Signs of Hemlock Woolly Adelgid: What to Look For on Your Property
Last updated: 2026-04-27
Hemlock woolly adelgid is one of the few forest pests you can identify yourself, without laboratory testing, on your own property. Here is exactly what to look for.
Found HWA on your property or want a professional assessment? Call Henry at (845) 754-8242.
How to Check Your Hemlock for HWA
Take a hand lens (10x magnification is ideal) and walk into your hemlock stand. Pull a branch down to eye level and examine the underside of the twigs — specifically the junction where each needle attaches to the twig.
In an uninfested tree, this area is clean. In an HWA-infested tree, you will see small white woolly masses — one per needle insertion point in heavy infestations, scattered in lighter ones.
No laboratory test is needed. The white woolly masses are the organism itself (the egg sacs produced by settled female adelgids). Finding them confirms HWA presence.
The White Woolly Masses
The defining sign of HWA is the white woolly egg sac attached to hemlock needles at the base where they connect to the twig, on the underside of branches.
They are approximately 1/16 inch in diameter — small, but visible to the naked eye in moderate to heavy infestations. In light infestations they may require a hand lens.
Two generations are present per year:
Sistens generation — overwinters from late fall through spring. This is the generation to look for from October through April. The sistens egg sacs are smaller but persistent and clearly visible against the dark green needles throughout winter.
Progrediens generation — produced in spring, completes development by early summer. These are larger and fluffy-looking but present for a shorter window.
Needle Discoloration and Premature Drop
Needle discoloration — HWA-infested needles shift from the normal deep green of healthy hemlock to a grayish-green or yellowish-green. This is a stress response to the repeated feeding on needle bases.
Premature needle drop — HWA feeding disrupts nutrient uptake to individual needles. Affected needles drop prematurely, creating a thinning appearance in the canopy that begins in the lower and interior crown and progresses outward and upward.
Canopy Thinning and Branch Dieback
Canopy thinning — in moderate to advanced infestation, the overall canopy becomes noticeably thinner and more open. Branches in heavily infested sections may show bare twigs where needles have dropped entirely.
Twig dieback — in advanced infestation, individual branch tips die back as the needle drop leaves entire twigs without photosynthetic capacity.
Where to Look on Your Property
HWA tends to establish first in lower-elevation hemlock, south-facing slopes, and stands that are moisture-stressed during summer. Riparian hemlocks — growing along streams and in wet drainages — often show HWA infestation before hilltop or ridge hemlock on the same property.
The best confirmation strategy is to check multiple hemlock trees in different parts of your property, focusing on south-facing exposures and lower-elevation stands first.
Henry Kowalec checks for HWA on every property he walks with eastern hemlock present — including during assessments requested for other purposes such as timber appraisals, 480-a management plan work, or general woodlot assessments.
Call (845) 754-8242 if you find HWA on your property or want a professional assessment of your hemlock stands.
→ Related: Hemlock Woolly Adelgid Treatment: What Works → Service: Hemlock Woolly Adelgid Assessment — EFP → Service: Woodlot Management
Henry Kowalec — Certified Consulting Forester — Environmental Forest Products, Westbrookville, NY
Frequently Asked Questions
What do hemlock woolly adelgid egg sacs look like?
HWA egg sacs appear as tiny white woolly masses attached to hemlock twigs at the base of individual needles on the undersides of branches. They are roughly 1/16 inch in diameter — small but visible to the naked eye in moderate to heavy infestations, and clearly visible with a 10x hand lens. Two generations of egg sacs are produced per year: a smaller sistens generation that overwinters and is visible from fall through spring, and a larger progrediens generation visible in spring. The sistens egg sacs are the easiest to find because they persist through winter when the tree has no competing visual distractions.
How is hemlock woolly adelgid different from other hemlock insects?
The white woolly appearance of HWA egg sacs is distinctive and not easily confused with other insects. Elongate hemlock scale also infests hemlock but produces flat, brown, oyster-shell shaped coverings on needles rather than white woolly masses. Hemlock woolly adelgid produces a characteristic white mass specifically at the base of needles on twig undersides — this location and appearance combination is the primary field ID characteristic.
How fast does hemlock woolly adelgid spread on a property?
HWA spreads within a property primarily by wind, birds, and deer movement. A single infested stand can spread to adjacent hemlock within 1 to 3 years under normal conditions. Because HWA populations grow exponentially — two generations per year, each female producing up to 300 eggs — infestations that appear manageable in one season can become severe the following year. Early detection allows earlier treatment intervention, which produces better outcomes.
Can I see hemlock woolly adelgid in winter?
Yes — the sistens generation of HWA overwinters as settled nymphs with their distinctive white woolly covering intact. Winter is actually one of the best times to look for HWA because the white masses stand out against the dark green hemlock needles and the absence of leaf litter and competing vegetation makes it easier to work under the trees. A walk through your hemlock stands in December through March with a hand lens takes 15 to 20 minutes and can identify whether HWA is present.