Hemlock woolly adelgid (Adelges tsugae)

Common Names: hemlock woolly adelgid, HWA, woolly adelgid, eastern hemlock woolly adelgid

Description:

Hemlock woolly adelgid (Adelges tsugae) is a tiny, aphid-like insect from Japan and China that feeds on the sap of hemlock trees [1]. Since it was first found in the eastern United States in 1951, it has killed hundreds of thousands of hemlocks and spread across much of the East [1]. MISIN tracks it because it threatens the more than 170 million hemlock trees in Michigan's forests, yards, and parks [1].

Identification:

The insect itself is hard to spot. It is less than 1/16 inch long (about 1.5 mm) and dark reddish-brown to purplish-black [1]. What catches your eye first is the white, woolly wax it makes to cover itself and its eggs. These cottony masses cling to the base of the needles on the underside of the twigs, right where each needle joins the branch. Each one is round and about the size of the tip of a cotton swab [1].

The white wool is easiest to find from late fall through early spring, when the insects are active and the masses are at their largest. The best way to check a tree is to turn over a low branch and look along the underside of the twigs [1].

At-a-glance cues:

1. White, woolly masses about the size of a cotton swab tip, lined up at the base of the needles on the underside of twigs [1]
2. Masses sit right where each needle joins the branch ? not out on the needles themselves [1]
3. Found only on hemlock trees, never on pines, spruces, or firs [1]
4. Easiest to see from late fall through early spring [1]

Hosts:

In North America, hemlock woolly adelgid lives only on hemlock trees ? it cannot survive on pines, spruces, or firs [1]. Eastern hemlock and the rarer Carolina hemlock are the most at risk. Western hemlock and the Asian hemlocks the insect evolved alongside are far more resistant and rarely suffer serious harm [1].

The insect is native to Japan and China, where natural predators keep its numbers in check [1]. In the eastern United States it has no such controls and has spread across much of the region. It settles on the twigs at the base of the needles and, once it begins feeding, never moves again [1]. Hemlocks grow both in home landscapes and in forests, where they often line streams and rivers and cast deep, year-round shade [1].

Life Cycle:

Hemlock woolly adelgid spreads easily, and it is almost always carried to a new tree rather than getting there on its own [1]. The young insects, called crawlers, are blown by the wind and hitch rides on birds, deer, and other animals that brush against an infested branch [1]. People move it the longest distances ? on infested nursery hemlocks, on firewood and logs, and on clothing or equipment that has touched a woolly tree [1].

Every adelgid is female, and the insect does not need a mate to reproduce [1]. It completes two generations each year, so even a few survivors can rebuild a population quickly. One generation feeds and lays eggs in spring; the next hatches in summer, rests through the warm months, then feeds and grows right through the winter [1]. That winter activity is one reason cold weather helps hold it back ? studies in the Northeast found that more than 90 percent of the insects can die when temperatures fall below ?22 ?F (?30 ?C) [1].

Impact and Damage:

For a homeowner, the main harm is to the hemlocks themselves. As the insects feed, needles dry out and drop, new buds fail to form, and the tree fades to a grayish-green [1]. Infested trees can die in as little as four years, though some hold on for ten years or more [1]. Losing a mature hemlock means losing its shade and privacy screen, and a dead tree near a house or driveway can become a hazard. The insect does not bite people or pets and poses no health risk to them [1].

In natural areas the loss runs deeper. Hemlock is a "foundation" tree ? its dense, evergreen shade shapes the whole forest around it, cooling streams and giving birds and mammals winter cover [3]. When hemlocks die off, soil temperatures, nutrient cycling, and decomposition all change, and those shifts ripple out to other plants and animals [3]. Because hemlocks often grow along streams and have shallow roots, widespread die-off in eastern forests has increased erosion, warmed stream water, and harmed the aquatic insects and other life living in those waters [1].

The costs add up as well: homeowners pay to treat or remove valued trees, and communities lose the forested parks and shaded recreation areas that hemlocks provide [1].

Similar species:

Several insects make white, fuzzy, or waxy patches that people mistake for hemlock woolly adelgid. The two quickest checks are the kind of tree you are looking at and exactly where on the branch the white material sits.

Beech blight aphid. This fuzzy white aphid feeds on the sap of American beech, not hemlock [4]. Big colonies are famous for raising their rear ends and swaying together when disturbed ? the "boogie-woogie aphid" dance ? something hemlock woolly adelgid never does. The host tree alone settles it: if it is a beech, this is not your pest.

Balsam woolly adelgid. A close relative that also wraps itself in white, woolly wax, but it lives on true firs (Abies species) rather than hemlock [4]. On firs it tends to cluster on the trunk and on swollen spots along the branches rather than lining the base of the needles. Once again, the host tree is the fastest way to tell which adelgid you have found.

Elongate hemlock scale. Another invasive pest of hemlock that often shares the very same trees, which makes it especially confusing [1]. Its waxy covers are small, flat, and yellow-brown to off-white, and they lie flattened along the needles themselves instead of forming the round, woolly tufts that an adelgid makes at the needle base [1].

Pine bark adelgid. Makes similar white, woolly wax, but it is found on the trunk and branches of eastern white pine, not on hemlock [4]. The host tree tells them apart at a glance. It is also worth knowing that non-insect things fool people too ? spider egg sacs and the frothy spittle of spittlebugs are both mistaken for this pest, but neither sits tightly at the base of the needles the way the adelgid's wool does [4].

Monitoring and rapid response:

Signs to watch for:

1. White, woolly masses at the base of the needles on the underside of twigs ? easiest to see from late fall through early spring [1]
2. Needles turning grayish-green, drying out, and dropping [1]
3. A thinning crown with few or no new buds at the branch tips [1]

What you can do:

1. Check the hemlocks on your property a few times a year, especially in late winter and early spring when the white wool stands out most [1]
2. On a small tree with only a few infested twigs, prune those branches, seal them in a bag, and put them in the trash [1]
3. Clean your tools, gloves, and clothing after working around an infested tree so you don't carry crawlers to healthy ones [1]
4. Do not move hemlock branches, logs, or firewood off your property ? this is how the insect travels the farthest [1]

Natural area crews can survey hemlock stands across a whole preserve, map the infested trees, and focus first on small, outlying patches to slow the spread into healthy stands [1]. In some regions, land managers also support the release of tiny predator beetles from Asia and the Pacific Northwest that feed on the adelgid ? a long-term biological control approach led by federal and university programs [2].

When to call a professional:

1. A large or mature hemlock you want to save ? protecting big trees takes professional equipment and timing [1]
2. Heavy or widespread infestations, or many affected trees across a property or woodlot [1]
3. Trees already in steep decline, where any treatment has to be matched to the tree's remaining health [1]
4. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. McCullough DG (2015). Hemlock Woolly Adelgid: A little insect that means big trouble for hemlock trees in Michigan. Michigan State University Extension Bulletin E-3300. canr.msu.edu
2. USDA Forest Service, Northern Research Station. Biological Control of Hemlock Woolly Adelgid. research.fs.usda.gov
3. Ellison AM, Bank MS, Clinton BD, et al. (2005). "Loss of foundation species: consequences for the structure and dynamics of forested ecosystems." Frontiers in Ecology and the Environment 3(9):479?486. research.fs.usda.gov/treesearch/22218
4. Michigan Department of Natural Resources / EGLE. Hemlock Woolly Adelgid Look-Alikes. michigan.gov/invasives

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