Adelges tsugae (hemlock woolly adelgid)

Pictures

Picture	Title	Caption	Copyright
Title Adults on Hemlock sprig Caption Adelges tsugae (Hemlock woolly adelgid); adults on Hemlock sprig. USA. Copyright ©Tom Coleman/USDA Forest Service/Bugwood.org - CC BY 3.0 US	Adults on Hemlock sprig	Adelges tsugae (Hemlock woolly adelgid); adults on Hemlock sprig. USA.	©Tom Coleman/USDA Forest Service/Bugwood.org - CC BY 3.0 US
Title Damage symptoms on Eastern hemlock Caption Hemlock woolly adelgid (Adelges tsugae); damage symptoms on Eastern hemlock (Tsuga canadensis). Joyce Kilmer area, North Carolina, USA. Copyright ©Jason Van Driesche/Bugwood.org - CC BY-NC 3.0 US	Damage symptoms on Eastern hemlock	Hemlock woolly adelgid (Adelges tsugae); damage symptoms on Eastern hemlock (Tsuga canadensis). Joyce Kilmer area, North Carolina, USA.	©Jason Van Driesche/Bugwood.org - CC BY-NC 3.0 US
Title Egg masses Caption Hemlock woolly adelgid (Adelges tsugae); egg masses on Tsuga sp. Laboratory photograph, Charlotte, North Carolina, USA. Copyright ©Lorraine Graney/Bartlett Tree Experts/Bugwood.org - CC BY-NC 3.0 US	Egg masses	Hemlock woolly adelgid (Adelges tsugae); egg masses on Tsuga sp. Laboratory photograph, Charlotte, North Carolina, USA.	©Lorraine Graney/Bartlett Tree Experts/Bugwood.org - CC BY-NC 3.0 US

Identity

Preferred Scientific Name

• Adelges tsugae Annand

Preferred Common Name

• hemlock woolly adelgid

Other Scientific Names

• Chermes tsugae Annand

International Common Names

• French: puceron lanigère de la pruche

Local Common Names

• Germany: Laus, Hemlockstannen; Tannenlaus, Hemlocks-

EPPO code

• ADLGTS (Adelges tsugae)

Summary of Invasiveness

The hemlock woolly adelgid (Adelges tsugae) is a small, aphid-like insect that has become a serious pest of eastern hemlock and Carolina hemlock. The most obvious sign of infestation is the presence of white, woolly egg masses on the underside of hemlock needles. Infested eastern North American hemlocks defoliate prematurely and will eventually die if left untreated. A. tsugae is a difficult insect to control as the white waxy secretion protects it from pesticides. It is dispersed to new habitats through the nursery trade and locally by wind, birds, mammals and humans. Hemlock trees provide important habitats for many wildlife species and A. tsugae has severe adverse ecological impacts which will become more severe as its distribution expands. Explanations for the rapid and invasive expansion of A. tsugae include a lack of natural enemies, a lack of resistance or tolerance to infestation by eastern hemlock and Carolina hemlock, and large reproductive output (Trotter and Shields, 2009).

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Metazoa
•         Phylum: Arthropoda
•             Subphylum: Uniramia
•                 Class: Insecta
•                     Order: Hemiptera
•                         Suborder: Sternorrhyncha
•                             Superfamily: Aphidoidea
•                                 Family: Adelgidae
•                                     Genus: Adelges
•                                         Species: Adelges tsugae

Description

Adelges tsugae is a small (0.74mm), reddish-purple, aphid-like insect that covers itself with a white, waxy secretion. Both winged and wingless forms are present. Their mouthparts are thread-like and about 1.5mm long and used to suck sap. Eggs are brownish-orange but darken as the embryo matures. When the eggs hatch, reddish-brown crawlers move about actively in search of a suitable site to settle. The tiny crawlers can only be seen with a hand lens as they are barely visible to the naked eye. Once the crawlers settle, they insert their mouthparts into the plant at the base of the hemlock needles and remain in the same place for the duration of their life. Dormant first instar nymphs are black with a white fringe around the edge and down the centre of the back. The developing nymphs produce white, cottony, waxy tufts that cover their bodies. The white masses are 3mm or more in diameter. The presence of these masses on the twigs and bark of hemlock is a sure sign of A. tsugae.

Distribution

Native range: China and Japan. A. tsugae were first described in western North America in 1924 and may be native to western North America (Havill and Footitt, 2007; US Forest Service, 2012).

Introduced range: Unintentionally introduced to eastern USA from Japan sometime before 1951, when it was first documented on ornamental hemlocks in Richmond, Virginia. Initially a pest of ornamental hemlock, but in the 1980s, A. tsugae was recognized as a threat to eastern and Carolina hemlocks as populations moved north through New Jersey, Long Island, and southern Connecticut and ultimately all of New England (Trotter and Shields, 2009). For distribution maps see the US Forest service website on hemlock woolly adelgid.

A. tsugae have moved south almost twice as fast as they have moved north, and some have linked this with a potential role of low temperatures in limiting the expansion of populations (Trotter and Shields, 2009).
 

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Habitat

The secondary host of Adelges tsugae is hemlock (where the asexual cycle occurs), and spruce is the primary host where the sexual cycle occurs. Experts hope that an aversion to cold weather will slow or stop its northern movement. In New Jersey and Connecticut, a large population of Adelges tsugae died as a result of a cold period in the winter of 2000.

Habitat List

Hosts/Species Affected

Some of the adults produced during the spring generation are winged individuals that are unable to reproduce on hemlock, therefore they leave the hemlock tree in search of spruce, the alternate host. But because no suitable spruce host is available in North America, they soon die. Hemlocks growing in poor conditions (compacted soils, ledgy soils, poor drainage, drought prone, etc.) are much more likely to succumb within 3-5 years from invasion. Hemlocks growing under better growing conditions have been shown to withstand infestations longer.

Host Plants/Plants Affected

Biology and Ecology

Nutrition
The hemlock woolly adelgid (Adelges tsugae) utilizes hemlock as its secondary host, where the asexual cycle occurs and spruce as its primary host, where the sexual cycle occurs. In North America the sexual cycle has not been previously recorded. Young twigs are the preferred feeding sites. Immature nymphs and adults feed on trees by sucking sap from the twigs. They attach themselves just below the base (abscission layer) of newly developed needles and feed on xylem parenchyma cells (tissue that manufactures and stores plant food).

Reproduction
A. tsugae has both sexual and asexual (parthenogenic) reproduction. In its native range the asexual cycle occurs on hemlock while the sexual stage occurs on spruce. However, because of a lack of suitable spruce species to act as a host, in its introduced range, A. tsugae has two asexual generations per year on hemlock (Trotter and Shields, 2009). Each adult can produce between 50-300 eggs within its lifetime. The large reproductive output of hemlock woolly adelgid contributes towards its invasive expansion (see also ‘Summary of Invasiveness’ section for explanations of invasiveness).

Lifecycle stages
Spring generation adults lay numerous eggs within large, white, woolly sacs. The eggs hatch and crawlers from the second generation move to attach themselves to new needles. Once they find a site, they settle and become dormant (no feeding) until fall (autumn). At that time, they end their dormancy and begin to feed and develop through the fall and winter. A. tsugae is atypical of most insect species in that it is inactive for much of the growing season and very active throughout the winter.
 

Natural Enemies

Means of Movement and Dispersal

Introduction pathways to new locations
Forestry:
Nursery trade: Adelges tsugae was inadvertently shipped to Maine from Connecticut on untreated nursery stock in 1999.
Other: Adelges tsugae was accidentally introduced to the North American continent earlier this century (McClure 1987).

Local dispersal methods
Garden escape/garden waste:
Hikers' clothes/boots: Humans can disperse the insect through recreational activities.
Horticulture (local): Adelges tsugae can be spread via infested nursery stock (Ward et al. 2004).
On animals: The eggs and nymphs can be readily dispersed by birds, deer and other mammals.
On animals (local): The eggs and nymphs can be readily dispersed by birds.
On animals (local): The eggs and nymphs, which are not attached to the plant via mouthparts can be readily dispersed by wind.
People sharing resources (local): Humans can also disperse the insect through various activities, including logging and recreational activities.
Road vehicles: Humans can disperse the insect through recreational activities.

Pathway Causes

Pathway Vectors

Plant Trade

Impact Summary

Impact

General Impacts

The hemlock woolly adelgid (Adelges tsugae) is damaging hemlock ecosystems in eastern North America where both eastern hemlock (see Tsuga canadensis in IUCN Red List of Threatened Species) and Carolina hemlock (T. caroliniana) serve as hosts. To date, approximately 25% of the 1.3 million hectares of host type has been infested. The entire range of eastern hemlock is at risk within the next 20 to 30 years. Immature nymphs as well as adults damage trees by sucking sap from the twigs. The trees lose vigour and prematurely drop their needles, to the point of defoliation, which may lead to death. If left uncontrolled, the adelgids can kill a tree within three to four years. Trees of all sizes and ages are attacked, but natural stands of hemlock are at greatest risk for death. The value of ornamental hemlocks is reduced by the presence of the dirty, white woolly masses attached to the twigs or base of needles. Eastern hemlock is economically important in several areas of the eastern United States. The nursery industry in North Carolina and Tennessee currently maintains approximately $34 million in hemlock growing stock. This industry is feeling the effects of A. tsugae in reduced sales of native hemlock for ornamental use.

Hemlock trees are ecologically important and provide a unique environment. Eastern hemlock trees can reach a lifespan of 900 years and this tree is a component of many old growth communities. The hemlock forest also provides nesting sites and a foraging habitat for Neotropical migratory bird species. For example, the Acadian flycatcher (Empidonax virescens) uses Eastern hemlock for nesting and populations are thought to be negatively affected by A. tsugae infestation in Appalachian riparian forests (Allen et al., 2009). In addition, lepidopterans, such as the hemlock angle (Semiothisa fissinotata), which feed solely on hemlocks, will be affected. Several threatened or endangered species of flora and fauna require hemlock forests to survive. These forests are normally stable and resistant to plant invasions, but the loss of hemlocks from such forests will greatly affect the microclimate and soil conditions. Large-scale hemlock die-offs will affect species diversity, vegetation structure, stand environmental conditions and ecosystem processes.  Numerous studies have suggested that A. tsugae and associated loss of hemlock trees can impact upon ecosystem processes such as nitrogen and carbon cycling (see Cobb, 2010; Nuckolls et al., 2009; Orwig et al., 2008; Hadley et al., 2008). The loss of hemlock trees from the forests can lead to increased dominance of deciduous hardwood species, which may in turn have ecosystem effects (Spalding and Rieske, 2010).

Threatened Species

Risk and Impact Factors

Impact mechanisms

• Herbivory/grazing/browsing

Impact outcomes

• Ecosystem change/ habitat alteration
• Host damage
• Negatively impacts forestry
• Threat to/ loss of endangered species
• Threat to/ loss of native species

Invasiveness

• Proved invasive outside its native range

Prevention and Control

Management information

Bibliography

References from GISD

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Principal Source

Global Invasive Species Database

Contributors

Reviewed by: Gabriella (Riella) Zilahi-Balogh, PhD Entomologist/Entomologiste Programme Officer, Plant Health/agent de programme, protection des végétaux Canada

Principal sources: USDA Forest Service - Hemlock Woolly Adelgid website. Northeastern Area Forest Health Protection

Distribution Maps

• = Present, no further details
• = Evidence of pathogen
• = Widespread
• = Last reported
• = Localised
• = Presence unconfirmed
• = Confined and subject to quarantine
• = See regional map for distribution within the country
• = Occasional or few reports

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