and ethno-cultural terms. The modern states of East Asia include China, Japan, Mongolia, North Korea, South Korea, and Taiwan. China, North Korea, South Korea and ...
. It feeds by sucking sap from hemlock and spruce trees (''
Tsuga
''Tsuga'' (, from Japanese (), the name of ''Tsuga sieboldii'') is a genus of conifers in the subfamily Abietoideae of Pinaceae, the pine family. The common name hemlock is derived from a perceived similarity in the smell of its crushed folia ...
'' spp.; ''
Picea
A spruce is a tree of the genus ''Picea'' (), a genus of about 35 species of coniferous evergreen trees in the family Pinaceae, found in the northern temperate and boreal (taiga) regions of the Earth. ''Picea'' is the sole genus in the subfami ...
'' spp.). In its native range, HWA is not a serious pest because populations are managed by natural predators and parasitoids and by host resistance.
In eastern
North America
North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
it is a destructive
pest that threatens the eastern hemlock (''
Tsuga canadensis
''Tsuga canadensis'', also known as eastern hemlock, eastern hemlock-spruce, or Canadian hemlock, and in the French-speaking regions of Canada as ''pruche du Canada'', is a coniferous tree native to eastern North America. It is the state tree o ...
'') and the Carolina hemlock (''
Tsuga caroliniana
''Tsuga caroliniana'', the Carolina hemlock, is a species of hemlock endemic to the United States. Distribution and habitat
Carolina hemlock is native to the Appalachian Mountains in southwest Virginia, western North Carolina, extreme northeas ...
''). HWA is also found in western North America, where it has likely been present for thousands of years. In western North America, it primarily attacks western hemlock ''
Tsuga heterophylla
''Tsuga heterophylla'', the western hemlock or western hemlock-spruce, is a species of hemlock native to the west coast of North America, with its northwestern limit on the Kenai Peninsula, Alaska, and its southeastern limit in northern Sonoma ...
'' and has only caused minor damage due to natural predators and host resistance.
Accidentally introduced to North America from
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
, HWA was first found in the eastern United States near Richmond, Virginia, in 1951.
The pest is now found from northern Georgia to coastal Maine and southwestern Nova Scotia.
As of 2015, 90% of the geographic range of eastern hemlock in North America has been affected by HWA.
Characteristics
An adult individual body length is typically 0.8 mm, and is oval in shape.
The tiny brown-colored insect has four thread-like stylets that are bundled together and function as a mouthpart. Three times the length of its body, the stylet bundle pierces the host plant's
parenchymatic ray tissue to derive nutrition from stored reserves.
It may also inject a toxin while feeding. The resulting desiccation causes the tree to lose needles and not produce new growth. Hemlocks stricken by HWA frequently become grayish-green rather than a healthy dark green. In the northern portion of the hemlock's range, death typically occurs 4 to 10 years after infestation. Trees that survive the direct effects of the infection are usually weakened and may die from secondary causes.
The presence of HWA can be identified by its egg sacs, which resemble small tufts of cotton clinging to the underside of hemlock branches. In North America, the hemlock woolly adelgid asexually reproduces and can have two generations per year. Both generations are
parthenogenetic
Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and development ...
and exclusively female.
In its native Asian habitat, a third winged generation called sexupera occurs; although this generation's sexual reproduction requires a species of spruce not found in the Eastern United States, and therefore dies, Between 100 and 300 eggs are laid by each individual in the woolly egg sacs beneath the branches. Larvae emerge in spring and can spread on their own or with the assistance of wind, birds, or mammals. In the nymph stage, the adelgid is immobile and settles on a single tree.
Control methods
Forest level
The current leading
biological control
Biological control or biocontrol is a method of controlling pests, such as insects, mites, weeds, and plant diseases, using other organisms. It relies on predation, parasitism, herbivory, or other natural mechanisms, but typically also invo ...
method of hemlock woolly adelgid is ''
Sasajiscymnus tsugae'',
riginally called ''Pseudoscymnus tsugae'' ''S. tsugae'' is a black
lady beetle
Coccinellidae () is a widespread family of small beetles ranging in size from . They are commonly known as ladybugs in North America and ladybirds in Great Britain. Some entomologists prefer the names ladybird beetles or lady beetles as they ...
that is relatively host-specific, feeding only on three known aldegid species, including HWA. This beetle was discovered in 1992 while feeding on hemlock woolly adelgid in its natural range of Japan. Since 1995, the
Pennsylvania Department of Conservation and Natural Resources
The Pennsylvania Department of Conservation and Natural Resources (DCNR), established on July 1, 1995, is the agency in the U.S. State of Pennsylvania responsible for maintaining and preserving the state's 124 state parks and 20 state forests; pr ...
's Bureau of Forestry has released hundreds of thousands of adult ''S. tsugae'' beetles into affected hemlock forests of the eastern United States to determine its effectiveness at controlling the spread of the adelgid.
From 1995 to 1997, experiments in Connecticut and Virginia found that releasing adult ''Sasajiscymnus tsugae'' beetles into infested hemlock stands resulted in a 47 to 88% reduction in adelgid densities within 5 months of introduction.
The beetle's lifecycle is in parallel to the lifecycle of the hemlock woolly adelgid. Both lay eggs in the spring and hatching occurs nearly simultaneously. When hatched, ''S. tsugae'' larvae are highly mobile and feed on hemlock woolly adelgid eggs and larvae. Each ''S. tsugae'' larva can effectively consume about 500 adelgid eggs or nearly 100 developing adelgid nymphs.
''
Laricobius nigrinus'' is another predatory beetle used as a biological control in response to hemlock woolly adelgid. Native to the western United States and Canada, ''L. nigrinus'' is known to prey exclusively on various woolly adelgids.
''L. nigrinus'' adults lay their eggs on top of wintering adelgid larvae in early spring, and upon hatching, the larval beetles feed on hemlock woolly adelgid.
Also under study is ''
Laricobius osakensis'' from Japan, a relative of ''L. nigrinus''. They have shown promise in field trials.
Individual trees
The environmentally safest chemical control methods for treating individual trees are nontoxic insecticidal soap and
horticultural oil Horticultural oils or narrow range oils are lightweight oils, either petroleum or vegetable based. They are used in both horticulture and agriculture, where they are applied as a dilute spray on plant surfaces to control insects and mites. They are ...
. These are sprayed on the foliage and smother the insects as they dry. Most trees need to be treated on a yearly basis.
Toxic systemic insecticides may be applied to the foliage and bark of a tree and can persist in killing the adelgid for up to four years after application. Caution must be used,
and restraint exercised around bodies of water.
Soil drenches/soil injections/bark sprays are used in larger trees that cannot be completely sprayed with insecticidal soaps or foliage insecticides. The most common insecticide is
imidacloprid
Imidacloprid is a systemic insecticide belonging to a class of chemicals called the neonicotinoids which act on the central nervous system of insects. The chemical works by interfering with the transmission of stimuli in the insect nervous system. ...
, which can be effective for several years if absorbed through the soil. Tree roots absorb and transport the product into the foliage and kill hemlock woolly adelgid. Soil drenches must be applied when soil moisture is adequate for the tree roots to absorb the product. These products should not be used in close proximity to bodies of water.
Trunk injections are used for large trees that are near water or where soils are too rocky for soil injections or drenches. The chemical is injected directly into the tree and transported to the twigs and needles where the hemlock woolly adelgids are feeding. Adequate soil moisture is also necessary for the tree to take up these products.
Significance
Hemlock is a vital component of the
New England
New England is a region comprising six states in the Northeastern United States: Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont. It is bordered by the state of New York to the west and by the Canadian provinces ...
forest
A forest is an area of land dominated by trees. Hundreds of definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing, and ecological function. The United Nations' ...
system, and is the third-most prevalent tree in
Vermont
Vermont () is a state in the northeast New England region of the United States. Vermont is bordered by the states of Massachusetts to the south, New Hampshire to the east, and New York to the west, and the Canadian province of Quebec to ...
. It provides protection from erosion along stream banks, food for deer and wildlife, and shelter for deer in winter. The tree is also valued both as an ornamental and as an important source of
lumber
Lumber is wood that has been processed into dimensional lumber, including beams and planks or boards, a stage in the process of wood production. Lumber is mainly used for construction framing, as well as finishing (floors, wall panels, wi ...
. Unlike the
balsam woolly adelgid
Balsam woolly adelgids (''Adelges piceae'') are small wingless insects that infest and kill firs, especially balsam fir and Fraser fir. They are an invasive species from Europe introduced to the United States around 1900.
Because this species ...
that attacked only mature balsam fir, HWA infests hemlocks of all ages. Where hemlocks occur in pure stands in that region, the most commonly observed tree species to succeed it is
black (sweet) birch. In the southern extreme of its range, hemlock typically occurs not in pure stands, but in linear
riparian areas
A riparian zone or riparian area is the interface between land and a river or stream. Riparian is also the proper nomenclature for one of the terrestrial biomes of the Earth. Plant habitats and communities along the river margins and banks a ...
and other moist sites. Succession in these areas is affected by the presence of ''
Rhododendron maximum
''Rhododendron maximum'' — its common names include great laurel, great rhododendron, rosebay rhododendron, American rhododendron and big rhododendron — is a species of ''Rhododendron'' native to the Appalachians of eastern North America, fro ...
'', which often coexists with hemlock, and because of a combination of influences restricts regeneration to shade and otherwise
understory-tolerant plant species. Major changes in ecosystem structure and function, including hydrologic processes, are expected with the loss of hemlock.
Loss of the eastern and Carolina hemlock from hemlock woolly adelgid infestation will likely result in many ecological shifts in eastern North America. The understory of hemlock forests is characterized as dark, damp, and cool and is an ideal habitat for various other organisms.
The moist environment is preferred by many native amphibian species, particularly newts and salamanders. Some species of birds have close association with the hemlock, especially during mating and nesting periods. Aquatic systems adjunct to hemlock stands are also affected by the trees' decline.
Brook trout
The brook trout (''Salvelinus fontinalis'') is a species of freshwater fish in the char genus ''Salvelinus'' of the salmon family Salmonidae. It is native to Eastern North America in the United States and Canada, but has been introduced elsewhere ...
is a native fish species to the eastern United States and is known to prefer the cool, shaded streams of hemlock forests during spawning events. Vulnerable animal populations are expected to diminish as a result of loss of hemlock habitat to the invasive hemlock woolly adelgid.
One factor giving hope is that the adelgid does not seem able to survive prolonged or bitter cold. Following the winter of 1999-2000, a considerable dieback of adelgids and subsequent regrowth of infested trees was observed across Connecticut. The same phenomenon was repeated after the prolonged winter of 2013-2014, in time to save numerous nearly succumbed forests.
A 2009 study conducted by scientists with the
U.S. Forest Service
The United States Forest Service (USFS) is an agency of the U.S. Department of Agriculture that administers the nation's 154 national forests and 20 national grasslands. The Forest Service manages of land. Major divisions of the agency in ...
Southern Research Station suggests the hemlock woolly adelgid is killing hemlock trees faster than expected in the southern Appalachians, and rapidly altering the
carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...
of these forests. According to ''
Science Daily
''Science Daily'' is an American website launched in 1995 that aggregates press releases and publishes lightly edited press releases (a practice called churnalism) about science, similar to Phys.org and EurekAlert!.
The site was founded by mar ...
'', the pest could kill most of the region's hemlock trees within the next decade. According to the study, researchers found "hemlock woolly adelgid infestation is rapidly impacting the carbon cycle in
emlocktree stands", and "adelgid-infested hemlock trees in the South are declining much faster than the reported 9-year decline of some infested hemlock trees in the Northeast."
In fact, as of 2007, the rate of HWA expansion was recorded as 15.6 km/year south of Pennsylvania and 8.13 km/year (or less) in the northern section of the HWA's range.
References
External links
University of Vermont entomology websiteSpecies Profile- Hemlock Woolly Adelgid (''Adelges tsugae'') National Invasive Species Information Center,
United States National Agricultural Library
The United States National Agricultural Library (NAL) is one of the world's largest agricultural research libraries, and serves as a national library of the United States and as the library of the United States Department of Agriculture. Located ...
. Lists general information and resources for Hemlock Woolly Adelgid.
US Department of Agriculture HWATree-Savers Biological Control for HWA
How Was HWA Introduced?: The Gilded Age Garden Hypothesis How When & Where was HWA introduced to Eastern US?
New York State Hemlock Initiative
{{DEFAULTSORT:Hemlock Woolly Adelgid
Adelgidae
Parasitic bugs
Insect pests of temperate forests
Conifer pathogens and diseases
Insects described in 1928
Insects of Japan