What Is Hemlock Woolly Adelgid?
Hemlock woolly adelgid is an invasive pest that threatens the health of our forests by killing hemlock trees. It was likely introduced to the US on infested nursery stock. Eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tusga caroliniana) lack natural predators and resistance to HWA and therefore are the only hemlock species known to be a fatal risk to HWA infestations.
How Does HWA Damage Hemlock Trees? 
HWA damage hemlocks by inserting their straw-like piercing mouthparts into hemlock twigs which creates a wound in the woody tissue of the tree. Over time, the density of HWA individuals on the tree increases which compounds the damage caused to the tree. Eventually, the damage impedes the flow of water and nutrients to the tree’s twigs which leads to needle loss. If left untreated, hemlocks infested with HWA can die within 3-5 years of infestation.
Where are Most of New York’s Hemlocks & Where is HWA in NYS?
In New York, most hemlock trees are concentrated in the Adirondacks, the Tug Hill Plateau, and the Catskills. At this time, HWA has become well established in the Catskills, the Finger Lakes region, and parts of Central & Western NY, and the Lower Hudson region. Small populations have also been found near Lake George and Oswego County. There is a state-wide effort to search for HWA populations. Learning where HWA is present helps limit its spread as trees can be treated with an insecticide.

What Can You Do? 
  • Learn to recognize the signs of a Hemlock Woolly Adelgid Infestation & report observations using the mobile app iMapInvasives.
  • View a detailed guide to HWA seasonal signs of infestation and hemlock identification. 


  • Take hikes & check hemlock trees for signs of HWA. Watch this video below to get tips on where to look on the tree. 

Take the #VirtualHikeChallenge!

  • Participate in the #VirtualHikeChallenge- win prizes & learn of local hiking trails with hemlocks near the path! 

Hiking Tips

Hemlock Woolly Adelgid Learning Resource

The NYS Hemlock Initiative is a leading collaborator for HWA management and biocontrol research. Check out their website to learn more about HWA phenology, biology, management and state-wide control efforts.