Introduced Species Summary Project
Hemlock Woolly Adelgid (Adelges tsugae)

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Common Name: Hemlock Woolly Adelgid

Scientific Name: Adelges tsugae


Phylum or Division: Arthropoda
Class: Insecta
Order: Homoptera
Family: Adelgidae

The Hemlock Woolly Adelgid (HWA) is a small aphid-like insect, about 0.8 mm in length. It is oval-shaped and brownish-reddish in color. Even though HWA is very small (about the size of a period on this page) it is recognized by a covering of dry, white, woolly/cottony/waxy substance on its body and egg masses. This covering is usually 3 mm or more in diameter and is present throughout the life of the adelgid. The eggs are brownish-orange, but darken as they mature, and are covered with the white, woolly secretion described above. Flat, naked reddish-brown crawlers hatch and are very active. Once they settle down they turn black with a white fringe around the edge and down the center of the back.

HWAs have thread-like mouthparts, which are 1/16th of an inch long. These feeding stylet bundles are made up of four separate stylets each enclosed within a sheath, located on the underside of the HWA. The stylet bundles are more that three times the length of the HWA and can penetrate deep into plant tissue and are used to suck sap and inject toxic saliva.

HWA is parthenogenetic and goes through two developmental stages every year. The first cycle or the spring generation develops from March to June. During this cycle over-wintering adults lay eggs, which develop into two forms - sexual winged forms and asexual wingless forms. The winged forms require spruce trees to complete their life-cycle. As such, due to a lack of spruce in eastern United States, the majority of the winged forms die due to lack of suitable hosts. The wingless forms continue feeding and developing on hemlocks. The spring generation reaches maturity by mid-June, when they are ready to lay eggs. Crawlers that hatch from these eggs in early July find new needles to settle on, where they attach and become dormant until October. They feed and continue to develop during the winter and by March of the next year are ready to lay eggs again.

HWA infested branch

Original Distribution:
HWA is a native of Asia, where it inhabits several species of hemlock and spruce trees. Its original distribution includes Japan, India, southwestern China and Taiwan.

Current Distribution:
Since its introduction into Virginia in the early 1950’s, the HWA has spread all over the eastern seaboard. The eastern North American distribution of the HWA, where it is a major pest, includes the following 15 states – Connecticut, Delaware, Georgia, Massachusetts, Maryland, North Carolina, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, South Carolina, Tennessee, Virginia and West Virginia. The rate of spread has been very fast during the past decade, at about 20-30 km/year.

In western North America, where HWA is not considered to be a pest, its range extends from northern California to southeastern Alaska.

HWA distribution in eastern US

Site and Date of Introduction:
The HWA was first observed in western North America on western hemlocks and mountain hemlocks in Oregon and British Columbia in the early 1920’s. It was first discovered in Oregon in 1924. In the eastern US, HWA was first observed in Richmond, Virginia around 1950.

Mode(s) of Introduction:
HWA is believed to have been accidentally transported to western United States from Asia, from either Japan or China. The insect then spread throughout the US probably by the transport of infested hemlock trees. Spread occurs from planted trees to native trees. HWA dispersal in the northeastern US was and is aided by wind, birds, deer and humans.

Reason(s) Why it has Become Established:
Several factors are responsible for the success of the HWA in eastern United States and for their rapid spread. The reproductive behavior of the insect causes high and fast growth. Reproduction is asexual and parthenogenetic, meaning all individuals are female and capable of reproducing. The two annual development stages of HWA discussed above leads to two generations of HWA being produced during each annual cycle. Their reproductive rate is very high, with each female laying up to 300 eggs. HWA multiply very rapidly when they first attack a hemlock tree with high nutritional quality. Thus in the initial stages of infestation, they multiply and spread very fast. Adelgids are very capable at locating and colonizing new young trees rapidly.

They have no natural predators in the eastern US. As such there is nothing to keep their growth in check and they multiply without any competition or predation, on hosts that are abundant.

HWA has adaptive qualities which aids its spread in new areas. It can tolerate extremes of temperature. In its native range in central Japan, it can survive winter temperatures that fall below -35 degrees Celsius and summer temperatures that exceed 40 degrees Celsius. It can also survive prolonged periods of low temperatures. As such its spread and establishment all along the east coast of the US with many variations of temperature is explained. HWA is also adapted to high elevations.

HWA has multiple modes of dispersal. It is carried by wind, birds, other forest animals, and people. It is also transported from site to site on infected trees, where they can spread from planted to native trees, from infested to uninfected trees.

Ecological Role:
In Japan and other parts of Asia, HWA inhabits ornamental and forest hemlocks and spruce, but their numbers are kept in check by the presence of natural enemies and by host resistance. HWA is found in western US on western hemlock and mountain hemlocks, but these trees are resistant to the adelgid.

In the eastern US, where they are considered to be a major threat to forest ecosystems, they destructively inhabit eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana). They destroy hemlocks by their feeding habits. Crawlers settle at the base of needles where they insert their stylus bundles deep within plant tissues, so that they penetrate the vascular tissue and reach the parenchyma cells of the xylem, which transfer and store nutrients in the plant and connect the xylem to the phloem and pith. HWA secretes saliva which hardens and remains in the plant after the stylet is withdrawn. The HWA thus depletes nutrients by depleting the plant’s nutrient stores. This desiccates the needles, causing them to turn a grayish-green color and drop from the tree. Defoliation follows and most buds are also killed, leading to almost no new growth on infested braches. This usually occurs within a few months of original infestation. Within a couple of years, major limbs start dying off, starting from the bottom, and the entire tree can be dead within four years.

These pests are in turn preyed upon by the ladybug Pseudoscymnus tsugae which are native to Japan. P. tsugae has a very narrow host range and feed mostly on adelgids, particularly the HWA. Their lifecycle is very closely connected to that of the HWA. These beetles are responsible for keeping HWA populations in check in Japan and preventing them from becoming pests.

Apart from the fact that HWA is preyed upon by Pseudoscymnus tsugae in its native range and is their main food source and thus has a place in the food web, no actual or possible benefits of the HWA can be perceived, particularly in the eastern US.

Feeding of HWA on hemlocks causes rapid decline in tree health, followed by quick mortality. Usually hemlocks die within four years of infestation. If the adelgid is left completely uncontrolled, sometimes the tree can be killed in a year or less. Sometimes death does not occur fast and infested trees can survive for relatively long periods of time with very little foliage at the top of the crown. Diseased trees are not very aesthetically pleasing. Such trees are more susceptible to attacks by other insects or diseases and are felled easily by strong winds.

Hemlock tree death is occurring at a fast rate in eastern forests due to HWA invasion. Eastern hemlock is an ecologically important species and often irreplaceable. Eastern hemlock stands provide unique habitat to many forest species which are dependent on the dense canopy of hemlock stands. Such wildlife species include ruffed grouse, turkey, deer, snowshoe hare and rabbit. Many species of birds use hemlocks as nesting site, food source, roost site and winter shelter. Many plant species also inhabit hemlock stands and streams shaded by hemlocks provide habitat to brook trout which flourish in cool waters. Hemlocks help regulate and maintain water temperatures in riparian habitats where they are commonly found. In winter, hemlock stands are warmer than hardwood forests since their dense canopy provides shelter from the wind and as such they provide essential shelter for many forest species. Hemlocks are necessary for the survival of many species of animals and plants that are considered endangered. Apart from these biotic effects, many ecosystem services will be affected as well. It has been suggested that HWA infestation can lead to drastic changes in nitrogen cycling. Substantial increases in nitrogen mineralization and nitrification rates have already been observed in infected hemlock stands. The loss of hemlock canopy would increase light availability on the forest floor and also reduce temperature. Thus the destruction of hemlocks by the HWA would have cascading effects that would affect and disrupt the entire ecosystem of eastern forests.

Hemlock is an economically important tree since it is used for pulpwood and to build structures such as barns and sheds. It is also used as an ornamental tree. Thus the loss of hemlock trees leads to economic losses. Even if trees do not die, the occurrence of the dirty woolly masses of HWA on twigs and needles reduces the value of hemlocks as ornamental trees.

Control Level Diagnosis:
Highest Priority.
HWA can damage trees very quickly and they are spreading very rapidly. Their rate of spread has been about 15 miles/year during the past decade. In many northeastern forests, the HWA has already infested all available host trees. Their potential for further spread is great due to their wide environmental tolerance. In Japan, HWA is adapted to high elevations and wide temperature ranges. This is an indication of possible future spread in the US, with possible spread and occupation throughout the entire hemlock range in the eastern US, ranges that are currently unoccupied by the HWA.

Control Method:  
Different control methods should be used depending on the location of the hemlocks. Infestations on ornamental trees need to be controlled by methods different from those used to control HWA infestation in natural forest systems.

In a landscaped environment, an integrated pest management approach should be taken. Cultural and chemical control methods can be combined using the following control techniques –
•    Invasion by adelgids can be reduced by controlling their dispersers or preventing access of dispersers to HWA.
•    Infested trees can be selectively removed to prevent further spread.
•    HWA can be mechanically removed from infested trees by spraying with strong jets of water or by clipping of infested twigs and needles.
•    Resistant hemlocks can be planted to minimize infestation by HWA.
•    Chemical insecticides can be applied. The most common method is to use horticultural oil, insecticidal soap, or any of several petrochemical insecticides designed to kill HWA. Insecticides can be injected or implanted directly into the stem of infested hemlocks, or they can be applied to the roots. Multiple, repeated applications are usually necessary.

Many of the control options listed above may not be viable for forest environments. The best control method for forest settings would be the use or introduction of biological control agents. There are hardly any natural predators of HWA in eastern forests and the predators that do exist have proved to be very ineffective in controlling HWA. Several exotic insects have been identified that are natural predators of the HWA in their native habitats. The most significant of such insects is the beetle Pseudoscymnus tsugae, which has been the most effective predator of HWA in Japan and has been mainly responsible for keeping HWA populations in check. Several characteristics of P. tsugae make it an effective predator of HWA. It is extremely mobile and feed voraciously and primarily on the HWA. It feeds on all life-stages of the adelgid and P. tsugae’s lifecycle is synchronous with that of HWA. It is adapted to a wide range of climatic conditions and as such can be introduced in the wide range of climates that the HWA invades. P. tsugae has high dispersal ability and searching efficiency.

Several other candidates, such as three lady beetles belonging to the genus Scymnus, have been identified as possible biological control agents of HWA but none possess all the qualities of P. tsugae.

P. tsugae feeding on HWA


Cheah, C.A.S.J. and McClure, M.S. 2000. Seasonal synchrony of life cycles between the exotic predator, Pseudoscymnus tsugae, (Coleoptera: Coccinellidae) and its prey, the hemlock woolly adelgid Adelges tsugae (Homoptera: Adelgidae). Agricultural and Forest Entomology 2:241-251.

Smith-Fiola, D. The Hemlock Woolly Adelgid: Life Cycle, Monitoring, and Pest Management in New Jersey. Rutgers Cooperative Extension. New Jersey Agricultural Experiment Station.

Salom, S.M. 1999. Hemlock Woolly Adelgid: A Major Threat to Eastern Hemlock. Virginia Forest Landowner Update 13(2).

Mayer, M., Chianese, R., Sheppard, J. and Palmer, D. 2000. Release of Pseudoscymnus tsugae (Coleoptera: Coccinellidae) on the Hemlock Woolly Adelgid, Adelges tsugae (Homoptera: Adelgidae) in NJ. Annual Report 2000. New Jersey Department of Agriculture. Division of Plant Industry, Phillip Alampi Beneficial Insect Laboratory, Trenton, NJ.

Orwig, D.A. and Foster, D.R. Stand, Landscape, and Ecosystem Analyses of Hemlock Woolly Adelgid Outbreaks in Southern New England: An Overview. Proceedings: Symposium on Sustainable Management of Hemlock Ecosystems in Eastern North America.

McClure, M.S. 2001. Biological Control of Hemlock Woolly Adelgid in the Eastern United States. Forest Health Technology Enterprise Team. USDA Forest Service.

McClure, M.S., Salom, S.M. and Shields, K.S. Hemlock Woolly Adelgid.
Baker, J.R., Bambara, S. and Baldwin, R.C.
“Hemlock Woolly Adelgid”
Ornamental and Turf
Department of Entomology Insect Note
Updated 9/02

“List of State and Counties with Known Hemlock Woolly Adelgid Infestations”
State surveys and reporting provided by State forest health specialists, from respective State Departments of Agriculture and Forestry agencies
USDA Forest Service, Forest Health Protection, Morgantown, WV
Updated 9/9/02

“Special Alert: Help Keep Hemlock Woolly Adelgid Out of Maine”
Press Release 6/26/02
Maine Forest Service

“Non-Indigenous Pest Threatens Hemlock Forests”
USDA Forest Service

Rhea, J.R.
“Economic and Environmental Impacts of the Hemlock Woolly Adelgid, Adelges tsugae on the Hemlock Resources of Eastern North America”
Theme: Environmental and Economic Impact of Forest Insect Pests, Part 1

Day, E.
“Hemlock Woolly Adelgid”
Distribution, Description, Identification, Life History, Control
Entomology Publication 444-244
Virginia Cooperative Extension

McClure, M.S.
“Hemlock Woolly Adelgid, Adelges tsugae (Annand)”
The Connecticut Agricultural Experiment Station

Photo Credits:  

HWA infestation on branch.
Virginia Cooperative Extension, Virginia Polytechnic Institute and State University.

HWA Distribution in eastern US.
USDA Forest Service.

P. tsugae feeding on HWA.
USDA Forest Service.

Author: Shahrina Chowdhury
Last Edited: 11/18/2002

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