woolly adelgid

What is the threat?

The hemlock woolly adelgid (HWA) is a tiny insect native to Japan that drains the water and nutrient stores from the Eastern Hemlock, killing the host tree within three to five years of infestation. If you see white, cottony clusters on the underside of needles on a hemlock, those are the woolly adelgids – they protect their eggs with a “wool-like wax” which resembles cotton and is most easily seen from November through April. The HWA itself is a mere 1.5 millimeters long, and can spread from tree to tree via birds, mammals, and even the wind. HWA also spread when firewood is moved, or stock from an infected nursery is brought into a previously pest-free area.

Already the HWA has caused “widespread hemlock mortality throughout the Appalachian and southern Catskill mountains,” causing “considerable ecological damage, as well as economic and aesthetic losses.”  The insect has been spotted from Maine to Georgia, and is killing trees as far north as Nova Scotia, where the colder temperatures do not seem to present a major obstacle to the insect.

Please watch the trailer for an astounding documentary by Wild Excellence Films, created pro bono by the skilled owners, out of a sense of love for the forest and its hemlocks —  Cathedral: The Fight to Save the Ancient Hemlocks of Cook Forest:

Why is it so critical to stop the hemlock woolly adelgid (HWA)?

Eastern hemlock (Tsuga canadensis) is a foundation species, upon which many other species depend for shelter and sustenance. Hemlocks along streams prevent soil erosion, filter pollutants, and keep the water clean and cool for fish and other aquatic species.

“The threat is huge.  Hemlock is an iconic, foundation species. It’s very important ecologically and aesthetically. I’ve heard the hemlock woolly adelgid described as the worst ecological disaster, after climate change.Mark Whitmore, Cornell entomologist

In contrast to the more deciduous forests dotted across the northeast, Cook Forest – known once as “the Black Forest” — is predominately comprised of stands of Eastern Hemlock and White Pine, many of them old-growth. Some individuals in the Forest Cathedral, a National Natural Landmark area, exceed 350 years in age and are among the nation’s very tallest and largest specimens.

While the loss of hemlocks is an ecological and aesthetic disaster wherever it occurs, their defoliation would be particularly catastrophic in Cook Forest – the very character of the park would change immediately and drastically. Rather than the cool, dark, mossy “black forest” which presently supports such a unique biodiversity and stands as a last bastion of the primeval woods of the 1600s, Cook Forest could become a sad, skeletal stand comprised of the husks of giants.

What can be done?

Pesticide treatment can be effective, but the practicalities of repeated applications over large swaths of remote forest land limit the efficacy of this approach — coverage is not complete, and is costly.

Individual trees can be inoculated, and this should approach should be taken for any particularly notable specimens – unfortunately, it is both time-intensive and costly. Each subject tree must be located, hiked to, and the pesticide applied – and this procedure must be repeated every seven years.

Biological controls could present a possibly viable and self-perpetuating alternative to chemicals – of course, just as pesticides present long-term dangers, there could be unforeseen consequences following the introduction of a “predator pest.” In New York, Cornell University scientists are raising for release a beetle from the Pacific Northwest which feeds on eggs of the HWA.  If viable, this may be an excellent option, but the beetles are not widely available in sufficient numbers.

Whichever combination of approaches is pursued, a quick and decisive response is of paramount importance. Not only does HWA infestation progressively weaken the tree, which will eventually reach a point from which regeneration is impossible, but the infestation also renders the tree susceptible to other pests and stressors, including droughts.

“[T]here are many factors that lead us to believe that early efforts can contain the infestation and prevent the major loss of forest, but immediate action must occur if we are to remain optimistic.” — Melanie Manion, representative for the parks department in Ottawa County, Michigan.

The DCNR is already treating parts of Cook Forest, which has been identified as a high priority site due to the density and quality of its hemlocks – there will be an ongoing need for funds for treatment and monitoring in the foreseeable future.  The CFC supports state efforts, and coordinates outreach programs for the individual private landowners in the area – because bugs don’t respect borders. 


The hemlock woolly adelgid is not the only foe the primeval forest faces, but presently it is among the most urgent, and the most dire.

How can you get involved? 

Join the CFC in supporting the High Allegheny Hemlock Conservation Partnership (HAHCP), a project of the Allegheny Forest Health Collaborative, by monitoring sites near your home and reporting both HWA positive and negative areas. The introductory video below by the Roger Tory Peterson Institute (RTPI), which coordinates the Hemlock Woolly Adelgid Volunteer Monitoring Program, explains the program further.   The CFC & RTPI will be hosting citizen scientist training on field work and the use of the imapinvasives program as frequently as possible – please subscribe to the News feed on our website, or follow the CFC on Facebook, for updates on program dates and locations.  (So many acronyms!)