Hemlock Woolly Adelgid


The Hemlock Woolly Adelgid (HWA) (Adelges tsugae), an aphid-like insect, was first described in western North America in 1924 and first reported in the eastern United States in 1951 near Richmond, VA.

Researchers have determined that several distinct populations of HWA occur in Asia and western North America and we now know that HWA populations found in the East originated from the Pacific Northwest, not southern Japan, as previously suggested. In their native range, populations of HWA cause little damage to the hemlock trees they feed on due to natural predators that have evolved with HWA.

HWA is not native to eastern North America, and in the absence of a natural predator, HWA, an introduced insect pest, attacks both Eastern Hemlock (Tsuga canadensis) and Carolina Hemlock (Tsuga caroliniana) which are often damaged and killed within a few years of becoming infested.  HWA is now established from northeastern Georgia to southeastern Maine and as far west as southeastern Ohio and Tennessee.


HWA is tiny, less than 1/16-inch (1.5-mm) long, and varies from dark reddish-brown to purplish-black in color. As it matures, it produces a covering of wool-like wax filaments to protect itself and its eggs from natural enemies and prevent them from drying out. This “wool”  (ovisac) is most conspicuous when the adelgid is mature and laying eggs. Ovisacs can be readily observed from late fall to early summer on the underside of the outermost branch tips of hemlock trees (see photo above).  HWA can spread 20 Km per year.

Life History

HWA is parthenogenetic (all individuals are female with asexual reproduction) and has six stages of development: the egg, four nymphal instars, and the adult. The adelgid completes two generations a year on hemlock. The winter generation, the sistens, develops from early summer to mid-spring of the following year (June–March). The spring generation, the progrediens, develops from spring to early summer (March–June). The generations overlap in mid to late spring.

HWA is unusual in that it enters a period of dormancy during the hot summer months. The nymphs during this time period have a tiny halo of woolly wax surrounding their bodies

FIGURE 2.—Hemlock woolly adelgid nymphs in dormancy.

Hemlock woolly adelgid nymphs in dormancy.

The adelgids begin to feed once cooler temperatures prevail, usually in October, and continue throughout the winter months.  HWA is a xylem feeder (most aphids feed on phloem) which makes it particularly detrimental to hemlocks in our region.  Hemlock in the East have evolved to deter chewers (such as scale).  Not only that, but during the time that hemlock are producing terpenoids (a sucker deterrent), HWA isn’t feeding.

In eastern forests, the ovisacs of the winter generation contain fewer eggs (dependent on temperature and elevation) than initially suspected (initially, it was thought that HWA in eastern forests laid up to 300 eggs per ovisac, like their Asiatic cousins). When hatched, the first instar nymphs, called crawlers, search for suitable feeding sites on the twigs at the base of hemlock needles. Once settled, the nymphs begin feeding on the young twig tissue and remain at that location throughout the remainder of their development. Unlike closely related insects that feed on nutrients in sap, the hemlock woolly adelgid feeds on stored starches. These starch reserves are critical to the tree’s growth and long-term survival.

Dispersal and movement of HWA occur primarily during the first instar crawler stage as a result of wind and by birds, deer, and other forest-dwelling mammals that come in contact with the sticky ovisacs and crawlers. Isolated infestations and long-distance movement of HWA, though, most often occur as the result of people transporting infested nursery stock.


Chemical and biological controls can reduce the HWA’s rate of spread and protect individual trees.

Chemical control options, such as foliar sprays using horticultural oils and insecticidal soaps, are effective when trees can be saturated to ensure that the insecticide comes in contact with the adelgid. Several systemic insecticides have also proven effective on large trees when applied to the soil around the base of the tree or injected directly into the stem. Chemical control is limited to individual tree treatments in readily accessible, nonenvironmentally sensitive areas; it is not feasible in forests, particularly when large numbers of trees are infested. Chemical treatments offer a short-term solution, and applications need to be repeated in subsequent years.

The best option for managing HWA in forests is biological control, using natural enemies (predators) from the adelgid’s native environment. Since the discovery of HWA along the East Coast, research into predator beetles by government, university, and private entities has continued, with variable results. 

Researchers began visiting areas abroad, where other species of Hemlock grow (there are nine species of Hemlock in the world; four in the US – two in the Pacific Northwest (PNW) and two along the East Coast) but do not succumb to thier adelgid pest.  Researchers were interested in determining what kept hemlocks in these other regions viable.  Three predator beetles were found to prey on HWA; in Japan, Sasajiscymnus tsugae (“Sassy”), in China, Scymnus sinuanodulus, and in the Pacific Northwest, Laricobius nigrinus (“Lari”). 

FIGURE 4.—Predators introduced for control in the Eastern United States, left to right (origin): Sasajiscymnus tsugae (Japan), Scymnus sinuanodulus (China), and Laricobius nigrinus (Western North America).

Predators introduced for control in the Eastern United States, left to right (origin): Sasajiscymnus tsugae (“Sassy”) (Japan), Scymnus sinuanodulus (China), and Laricobius nigrinus (“Lari”) (Northwestern North America).

Researchers tested these beetles in the lab, offering HWA and similar native species as prey to see if the beetles would choose HWA over other, native species found in our eastern forests.  The beetles were also tested for hardiness and ease of reproduction in the lab which could result in mass production and release of the beetles in the field as a form of biocontrol of HWA.  At the same time, many people, wanting to do whatever possible to save their hemlocks, were using chemicals, which were more accessible and more affordable to control HWA.

For large-scale release as potential biological control of HWA at the time, the “Sassy” beetle showed the most promise and millions of these Asiatic beetles were released in eastern forests where high concentrations of hemlock occurred.  Unfortunately, the results were not as hoped for; the Hemlock are still suffering, if not dead, and very few of these “Sassy” beetles have been relocated at release sites; release sites that still offer plenty of food (HWA).

Dr. Richard C. McDonald, an entomologist with Symbiont Biological Pest Management, has been studying HWA since it became a major issue in North Carolina, and he’s made some groundbreaking discoveries.  For years, Dr. Richard C. McDonald, “Dr. McBug,” has been visiting the PNW to study the interaction of HWA and the “Lari” beetle, and he has found that the “Lari” beetle is the reason that HWA does not kill Hemlocks in the PNW; the “Lari” beetle keeps HWA in check.  Too, he discovered that the HWA found in the PNW is native to North America, just like its predator, the “Lari” beetle!

Of course, Dr. McDonald couldn’t wait to test the “Lari” beetle in our eastern forests, and that he did!  With an initial, small release in Banner Elk, NC, Dr. McDonald watched and waited, only to discover that these “Lari” beetles, native to North America, were demolishing the HWA on the test trees.  Not only that, but he was able to recover adult beetles from year to year, and after a few years of feeding on the buffet of HWA on test trees, he was finding larvae and beetles on surrounding Hemlock, away from the test trees!  Too, the hemlock were recovering from their weakened state and showing vigor! More releases were made in the area and the hemlock in Banner Elk and beyond are living…and thriving!

HWA is not eradicated from these areas, and it probably never will be, but now, there is a predator in place that creates a natural balance.  The “Lari” beetle and its larvae prey on HWA, decreasing the number of HWA to the point that dieback ceases and the Hemlock produce new growth.  The optimum level of infestation for hemlock growth is 45%; “Lari” often reduced the infestation to 30% or less.

The “Lari” beetle is a winter feeder of HWA, relying on HWA for all stages of development; they feed on HWA from October through May. Adult “Lari” beetles eat 6-8 HWA per day during this time.  Also during this time, a female “Lari” beetle lays 200-400 eggs, and each “Lari” larvae needs to eat approximately 235 HWA eggs to reach maturity.  Research continues with the discovery of a summer predator of HWA, as well.

HWA and Lari shown (2)The adult “Lari” beetle (Laricobius nigrinus) feeding on HWA.

“Lari” beetles can feed on any adelgid, but it can only reproduce on HWA and Pine Bark Adelgid (native).  “Lari” will also feed on Balsam Woolly Adelgid and Coolley Spruce Adelgid (non-native).

The goal of the local NC collection/release is to redistribute “Lari” beetles into every western North Carolina watershed with HWA-infested Hemlocks.  Releases, since 2006, and natural dispersion have established the “Lari” beetle in a 35-mile area  (and growing)…spreading 2+ miles on all fronts, each year.

PAC’s participation in local collection/release efforts to introduce “Lari” beetles in this portion of western NC will aid in creating a manageable time frame for the spread and establishment of “Lari” and the preservation of our area Hemlocks!

PAC’s Pam Torlina received training to monitor the predation of HWA by “Lari” beetles, how to collec “Lari” beetles from areas where they are actively feeding, and also how to release “Lari” beetles into other HWA infested areas.

PAC will be receiving a “starter kit” of “Lari” beetles to release at a preselected site in Polk County.  This initial group of beetles will be the start of a “Lari beetle farm” armed to prey on HWA in our area.  Over time, PAC will be able to collect “Lari” beetles and release them in other hemlock stands in the area to continue preserving our native forest health.

Even with all of this progress in the fight to save our hemlocks, there is much more research that needs to be conducted.  If people are interested in donating in support of further research development in this field, please contact Dr. Richard C. McDonald at drmcbug@skybest.com, and if people are interested in contributing to PAC to secure more “Lari” beetles for release in Polk County, to save our hemlocks, please contact PAC at 828-859-5060 or email info@pacolet.org.

On September 29, 2015, PAC received 53 Laricobius nigrinus beetles (“Lari”), predator beetles native to North America and proven to feast on the non-native and invasive Hemlock Woolly Adelgid (HWA) that is killing Eastern and Carolina Hemlock along the east coast.

These beetles have been released on a PAC protected property in hopes that they will not only feed on the HWA at the site, but also spread to other hemlock in the watershed as well as become a source to gather beetles in the future which can be moved to other locations in the county.

There is hope for the hemlock and now we’ve put beetles to needles, right here, in Polk County!

P1110967The shipment of Lari beetles hungry for Hemlock Woolly Adelgid (HWA)!

Lari release3PAC’s Director of Stewardship & Land Protection, Pam Torlina releasing Lari beetles on a PAC protected property.

beetles save needles2

Click here to read “9 Main Points about HWA and its Natural Enemies

Click here to learn more about the “Lari” beetle


Another interesting discover from Dr. McDonald is that HWA and predator activity can be detected using Ultraviolet-A light!

UV-A_2Eastern Hemlock (Tsuga canadensis) infested by HWA and “Lari” beetle (Laricobius nigrinus) activity as seen under UV-A light.

The yellow-green color is adult HWA blood

The bright yellow color is damaged HWA eggs

The blueish color is HWA “honeydew” (pee)

The orange color is “Lari” beetle frass (poop)

Click here to watch a video of Dr. McDonald speaking about “Laricobius nigrinus saving Hemlocks from Hemlock Woolly Adelgid.”

Dr. McBug

(more to come!)