Hemlock Woolly Adelgid

How to identify it

• The most obvious sign is white, cottony tufts of wax lined up like tiny cotton balls at the base of the needles on the twigs; NY DEC says to check the underside of branches.^[1][3][7]
• The wax stays on the twigs through the season and into the next year, even after the insects inside have died, and is easiest to see in spring and fall when egg masses are present.^[1][8]
• The wool is attached to the twig, is waxy rather than silky or stretchy, and looks wispy like a cotton ball rather than painted on like pine sap.^[5]
• Size figures vary by stage: overwintering females are black, oval and about 2 mm long (Penn State); NY DEC gives about 1.5 mm; UMass describes immatures as about a millimeter, while Penn State puts the settled crawler at about 0.3 mm.^[1][7][3]

Where it lives

Native to Asia; found in the western U.S. in the 1920s and first seen in the East near Richmond, Virginia, in 1951. It now spreads from Georgia to Maine, and Penn State's pest record list also includes Oregon and California, where it hasn't caused severe damage. GBIF's 1990-2026 U.S. records (about 6,800, from 22 states on the map) come mostly from Massachusetts, New York and Pennsylvania.^{[7][5][1][11]}

US map: public records from 22 states since 1990 (GBIF). View the map

Biology and life cycle

Unlike most insects, it is dormant through much of summer and active in winter: immatures settle by mid-July, resume feeding in mid-October and become adults by about early March.^[3][1]

Females lay clusters of brownish-orange eggs in the woolly masses from late winter into June, and on eastern hemlock there are two generations a year.^[1][5]

All are females and nearly all are wingless; a few winged ones fly off in search of spruce, but no spruce in this country supports them.^[3][5]

It moves between trees on wind, birds and mammals, and over long distances on nursery stock shipped from infested areas. Vermont suggests taking down bird feeders from early April to August to slow its spread.^[3][5]

Damage and signs

• It feeds by inserting its mouthparts at the base of the needles and drawing out sap, which reduces tree health.^[1][3]
• Heavy infestations cause yellowing and gray-tinted needles, needle drop, reduced twig growth, branch dieback and tree death; drought, elongate hemlock scale and hemlock borer can speed decline.^[1][7][8]
• Sources differ on how fast trees die: UMass says stressed hemlocks may die in 3 to 5 years while healthy ones on good sites may last 7 to 10 years before visible decline; Vermont says trees may die in four to six years.^[3][5]
• Hemlocks shade streams and keep water cool for species such as trout. A research review finds the adelgid has reset forest succession, homogenized biological diversity, altered water dynamics and turned hemlock stands from carbon sinks into carbon sources.^[7][9]

Health risk

It is a plant-sap feeder whose harm is to hemlock trees, through decline and death; none of these sources reports any risk to people or pets.^[3][1]

Control

Prevention

Check several twigs on all sides of each hemlock a few times a year. Early infestations are easier and cheaper to manage, and trees without adelgids need no treatment.^[2][3]

Water hemlocks deeply (about 1 inch a week) in hot, dry spells, and don't fertilize infested trees with nitrogen, which helps the adelgids thrive.^[3]

Biological control

Predatory insects are released as biological control, mainly the beetles Laricobius nigrinus and L. osakensis and silver flies. A 2023 survey found L. nigrinus established in Maine, New York and Pennsylvania but none recovered in Massachusetts, New Hampshire or Vermont. In New York these releases are made only on public lands.^[10][2][7]

Pesticides

Horticultural oil sprays work very well when applied thoroughly throughout the tree; homeowners can spray small hemlocks and hedges, while large trees need an arborist with the right equipment.^[3]

Sources differ on who may apply systemic insecticides. Cornell's NYS Hemlock Initiative says an imidacloprid soil drench is the one treatment landowners can use without a pesticide applicator license; NY DEC says imidacloprid and dinotefuran must both be applied by a licensed applicator. Imidacloprid acts slowly (up to a year to reach the canopy) but protects for up to seven years. Check your state's rules and the label.^[2][7][3]

Timing differs by source: Cornell says systemic treatments work best in spring or in fall while soils are moist; Penn State says late September through October is the best time to target overwintering females. Heavily damaged trees may not move a systemic insecticide well.^[2][1][3]

When to call a professional

Dinotefuran bark sprays reach the canopy within two to three weeks but last about a year, and only certified applicators may use them; they suit badly declining trees.^[2][7]

Quarantines: don't move hemlock

Some states regulate moving hemlock to slow the adelgid. Vermont's quarantine restricts hemlock nursery stock and seedlings (run by the Agency of Agriculture, Food and Markets) and hemlock logs, pulpwood, bark and chips from infested counties (run by the Department of Forests, Parks and Recreation), including hemlock coming in from other states.^[6][5]

Vermont notes that Maine and Canada also have hemlock woolly adelgid quarantines and that rules vary from state to state, so check before moving hemlock across state lines.^[6]

New York DEC asks people to leave infested material where it was found and clean equipment and gear used near an infestation.^[7]

Related profiles

• Aphid Flies (Silver Flies)
• Hemlock sawfly
• Hemlock scale
• Should I treat my hemlock for woolly adelgid?
• Bugs in Christmas trees: what they are and what to do
• Elongate Hemlock Scale

Common questions

What are the white cottony spots on my hemlock?

White waxy tufts at the base of hemlock needles are the classic sign of hemlock woolly adelgid; most people notice them in late winter and spring when the eggs are laid.^[1][3]

Can I treat my own hemlock?

Often, yes: UMass says homeowners can spray small trees with horticultural oil, and Cornell says landowners can apply an imidacloprid soil drench without a license, though NY DEC says it needs a licensed applicator. Check your state's rules, follow the label, and hire an arborist for large trees.^[3][2][7]

Is my infested hemlock doomed?

Not necessarily. It takes several years for the adelgid to kill a tree, and many trees with thinned canopies recover after treatment.^[2][3]

Can I move hemlock firewood or nursery plants?

Check first. Vermont, Maine and Canada have quarantines on moving hemlock, and Vermont's covers nursery stock and wood from infested counties.^[6]

Sources

1. Penn State Extension, “Hemlock Woolly Adelgid (updated June 24, 2026).” extension.psu.edu/hemlock-woolly-adelgid (accessed 2026-11-11)
2. New York State Hemlock Initiative, Cornell University, “HWA Management for Landowners.” blogs.cornell.edu/nyshemlockinitiative/hwa-management/hwa-management-for-landowners/ (accessed 2026-11-11)
3. UMass Extension, “Hemlock Woolly Adelgid Frequently Asked Questions (last updated April 14, 2026).” www.umass.edu/agriculture-food-environment/landscape/fact-sheets/hemlock-woolly-adelgid-frequently-asked-questions (accessed 2026-11-11)
4. iNaturalist, “Adelges tsugae taxon page (order and family).” www.inaturalist.org/taxa/61513 (accessed 2026-11-11)
5. Vermont Invasives (Vermont Agency of Natural Resources, Agency of Agriculture, Food and Markets, and UVM Extension), “Hemlock Woolly Adelgid (Adelges tsugae).” vtinvasives.org/invasive/hemlock-woolly-adelgid (accessed 2026-11-11)
6. Vermont Department of Forests, Parks and Recreation, “Hemlock Woolly Adelgid Wood Product Considerations (Hemlock Woolly Adelgid Quarantine).” fpr.vermont.gov/hemlock-woolly-adelgid-wood-product-considerations (accessed 2026-11-11)
7. New York State Department of Environmental Conservation, “Hemlock Woolly Adelgid.” dec.ny.gov/nature/animals-fish-plants/insects-and-other-species/hemlock-woolly-adelgid (accessed 2026-11-11)
8. Maryland Department of Agriculture, “Hemlock Woolly Adelgid Management and Suppression.” mda.maryland.gov/plants-pests/Pages/hemlock_woolly_adelgid.aspx (accessed 2026-11-11)
9. Insects, via PubMed, “Ellison AM, Orwig DA, Fitzpatrick MC, Preisser EL (2018), The past, present, and future of the hemlock woolly adelgid (Adelges tsugae) and its ecological interactions with eastern hemlock (Tsuga canadensis) forests, Insects 9(4):172.” pubmed.ncbi.nlm.nih.gov/30477155/ (accessed 2026-11-11). Cited from the PubMed abstract.
10. Environmental Entomology, via PubMed, “Crandall RS, Chandler JL, Dietschler NJ, Andersen JC, Elkinton JS (2023), Establishment and postrelease recovery of Laricobius nigrinus and Laricobius osakensis released for biological control of Adelges tsugae in the northeastern United States, Environmental Entomology 52(5):769-778.” pubmed.ncbi.nlm.nih.gov/37494455/ (accessed 2026-11-11). Cited from the PubMed abstract.
11. Global Biodiversity Information Facility (GBIF), “Adelges tsugae (hemlock woolly adelgid): occurrence records in the U.S., 1990-2026 (GBIF occurrence search).” api.gbif.org/v1/occurrence/search?taxonKey=2049739&country=US&hasGeospatialIssue=false&occurrenceStatus=PRESENT&year=1990%2C2026&facet=stateProvince&facetLimit=400&limit=0 (accessed 2026-11-11). State counts are records reported (iNaturalist research-grade, museum specimens, surveys), not abundance.