Hemlock woolly adelgid
), commonly abbreviated as HWA
, is a true bug native to East Asia
East Asia or Eastern Asia is a subregion of Asia that can be defined in either geographical or cultural terms...
that feeds by sucking sap from hemlock trees (Tsuga
Tsuga is a genus of conifers in the family Pinaceae. The common name hemlock is derived from a perceived similarity in the smell of its crushed foliage to that of the unrelated plant poison hemlock....
spp.). In eastern North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, it is a destructive pest that poses a major threat to the eastern hemlock (Tsuga canadensis
) and the Carolina hemlock (Tsuga caroliniana
). The range of Eastern Hemlock extends north of the current range of the adelgid, but there are fears that the adelgid could spread to infect to these northern areas too. Accidentally introduced to North America from Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
in 1924, HWA was first found in the eastern United States some decades later. In Pennsylvania, for example, the earliest record is from 1967. The pest has now been established in eleven eastern states from Georgia
Georgia is a state located in the southeastern United States. It was established in 1732, the last of the original Thirteen Colonies. The state is named after King George II of Great Britain. Georgia was the fourth state to ratify the United States Constitution, on January 2, 1788...
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...
, causing widespread mortality of hemlock trees. 50% of the geographic range of eastern hemlock has been impacted by HWA.
The presence of HWA can be identified by its egg sacs, which resemble small tufts of cotton clinging to the underside of hemlock branches. Hemlocks stricken by HWA also tend to have a grayish-green appearance, whereas healthy hemlocks are dark green. Hemlock woolly adelgid reproduces asexually and can have two generations per year. Between 100 and 300 eggs are laid 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 and/or mammals. In the nymph stage, the adelgid is immobile and settles on a single tree. The next stage is the adult, which needs a species of spruce not found in the Eastern United States to sexually reproduce, so the only populations from year to year are the two asexually produced generations.
The hemlock woolly adelgid feeds on the phloem
In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
sap of tender hemlock shoots. It appears to also inject a toxin while feeding. The resulting desiccation causes the tree to lose needles and not produce new growth. In the northern portion of the hemlock's range, death typically occurs four to ten years after infestation. Trees that survive the direct effects of the infection are usually weakened and may die from secondary causes.
Hemlock is a vital component of the New England
New England is a region in the northeastern corner of the United States consisting of the six states of Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut...
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...
system, and is the third most prevalent tree in Vermont
Vermont is a state in the New England region of the northeastern United States of America. The state ranks 43rd in land area, , and 45th in total area. Its population according to the 2010 census, 630,337, is the second smallest in the country, larger only than Wyoming. It is the only New England...
. Providing protection from erosion along stream banks, food for deer and wildlife, and shelter for deer in the winter, hemlock is also valued both as an ornamental and as an important source of lumber
Lumber or timber is wood in any of its stages from felling through readiness for use as structural material for construction, or wood pulp for paper production....
. Unlike the balsam woolly adelgid
Balsam woolly adelgids 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....
that attacked only mature balsam fir, HWA infests hemlocks of all ages. Where hemlock occurs in pure stands in that region, the most commonly observed tree species to succeed it is black (sweet) birch
Black Birch may refer to:* Betula lenta* Betula nigra* Black Birch Ridge outside of Blenheim on the South Island of New Zealand* the Black Birch Astrometric Observatory at Black Birch Ridge...
. In contrast, in the southern extreme of its range, hemlock does not typically occur in pure stands, but in linear riparian areas and other moist sites. Succession in these areas is affected by the presence of Rhododendron maximum
Rhododendron maximum — also called great rhododendron, great laurel, rosebay rhododendron, American rhododendron or big rhododendron — is a species of Rhododendron native to eastern North America, from Nova Scotia south to northern Alabama.-Description:R. maximum is an evergreen shrub growing to 4...
which often coexists with hemlock, and, due to 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.
A 2009 study conducted by scientists with the U.S. Forest Service 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 of these forests. According to Science Daily
Science Daily is a news website for topical science articles. It features articles on a wide variety of science topics including: astronomy, exoplanets, computer science, nanotechnology, medicine, psychology, sociology, anthropology, biology, geology, climate, space, physics, mathematics,...
, 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 [hemlock] tree 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 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.
Few options are available for control of this pest. The adelgids can be physically removed from the tree, either by vigorous washing or by cutting off infected branches. Various petrochemical pesticides are available for home use or by a licensed arborist, including diazinon
Diazinon , a colorless to dark brown liquid, is a thiophosphoric acid ester developed in 1952 by Ciba-Geigy, a Swiss chemical company...
Fluvalinate is a synthetic pyrethroid commonly used to control varroa mites in honey bee colonies....
Imidacloprid is a nicotine-based, systemic insecticide, which acts as a neurotoxin and belongs to a class of chemicals called the neonicotinoids. Although it is now off patent, the primary manufacturer of this chemical is Bayer CropScience,...
Malathion is an organophosphate parasympathomimetic which binds irreversibly to cholinesterase. Malathion is an insecticide of relatively low human toxicity, however one recent study has shown that children with higher levels of organophosphate pesticide metabolites in their urine are more likely...
. The use of pesticides is limited because hemlocks tend to grow within close proximity to aquatic environments such as rivers or streams, and the risk of contaminating these vulnerable ecosystems is great. Furthermore, the large-scale use of pesticides is often denounced in forest settings. Horticultural oil and insecticidal soap are another option.
Since 1997, the main approach has turned to biological control. The use of entomopathogenic fungi and certain insects as predators specific to the adelgid have potential. Several species of the coleopteran genus Laricobius
, a kind of winged insect, are being tested,
and the coccinellid Pseudoscymnus tsugae (a beetle) an effective predator of the Hemlock woolly adelgid in Honshū, Japan. Pseudoscymnus has been released in several states in the northeastern US. There is also evidence that another beetle, Harmonia axyridis, may be a predator of adelgids.
Scientists have also investigated the ability of Asian hemlocks to be resistant or deterrent to HWA. Mitochondrial DNA analysis suggests the western species of hemlock (Tsuga heterophylla
and T. mertensiana
), as well as Asian species (T. chinensis
, T. dumosa
, T. diversifolia
and T. sieboldii
), show long evolutionary history with Adelges tsugae
(HWA) and therefore are not damaged by the adelgid.
Scientists have successfully hybridized foreign hemlock species with the eastern North American hemlocks, maintaining the aesthetic look and canopy structure similar to the native eastern hemlocks, but retaining the resistance to HWA seen in foreign hemlocks. Scientists found that by hybridizing Tsuga chinensis
with T. caroliniana
, the resistance traits were inherited, thus adelgids were less likely to settle on the plants, where they expressed a slower growth and survival rate.
Steps that have been taken to eradicate this pest:
- A quarantine has been established in the tristate area of Maine, New Hampshire, and Vermont. Homeowners and nurseries cannot bring in hemlock seedlings or trees from adelgid-infested states into these states without an inspection permit certifying they are pest-free.
- Pathogenicity trials: isolates have been screened for pathogenicity against thrips, aphids and HWA.
- Isolation and identification: all fungi discovered were isolated, established in pure culture and identified.
- Exploration: cooperative efforts in ME, MA, CT, NJ and VA states led to the isolation of many insect-killing fungi associated with HWA populations. Exploratory activities were also conducted in China, where HWA originated.
- Characterization: isolates have been evaluated to determine their growth, sporulation and germination characteristics.
- Biological studies: hypothesizing that range expansion of HWA is limited, trials were designed to determine the low lethal temperatures of this pest. Tests are also being done to ascertain if HWA populations from different plant coldhardiness zones respond differently to low temperature exposures.
- Pseudoscymnus tsugae: Beginning in 1995, more than 100,000 adult beetles have now been released in infested hemlock forests in Connecticut, New Jersey and Virginia to evaluate P. tsugae as a biological control agent.