Guide

Look closerInsects

Hemlock woolly adelgid: the white wool on hemlock twigs

November to May

The tiny white cottony tufts on the underside of eastern hemlock twigs are hemlock woolly adelgid, an invasive insect that feeds on hemlock sap. The tufts start small in late fall and grow fuller all winter. They are well developed by March and easiest to see in spring.

Why this time of year

After a summer resting period, the overwintering generation wakes between late September and mid-October and begins feeding and making its wool. By November the new tufts are small but stand out against the dark green needles, and they keep growing through the cold months.

How to notice it

  1. Find an eastern hemlock and gently lift the tip of a low branch so you can see its underside. Not every hemlock has it. If you find no wool, try another tree.
  2. Look where each needle meets the twig for white tufts like the tip of a cotton swab, up to about 1/8 inch across when full grown. In November expect small, fresh tufts; by March they are puffy.
  3. Look but do not pick. Leave twigs and boughs on the tree, and do not bring hemlock greenery home from a grove that has the wool.
  4. Come back to the same branch every few weeks to watch the tufts grow. They are fullest in spring, when the eggs are laid inside them.

Try this with kids: Lift a low hemlock branch and count the white tufts on one twig, then let the branch go and leave every twig on the tree.

Word to know: “Adelgid”: a tiny sap-sucking insect related to aphids.

Safety

  • Leave hemlock twigs and boughs where they grow. Moving live hemlock from places that have the insect can spread it.
  • If you visit a grove with the wool in spring or early summer (March-July, when eggs and the tiny crawling young are out), brush off clothing and gear before visiting another wooded area.

Sources

  • umass.eduUMass Extension: woolly masses resemble the tip of a tiny cotton swab, apparent on the underside of hemlock branches at the base of needles; sistens generation July-March; first detected in Massachusetts in 1988.
  • umass.eduUMass Extension FAQ: immatures about a millimeter long at the needle base; moved by wind, birds and mammals including humans.
  • www1.maine.govMaine Forest Service: in mid-October feeding resumes and the woolly covering begins to develop; little wool in November, well developed by March; most conspicuous in spring; tufts up to 1/8 inch; eggs and crawlers March-July; never move live hemlock from infested areas; clean clothing after visiting infested hemlocks.
  • usanpn.orgUSA-NPN / NYS Hemlock Initiative photo guide: from late October through June HWA is easy to see with the naked eye as white woolly ovisacs at the base of the needles.
  • blogs.cornell.eduCornell NYS Hemlock Initiative: easiest to see late October through May; small cotton-ball-like woolly masses near the base of the needles.
  • portal.ct.govCT DEEP: look at the underside of needles for small clusters of white waxy wool; spread by wind, wildlife, humans and movement of infested plant material.
  • fs.usda.govMassachusetts Forest Health Highlights 2024: HWA identified in all Massachusetts counties and all communities with significant hemlock resources; populations build in mild winters.
  • bpb-us-e1.wpmucdn.comDietschler et al. 2024: estivation break synchronized between NY and VA, late September to late October; temperature had no significant effect in lab; daylength may be the cue (supports phenology fixed).

Checked against these sources on October 16, 2026.

In the guide for November, December, January, February, March, April, May

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