Hemlock Woolly Adelgid (HWA)

Adelges tsugae Annand









Adelgids are conifer-feeding insects, related to aphids.  Hemlock woolly adelgid, Adelges tsugae Annand, (HWA), feeds on hemlock species and was first described from samples originally from Oregon by P. N. Annand in California in 1924.  In the eastern United States, this non-native insect pest was initially reported at a private estate in Richmond, Virginia in the early 1950s. The hemlock woolly adelgid in eastern US originated from southern Honshu island, Japan and infestations have since spread widely to attack eastern hemlock, Tsuga canadensis, and Carolina hemlock, Tsuga caroliniana.  Currently, HWA has been reported in 20 eastern states, from the southern Appalachians in the Carolinas and Georgia, through the Mid-Atlantic States, westwards to Ohio and Michigan and northwards to northern New England.  Most recently, established HWA infestations were discovered in southwestern Nova Scotia in Canada in 2017. In Connecticut, HWA was first reported to the Connecticut Agricultural Experiment Station in New Haven in 1985 and by 1997, was found throughout the state. Hemlock woolly adelgid has two all-female generations a year and each female develops and lays eggs under a ball of protective waxy wool, most visible on the underside of the newest hemlock growth. The adelgid feeds along the stems targeting the hemlock’s storage cells and inhibiting the ability to produce new growth.  Unchecked in North American by native natural enemies, HWA populations can explode relatively quickly to impact hemlocks although long term research at the CAES has shown that severe winter events can markedly reduce infestations for the following spring, allowing trees to recover. Hemlock tree mortality from HWA attack can occur in a few years, often in conjunction with other serious pests, such as the non-native elongate hemlock scale, Fiorinia externa Ferris, especially on stressed, drought-prone sites. Weakened trees may then die when overwhelmed by the native hemlock borer, Phaenops (formerly Melanophila) fulvoguttata

But HWA-infested hemlocks can also survive HWA infestations if regularly controlled, especially in the garden landscape. Horticultural oil or insecticidal soap applications, once or twice annually in May and late June, depending on the level of HWA infestations, are the preferred method of control with the least impact on non-target and beneficial species but is generally costly and impractical for large scale control in the forest. Chemical control with neonicotinoids has been widely utilized in some other states for control of HWA but in Connecticut, this is now restricted to licensed use (CT Public Act No. 16-17 An Act Concerning Pollinator Health:


Connecticut has been managing HWA for over 34 years, implementing statewide biological control with the Japanese ladybeetle predator of HWA, Sasajiscymnus tsugae, and many of the state’s forest hemlocks have survived and persisted since the initial waves of tree mortality to this day. Hemlock woolly adelgid is at its lowest levels in 2019 since its first report in 1985.  Hemlocks which showed decline from the recent severe drought of 2015-2017 are now recovering after the wet year of 2018. However, concurrently, elongate hemlock scale has spread throughout much of Connecticut’s hemlock forests in recent years and is affecting recovery in some sites.


    Biological Control of HWA in Connecticut

In Connecticut, biological control of HWA has focused exclusively on laboratory rearing and mass releases onto state, town and private forests of the tiny Japanese coccinellid, Sasajiscymnus [formerly Pseudoscymnus] tsugae, discovered and described by Japanese and CAES scientists. This tiny ladybeetle, about 2mm in length, was the first non-native HWA predator species imported into the US from southern Honshu island, Japan.  It is the only HWA predator that is reared commercially and is now available to the public from Tree-Savers, a company from Pennsylvania (http://tree-savers.com/). This species feeds on all stages of HWA as larva and adult and studies have shown its ability to adapt and overwinter in Connecticut.
Between 1995-2007, >176,000 adult S. tsugae were reared at the Valley Laboratory in Windsor and released at 26 sites on state lands, town and private forests and open space. An additional 2,000 S. tsugae were donated to Connecticut by Tree-Savers in 2017 and released for further research in new towns. In total, from 1995-2018, > 178,000 S. tsugae were released in 35 sites throughout Connecticut to manage HWA populations in an effort to save our state’s hemlocks. Currently, an integrated assessment of the status of Connecticut’s hemlocks and the efficacy of these biocontrol implementations of S. tsugae in Connecticut at older sites is underway, funded by the USDA National Institute of Food and Agriculture. Field release sites are being revisited and reassessed 13-21 years after release

Sasajiscymnus tsugae (Coleoptera:Coccinelliadae)


S. tsugae
adult feeding on HWA eggs



Stages of S. tsugae on HWA: all larval and adult stages feed on all 

stages of HWA



Map showing locations of S. tsugae release sites in Connecticut from 1995-2018


For further information, contact carole.cheah@ct.gov

CAES Publications on hemlocks, hemlock woolly adelgid and biological control of HWA, elongate hemlock scale:



Eastern Hemlocks


Hicock, H. W.  1958    Hemlock seedlings: Getting Them Off to a Good Start Frontiers of Plant Science 11(1) Fall 1958



Nienstaedt, H. and Olson, J.S. 1955. Heredity and Environment: Short-cut study shows how both affect hemlock growth. Frontiers of Plant Science 7(2) Spring 1955



Olson, J. S. and Nienstaedt, H.  1953.   Hemlock in Connecticut. Frontiers of Plant Science 6(1) November 1953



Olson, J.S., Stearns, F. W. and Nienstaedt, H. 1959. Eastern Hemlock Seeds and Seedlings Response to Photoperiod and Temperature. Bulletin B620



Olson, J.S., Stearns, F. W. and Nienstaedt, H. 1959. Eastern Hemlock Growth Cycle and Early Years. Circular 205




Stephens, G. R. 1984 Heavily defoliated white pine has lower mortality than hemlock. Frontiers of Plant Science 37(1) Fall 1984



Life cycle, impacts and control of HWA


Cheah, C. 2018. Hemlock woolly adelgid (HWA) and other factors impacting eastern hemlocks.  CAES Factsheet



Cheah, C. 2016.  HWA winter mortality in Connecticut and implications for management and control.  CAES Factsheet



Cheah, C. 2010.  Connecticut's Threatened Landscape: Natural Enemies for Biological Control of Invasive Species. Frontiers of Plant Science 57(2) Spring 2010



Cowles, R.S. 2015. Use of Neonicotinoids in the Home Landscape. CAES Factsheet.



McClure, M.S. 1995. Managing hemlock woolly adelgid in ornamental landscapes.  Bulletin B925.



McClure, M.S. 1991. Pesticides will protect ornamentals from hemlock woolly adelgid. Frontiers of Plant Science 44(1) Fall 1991



McClure, M.S. 1987. Biology and control of hemlock woolly adelgid.

Bulletin B925



McClure, M.S. 1987. Hemlock woolly adelgid may also attack spruce. Frontiers of Plant Science 39(2) Spring 1987



McClure, M.S. 1995. Using natural enemies from Japan to control hemlock woolly adelgid. Frontiers of Plant Science 47(2) Spring 1995



McClure, M.S. and Cheah, C.A. S-J. 1998. Released Japanese ladybugs are multiplying and killing hemlock woolly adelgids. Frontiers of Plant Science 50(2) Spring 1998



Ward, J.S, Montgomery, M.E., Cheah, C.A.S-J., Onken, B.P. and Cowles, R.S. 2004. Eastern Hemlock Forests: Guidelines to minimize the impacts of Hemlock Woolly Adelgid.  USDA Forest Service Northeastern Area State and Private Forestry, NA-TP-03-04 28 pp.



Scales on hemlocks


McClure, M.S. 1987. Controlling hemlock scales with least environmental impact. Bulletin B844



McClure, M.S. 1978.  Two parasitic wasps have potential for controlling hemlock scales. Frontiers of Plant Science 30(2) Spring 1978




Other hemlock pests, diseases and associated phenomenon


Cheah, C. and Li, DeWei  2016.  The Red Bark Phenomenon. CAES Factsheet



Chris T. Maier, Carol R. Lemmon, Ronald M. Weseloh, and Theodore G. Andreadis 1993. Spring hemlock looper returns to attack hemlock forests in Connecticut. Frontiers of Plant Science 45(2) Spring 1993



Hemlock (Tsuga) Plant Health Problems:



Links to recent talks and interviews:



Danbury News Times:  Robert Miller 
Defeating scourges, hemlocks in state thriving once more…Mar 29, 2020: https://www.newstimes.com/opinion/article/Robert-Miller-Defeating-scourges-hemlocks-in-15161914.php

Connecticut Magazine May 2020: Todd McLeish 
Insects are being deployed in the war against invasive species in Connecticut… April 15, 2020: https://www.connecticutmag.com/health-and-science/insects-are-being-deployed-in-the-war-against-invasive-species-in-connecticut/article_3915e712-7b46-11ea-a637-b720a53c76e1.html?utm_medium=social&utm_source=email&utm_campaign=user-share


Danbury New Times: Robert Miller

Deep winter freezes a blessing for hemlocks….January 26, 2019



Connecticut Public Radio: Patrick Skahill

Cold Weather Provides Welcome Relief for Connecticut’s Troubled Hemlock Trees…January 30, 2019




Plant Science Day 2018 Short Talk

Climate impacts on hemlocks and hemlock woolly adelgid in the Northeast




The Holistic Nature of Us

All New Podcast from Judith Dreyer 2018



Biological control of invasives: hemlock woolly adelgid

The University of Connecticut Hot Topics series for the Master Gardeners Program June 20, 2018



Connecticut Post: John Burgeson

Cold snap gives hemlocks a fighting chance… January 26, 2018 

https://www.ctpost.com/local/article/Cold-snap-gives-hemlocks-a-fighting-chance-12528892.php?utm_campaign=email-tablet&utm_source=CMS Sharing Button&utm_medium=social#photo-14948007


Yale Daily News:  William Langhorne

An Invasion Intervention; March 6, 2018



The Day:   William Hobbs

Nature Notes: Cold snaps and special beetle help combat hemlock woolly adelgids;  Mar 9, 2018




Danbury New Times: Bob Miller





Danbury New Times: Bob Miller




Danbury New Times: Bob Miller



Connecticut Public Radio: Patrick Skayhill





Danbury New Times: Bob Miller




Danbury New Times: Bob Miller




Further Reading on HWA Biocontrol / Eastern Hemlocks in CT:


Cheah, C. 2019. Climate Impacts on Eastern Hemlock Sustainability. The Habitat 31 (1): 6-7, 13-14 


Cheah, C.A.S.-J. 2017. Predicting winter mortality of hemlock woolly adelgid in Connecticut by climatic divisions. Northeastern Naturalist Volume 24, Special Issue 7, 2017 B90-118 


Cheah, C. 2011. Sasajiscymnus (= Pseudoscymnus) tsugae: A ladybeetle from Japan Chapter 4. In: Cheah, C., Montgomery, M., Salom, S., Parker, B. L., Costa, S. and Skinner, M. 2004.  Biological control of hemlock woolly adelgid. USDA Forest Service. FHTET-2004-04, Reardon, R. and B. Onken [Tech. Coordinators], p.43-52


Cheah, C.A. S-J. & McClure, M.S. 2010.  Sasajiscymnus [formerly Pseudoscymnus] tsugae [Coleoptera:Coccinellidae].  In: Biological Control: A Guide to Natural Enemies in North America. A. Shelton.  Cornell University College of Agriculture and Life Sciences, Department of Entomology website. Online at:



Cheah, C. 2008.  The case for Sasajiscymnus tsugae: Biological control has helped save Connecticut’s hemlocks.  Abstract for a poster in Proceedings of the Fourth Symposium on Hemlock Woolly Adelgid in the Eastern United States. Hartford, CT.  Feb. 12-14, 2008. B. O. Onken and R. Reardon (Compilers) FHTET 2008-01. pp. 279-280.


Cheah, C. 2006.  Hope for hemlocks.  Connecticut Woodlands 71(3):13-15.


Cheah, C.A. S-J., Mayer, M. A., Palmer, D., Scudder, T. and Chianese, R. 2005.  Assessments of biological control of hemlock woolly adelgid with Sasajiscymnus tsugae in Connecticut and New Jersey.  In: Proceedings of the Third Symposium on the Hemlock Woolly Adelgid in the Eastern United States, Feb.1-3, 2005, Asheville, North Carolina. Onken, B. and Reardon, R. (Compilers) FHTET 2005-01 pp. 116-130.


Ward, J. S., Cheah, C. A. S-J., Montgomery, M.E., Onken, B. P. and Cowles, R.S. 2004

Eastern Hemlock Forests: Guidelines to minimize the impacts of hemlock woolly adelgid. USDA Forest Service Northeastern Area State and Private Forestry, NA-TP-03-04 28 pp.



Cheah, C.A. S-J. & McClure, M.S. 2002. Pseudoscymnus tsugae in Connecticut forests: the first five years. Proceedings of the Hemlock Woolly Adelgid Symposium, East Brunswick, NJ, February 5-7, 2002.  Eds. Onken, B., Reardon, R. Lashomb, J.  p. 150-165 


McClure, M.S. and Cheah, C. A. S-J. 2002.  Important mortality factors in the life cycle of hemlock woolly adelgid, Adelges tsugae Annand [Homptera:Adelgidae] in the Northeastern United States.  Proceedings of the Hemlock Woolly Adelgid Symposium, East Brunswick, NJ, February 5-7, 2002.  Eds. Onken, B., Reardon, R. Lashomb, J.  p. 13-22.


Cheah, C.A. S-J. & McClure, M.S. 2000. Seasonal synchrony between the exotic predator, Pseudoscymnus tsugae (Coleoptera:Coccinellidae) and its prey, the hemlock woolly adelgid, Adelges tsugae. Agriculture and Forest Entomology 2, 241-251 https://doi.org/10.1046/j.1461-9563.2000.00073.x 


McClure, M. S., Cheah, C.A.S-J. & Tigner, T. 2000. Is Pseudoscymnus tsugae the solution to the hemlock woolly adelgid problem? : An early perspective. Proceedings of a Symposium on Sustainable Management of Hemlock Ecosystems in Eastern North America 1999. (eds. K. McManus, K. Shields and D. Souto) p. 89-95. USDA Forest Service General Technical Report NE-267.


Cheah, C.A.S-J. & McClure, M. S.  1998.  Life history and development of Pseudoscymnus tsugae (Coleoptera:Coccinellidae), a new predator of the hemlock woolly adelgid (Homoptera: Adelgidae).  Environmental Entomology, 27, 1531-1536. https://doi.org/10.1093/ee/27.6.1531


Cheah, C.A.S-J. & McClure, M. S.  1996.  Exotic natural enemies of Adelges tsugae and their potential for biological control. Proceedings of the First Hemlock Woolly Adelgid Review (ed. by S. Salom, T. Tigner and R. C. Reardon), pp. 103-112. USDA Forest Service Forest Health Technology Enterprise Team 96-10.


Development of Artificial Diets for HWA Predator Rearing


Cohen, A. C and Cheah, C. A.S.J. 2015. Interim Diets for Specialist Predators of Hemlock Woolly Adelgids.  Entomol Ornithol Herpetol 4:153 doi: 10.4172/2161-0983.1000153


Cohen, A.C. and Cheah, C. 2011. Chap. 14. Development of artificial diets for predators of hemlock woolly adelgids. In: Implementation and Status of Biological

Control of Hemlock Woolly Adelgid. Tech. Coor. Onken, B. and Reardon, R. USDA Forest Service FHTET Publication FHTET-2011-04 pp. 148 – 160



Cohen, A.C., Cheah, C., Kidd, K. and Hodgson, T. 2011. Chap. 13. Defining PC/QC standards of mass-rearing HWA predators. In: Implementation and Status of Biological Control of Hemlock Woolly Adelgid. Tech. Coor. Onken, B. and Reardon, R. USDA Forest Service FHTET Publication FHTET-2011-04 pp. 139-147

Cohen, A.C. and Cheah, C. 2010.  Packaging and Presentation of Artificial Diets for Hemlock Woolly Adelgid Predators In: Proceedings of the Fifth HWA Symposium in the Eastern United States, Asheville, NC August 17-19, 2010. Compilers Onken, B.  and Reardon, R.  USDA Forest Service FHTET-2010-07. p 33-35. 


Cohen, A.C., Cheah, C. Hain, F., Kidd. K. and Hodgson. T. 2010. Developing process control and quality control in rearing systems for hemlock woolly adelgid predators. In: Proceedings of the Fifth HWA Symposium in the Eastern United States, Asheville, NC. August 17-19, 2010. Compilers Onken, B.  and Reardon, R.  USDA Forest Service FHTET-2010-07. P 141-143 


Cohen, A.C., Cheah, C.A.S-J., Strider, J., and Hain, F.  2008.  Diet development for hemlock woolly adelgids and their predators In:  Proceedings of the Fourth Symposium on the Hemlock Woolly Adelgid in the Eastern United States, Feb.12-14, 2008, Hartford, Connecticut. Onken, B. and Reardon, R.  [Compilers] USDA Forest Service FHTET 2008-01 p. 150-156