Hemlock Woolly Adegid – A 2019 update

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In mid-October 2019, Jim Chatfield, Amy Stone, and Thomas deHaas attended the Central Appalachian Spruce Restoration Initiative (CASRI) to discuss conifer health, specifically, Canadian Hemlock (Tsuga Canadensis) and hemlock woolly adelgid (Adelges tsugae) (HWA) at Blackwater Falls, in Davis, West Virginia.


Blackwater Falls


HWA was first discovered in West Virginia in 1992. State suppression began in 2004. CASRI is a diverse partnership of private, state and federal organizations who share a common goal of restoring historic red spruce - northern hardwood ecosystems across the Central Appalachians. Speakers included: John Perez, Biologist - New River Gorge and Gauley, Amy Hill – USDA Forest Service, Hessel Lab –University of West Virginia, and Ben Smith, Forest Restoration Alliance, North Carolina State University

HWA is devastating and can cause death to trees in the course of several years. HWA is being treated aggressively in the New River Gorge, Bluestone and Gauley River National Recreation Area as well as other parts of West Virginia.



HWA Kentucky

4588707 Ignazio Graziosi, University of Kentucky, Bugwood.org (Damage in Kentucky)


Native to Asia, HWA is an invasive, aphid-like insect that attacks North American hemlocks. HWA are very small (1.5 mm) and often hard to see, but they can be easily identified by the white woolly masses they form on the underside of branches at the base of the needles.


hwa 2

0718057 John A. Weidhass, Virginia Polytechnic Institute and State University, Bugwood.org 


Juvenile HWA, known as crawlers, search for suitable sites on the host tree, usually at the base of the needles.

They insert their long mouthparts and begin feeding on the tree's stored starches. HWA remain in the same spot for the rest of their lives, continually feeding and developing into adults. Their feeding severely damages the canopy of the host tree by disrupting the flow of nutrients to its twigs and needles. Tree health declines and mortality usually occurs within 4 to 10 years.


Signs of infestation:

  • White woolly masses (ovisacs) about one-quarter the size of a cotton swab on the underside of branches at the base of needles
  • Needle loss and branch dieback
  • Gray-tinted needles


Mortality of HWA begins at 13 degrees below zero F with total mortality at 25 below zero F. Several days of these cold temperatures causes even more mortality of the HWA.

Chemical controls are shown to be highly effective to control HWA. Imidacloprid in the form of Merit, Core-tech, and stem injection as well as Dinotefurin in the form of Safari.


hwa stem injection

1344148 Great Smoky Mountains National Park Resource Management , USDI National Park Service, Bugwood.org


The chemical applications are effective and trees respond with healthy growth the following year after treatment. Treatments can last up to 7 years.

Laricobius nigrinus is a beneficial insect native to the Pacific Northwest. There is hope that these beneficials will help to control the spread of HWA. The main focus right now for biological control are two beetles in the genus Laricobius (L. nigrinus and L. osakensis), and research into a potential additional biocontrol; a species of silver fly in the genus Leucopis.

 HWA was found on the border of Lake and Geauga County, Ohio on Little Mountain in Holden Arboretum. Those trees were treated and the adelgid populations have declined. The nurseries treat for HWA prior to shipping nursery stock.

If you believe you have found HWA in your Hemlock trees, contact your local OSU Extension Office or Ohio Department of Agriculture. Reports can also be made using the Great Lakes Early Detection Network App. HWA, as well as other pests can be reported using the App and confirmed reports populate an EDD maps that can be viewed to monitor the spread of these species.


As a side note, Hemlocks can become infested with Hemlock Elongate Scale, . Fiorinia externa Ferris.



hemlock elongate scale


These also respond to chemical treatments.

The key to your hemlock health is active scouting, several times a year and timely treatment.

Ohio Department of Natural Resources-Division of Forestry will present a workshop “Protecting Hemlock Trees” on November 14th,2019 at Holden Arboretum: