Hemlock Woolly Adelgid Overview

What is Hemlock Woolly Adelgid?

Light infestation of hemlock woolly adelgid.  Photo: Maine Forest Service 2007Heavy infestation of hemlock woolly adelgid.  Photo: US Forest ServiceHemlock Woolly Adelgid (Uh-dell-jid) (HWA) is an introduced, aphid-like insect from Asia that attacks eastern hemlock.  As of 2011, eighteen states from Maine to Georgia have HWA.  Many areas infested with HWA display extensive tree decline and mortality.  HWA affects all species of hemlock, but does not affect pine, spruce, fir or other conifers. 

The most obvious sign of HWA is the covering of wool-like wax filaments produced as the insect matures.  The woolly masses generally range from about 1/16-inch to 1/8-inch in diameter.  They are most visible from late fall to early summer on the undersides of the outermost branch tips of hemlock trees.

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Adelgid wool on hemlock.  Photo: Maine Forest ServiceLook for These Signs on Hemlocks:

  • Immobile, white woolly masses at the base of needles on undersides of hemlock twigs
  • Trees that have been impacted for years will also display off-color needles, often with a grayish-cast; thinning crowns, premature needle loss.

If you think you have seen a hemlock tree with adelgid, note its location and
call the Maine Forest Service at (207) 287-2431 or
use our on-line form

As of 2018 hemlock woolly adelgid has been found in forests of more than 40 southern and coastal Maine towns. Most detections are made by informed citizens.

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Don't be Fooled by These Imposters--They are not Hemlock Woolly Adelgid

Some hemlock woolly adelgid look-alikes. Photo: USFS

Many things look like hemlock woolly adelgid at first glance. On closer inspection, they can be ruled out either because of their texture, their location or another characteristic. Remember, HWA will usually be found on the undersides of twigs at the bases of needles and is coated with a waxy material. When in doubt, please contact us.



Elongate hemlock scale can also be mistaken for hemlock woolly adelgid, and is often found on hemlock trees already infected with HWA. Please let us know if you think you have found elongate hemlock scale.


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The Maine HWA Situation in 2020

  • Hemlock woolly adelgid appears to be widespread in the towns of Harpswell, Kittery, Westport Island, and York and may be widespread in other communities, especially on coastal peninsulas and islands. Elongate hemlock scale has been found in the forests of Kittery and on planted trees along the south coast. In 2016 it was found to be well-established in Frye Island. Small, scattered populations were also found in Standish and Raymond. It is likely more widely distributed in forests and landscapes. When scale and adelgid are found together, they may hasten the decline of hemlock. Some chemical treatments for adelgid are not effective against elongate hemlock scale. If you suspect you have found elongate hemlock scale, please contact us.
  • • As of January 2020, HWA is known to be established, in scattered areas, in the following Maine townships:
    • Cumberland County: Brunswick, Cape Elizabeth, Cumberland, Falmouth, Freeport, Frye Island, Great Diamond Island (part of Portland), Harpswell, Raymond, Scarborough, South Portland, Standish, Yarmouth
    • Knox County: Camden, Friendship, Owls Head
    • Lincoln County: Boothbay, Boothbay Harbor, Bristol, Edgecomb, South Bristol, Westport Island, Wiscasset
    • Sagadahoc County: Arrowsic, Bath, Georgetown, Phippsburg, Topsham, West Bath, Woolwich
    • York County: Alfred, Arundel, Berwick, Biddeford, Eliot, Kennebunk, Kennebunkport, Kittery, Lebanon, Ogunquit, Old Orchard Beach, Saco, Sanford, South Berwick, Wells, York

    (select image below to open a pdf version of the map)

  • We suspect HWA is established in other townships in that general climatic region. The mild winters since 2009 have allowed populations to build enough to reach detectable levels in areas where they had previously been established but at sub-detectable levels.

  • In March 2020, the hemlock woolly adelgid quarantine area in Maine was expanded to include 3 entire counties and 29 additional towns.  The quarantine area for hemlock woolly adelgid in Maine includes (map):
    • Androscoggin county: the Minor Civil Divisions of Auburn, Durham, Lewiston, Lisbon, Mechanic Falls, Minot, Poland and Sabattus;

    • Cumberland county:the Minor Civil Divisions of Baldwin, Brunswick, Cape Elizabeth, Casco, Chebeague Island Cumberland, Falmouth, Freeport, Frye Island, Gray, Gorham, Harpswell, Long Island, Naples, New Gloucester, North Yarmouth, Portland, Pownal, Raymond, Scarborough, Sebago, South Portland, Standish, Westbrook, Windham, and Yarmouth;

    • Hancock county: the Minor Civil Divisions of Bar Harbor, Blue Hill, Brooklin, Brooksville, Bucksport, Castine, Cranberry Isles, Deer Isle, Ellsworth, Franklin,Fletchers Landing TWP, Frenchboro, Gouldsboro, Hancock, Lamoine, Mount Desert, Orland, Penobscot, Sedgwick,Sorrento, Southwest Harbor, Stonington, Sullivan, Surry, Swans Island, T7-SD BPP, T9-SD BPP, T10-SD BPP, Tremont, Trenton, Verona Island and Winter Harbor;

    • Kennebec county: the Minor Civil Divisions of Litchfield and Pittston;

    • Knox county;

    • Lincoln county;

    • Sagadahoc county;

    • Waldo county: the Minor Civil Divisions of Belfast, Belmont, Frankfort, Islesboro, Lincolnville, Morrill, Northport, Prospect, Searsmont, Searsport, Stockton Springs, Swanville and Waldo; and

    • York County.

  • Counties in the Northeastern United states under quarantine for hemlock woolly adelgid (right) shaded in brown and yellow.
  • Hemlock Woolly Adelgid Distribution in the Eastern United States.  USFS

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Timeline of Hemlock Woolly Adelgid Detections in Maine

1999 - Hemlock woolly adelgid was inadvertently shipped to Maine from Connecticut on untreated nursery stock . The infested stock was treated and monitored for insect survival.

2000 - Hemlock woolly adelgid infested outplanted hemlocks were found in new locations. Some of the hemlocks exhibited relatively heavy infestations indicating that they were infested when planted.

2001 - Again infested outplanted hemlocks were found in new locations. New infestations were found at sites treated in previous years

2003 - Natural spread of hemlock woolly adelgid was detected in Kittery and York forests. These sites were considered to be the northern edge of the general infestation in eastern North America.  Spread was most likely from natural movement by wind, birds and/or mammals. Management tactics in this area focus on containment.

2004 - A small hemlock woolly adelgid infestation was found in Wells, Maine. Beginning in 2004, new control measures included pesticide treatments on sites with a high probability of further spread, high HWA populations and human activity, and HWA predator beetles released into remote infested areas as a biological control.

2005 - Infestations detected in Eliot and South Berwick

2006 - No new towns detected, but continued new discoveries in towns known to have established adelgid, and known populations began to intensify. Infested planted tree detected in Southport.

2007- Infested planted tree detected in Brooklin.

2008 - Park Ranger, Janet Mangion, discovers hemlock woolly adelgid during routine road-side maintenance at Ferry Beach State Park in Saco. Press coverage of the detection yields reports of infested outplanted hemlocks in South Portland and infested forest hemlocks in Kennebunkport. Follow up surveys reveal a low-level infestation throughout Ferry Beach State Park.

2010 - Hemlock woolly adelgid was detected in a large swath of coastal townships including: Arrowsic, Bath, Boothbay, Boothbay Harbor, Bristol, Brunswick, Cumberland, Edgecomb, Falmouth, Freeport, Georgetown, Great Diamond Island (part of Portland), Harpswell, Ogunquit,Phippsburg, South Bristol, South Portland, West Bath, Westport Island, Wiscasset, Woolwich, Yarmouth

2011- Hemlock woolly adelgid detected in Cape Elizabeth forest trees. Infested planted trees confirmed in two locations in Mount Desert.

2012 - Forest infestation found in Alfred, Arundel, Berwick, Biddeford, Kennebunk and Topsham during the Maine Forest Service detection survey. In August, a trained volunteer detected a forest infestation in Scarborough and a homeowner detected infested planted trees in Sedgwick and a caretaker reported infested trees in Northeast Harbor (the latter two locations had co-infestations of elongate hemlock scale).

2013 - Forest infestation found in Old Orchard Beach during the Maine Forest Service detection survey. Infestations on planted trees reported in Ellsworth and Blue Hill.  SEPTEMBER: Quarantine Revised

2014 – Forest infestations found in Lebanon and Sanford through remote sensing during Master’s thesis work by Justin Williams, UNH.   Forest infestation found in Friendship and Owls Head during Maine Forest Service detection surveys.

2016 – Well established infestation of HWA and EHS found on the northern end of Frye Island in Saco Lake.  EHS had been there since 1993.  Further survey detected isolated population of HWA east of the island in Raymond and Standish.  In 2017, the trees heavily infested with EHS were removed, and surrounding host treated with pesticides.  Areas of the island at high risk for spreading HWA were also treated.

2017 - Forest infestations found in Newcastle during Maine Forest Service detection surveys, and in Camden, detected by a volunteer.

2020 – Forest infestation found in Mount Desert, first detected by landowner.

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Slow-The-Spread program for Hemlock Woolly Adelgid

Four Management Principles:  Exclusion and Eradication used where HWA is not established and Containment and Mitigation used where HWA is established

Management Tools: Quarantine, Surveys and Monitoring, Chemical Control, Biological Control, Outreach

Quarantine External (between states) and internal (within Maine) quarantines regulate movement of hemlock materials to reduce unintentional movement of HWA.

HWA Quarantine Area in Eastern United States

Surveys & Monitoring Maine Forest Service personnel regularly survey to locate new infestations and monitor the spread of HWA. Impact assessment plots have been established at five sites in Maine. Data from these sites will add to the understanding of forest impacts of the adelgid, and will be analyzed along with information collected in Vermont, New Hampshire, Massachusetts and Connecticut (impact plot summary).

Maine Forest Service Employee Inspects Hemlock for Signs of Adelgid.  Photo: Maine Forest Service

Chemical Control In the spring and summer HWA can drop onto humans and vehicles and may be moved into uninfested areas.  Treating infested trees in high-traffic areas, such as roadsides and residential sites, can reduce accidental human spread of HWA.

Chemical control of hemlock woolly adelgid in a residential area.  Photo: Maine Forest Service

Biological Control Two species of beetle, which feed exclusively on adelgid, have been released in Maine. Release sites are monitored periodically for beetle establishment (monitoring protocol). Impact assessment plots have been established at two release sites. Maine Forest Service cooperates in ongoing research into fungal agents of disease.  Biological control is the best hope for long-term control of HWA in Maine’s forests. (Off-site: biological control of hemlock woolly adelgid)

Sasajiscymnus tsugae beetles released on infested hemlock in Ferry Beach State Park.  Photo: Maine Department of Agriculture, Plant Industry

Laricobius nigrinus, minutes after release on hemlock twig (hemlock nymphs are black, rimmed with white wool)


Public Outreach An informed public plays an important role in detecting HWA.  Programs such as “Take a Stand” enlist public support and educate volunteers in HWA detection. Maine Forest Service also cooperates with Ecosystem Investigation Network to increase citizen monitoring for the pest (see their HWA Field Mission). Press releases help raise awareness about the threat.

Volunteers learn how to identify hemlock woolly adelgid.  Photo: Maine Forest Service

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What Can You Do

While there is little you can do to prevent natural spread of this insect, you can help reduce the likelihood of hemlock woolly adelgid coming to your backyard and thriving in your trees. In our climate, there is usually time to “rescue” tree health on ornamental trees after detection of these insects. And, if forest trees are being monitored, there is usually time to plan a response before widespread damage from the insect.

  • Do not move hemlock trees, seedlings or nursery stock from infested areas (Maine Map).
  • Please check any hemlock plantings and forest trees for adelgids.
  • MFS employee inspecting hemlock for sign of adelgid (Photo: Amanda Sawyer)Monitor your trees regularly for signs of the insect.
    • Inspect newer growth for discrete white cotton ball-like masses on the undersides of the twigs, usually attached at the bases of needles and up to 1/8th" diameter.
    • Try to look twice a year; in late March (this is when eggs are present and wool is at its fluffiest) and early November (look on on new growth, where settled nymphs will have just started to feed and develop their waxy coating. Remnants of old wool may still be visible on older growth).
    • Use photos for a guide, for more photos visit our Hemlock Woolly Adelgid Web site.
    • Look at lots of branches (hundreds would be good).
  • Cover the area that you are concerned about. Do not fertilize hemlock yard trees if you are near infested areas (adelgids thrive on the nitrogen enrichment provided by fertilizer, much like aphids).
  • Do not fertilize hemlock yard trees if you are near infested areas (adelgids thrive on the nitrogen enrichment provided by fertilizer, much like aphids).
  • Remove bird feeders in the spring and summer (April through August) to reduce the number of birds drawn to your trees. Birds can transport HWA short and long distances.
  • Prune live hemlock twigs and branches likely to come in contact with delivery vehicles and other things that may have contacted infested trees. Within 20 miles of the coast, consider pruning hemlock branches near trails and resting spots where people can inadvertently introduce the insect.
  • If you plan on harvesting, harvest hemlock during August through February when eggs and crawlers are not present. For more information on management planning for hemlock woolly adelgid, see Silvicultural Options For Managing Hemlock Forests Threatened by Hemlock Woolly Adelgid (Orwig and Kittredge, off-site).
  • If you think you have found hemlock woolly adelgid,
  • Volunteer to survey for hemlock woolly adelgid in your community. If you are interested in joining our volunteer project Take A Stand For Hemlocks, you can find more information about it here.

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