What will be the fate of our Hemlocks in the Eastern US?

Healthy Hemlock


Or this?

Or this?

The imported Hemlock Woolly Adelgid (HWA) has brought widespread destruction to our native hemlocks in the eastern US. The loss of the hemlock would be an ecological disaster for our mountain coves and waterways, as well as our neighborhoods and parks. As a keystone species, on which many other species depend, hemlocks provide critical shelter and habitat for a whole range of animal species including birds, amphibians, fish and mammals. And there are no apparent successors, native or otherwise, that could fill the critical ecological niche of the Eastern hemlock. So all hemlock-dependent ecosystems would be threatened by the hemlock’s demise.

Good News for Hemlocks?

The threat of hemlock loss is real, but it is not inevitable. There are USDA-approved biological control agents, primarily predator beetles, that can reduce the threat of HWA to our native hemlocks and allow the long-term survival and recovery of our native hemlock ecosystems. This restoration and recovery process is already in evidence in both wild and residential hemlock areas … See hemlock restoration. But it requires human involvement to introduce the predator beetles! Hemlocks on public lands have received help from USDA, but protecting and restoring hemlocks on private lands requires your help.

Here is a report from NPS Forester Jesse Webster on the fate of hemlocks at Great Smoky Mountains National Park, ten years after initiating biological control with Sasajiscymnus tsugae predator beetles   

“This won’t be the hemlock equivalent of the chestnut blight,” Webster said. “The park’s hemlock forests will never be the same [because of the loss of old-growth trees], but we won’t have the ecological extinction everyone was worried about.

The HWA predator beetles approved for release in the eastern US are so highly specialized that they require adelgids in order to survive and reproduce. None of these predator beetles are native to the eastern US. But one of the approved beetles is the native predator for the HWA that threatens our eastern hemlocks. USDA-sponsored genetic research (below) has identified the origin point of our HWA import as Southern Japan, which is also the origin for the predator beetle – Sasajiscymnus tsugae (aka Sasi or St). And this predator beetle, with millions of USDA-sponsored releases on public lands, is now available to protect hemlocks on private lands as well. (Here is the commercial source for Sasi beetles.)

For an overview of USDA-approved HWA predator beetles, including Sasajiscymnus tsugae, Laricobius nigrinus and Laricobius osakensis, go to Predator Cocktail page.

The purpose of this site is to encourage private citizens’ and landowners’ involvement in saving our native hemlocks from the Hemlock Woolly Adelgid. Whether your HWA-threatened hemlocks are in a neighborhood, a local park, or a wild cove or waterway, you can do something to protect them. To learn more about community-based HWA control efforts, go to Community page.

Here is a report from the Smokies on the effectiveness of the Japan-origin predator beetle, Sasajiscymnus tsugae (Sasi, for short) in protecting hemlocks at long-term release sites:

Researchers at UT found one of the original predator beetle species — Sasajiscymnus tsugae — at eight of the 10 release sites in the Smokies and Cherokee National Forest. Some had spread 1.2 miles from the initial release site. Where the beetles were found, the hemlocks were alive.

Wherever these voracious little black ladybug predators go, their first objective is finding HWA to feed and reproduce on. So releasing them where there are HWA will benefit both you and your hemlocks!


To learn more about biological control of the Hemlock Woolly Adelgid, go to Biological Control page.

To learn how to use predator beetles to help your hemlocks, go to How You Can Help page

Where does the Hemlock Woolly Adelgid come from ?

Recent USDA-sponsored research by Nathan Havill and associates has transformed our understanding of the relationship between hemlocks and HWA around the world! Havill’s analysis of mitochondrial  DNA from all known HWA populations identified the five genetically distinct HWA populations depicted below. So now we know that there are (at least) five different HWA, not just one.


Note that 4 of the 5 HWA lineages identified here are based in Asia: one in China, one in Taiwan, and two in Japan. The HWA found in the US Pacific NW is a distinct lineage, which can be considered native (endemic) to the Pacific Northwest – not a recent introduction. However, the HWA introduced to the eastern US is not from the HWA population in the western US! Instead it is from the HWA originating in Southern Japan.  So it is to Japan that we must look for clues about controlling HWA in the eastern US!

And the good news is that the predator beetle (Sasajiscymnus tsugae) that controls HWA in southern Japan will also control that same HWA here in the eastern US. This predator is USDA-approved for US release and is readily available to private landowners during the Spring release season. Here is the commercial lab that rears predator beetles for private landowners.

Each of these HWA lineages is associated with a different hemlock population, in a different geographical location. So HWA is not a single organism, but at least 5 different organisms in 5 different ecosystems – each with a different geographical origin, each with a different hemlock host, and each with different HWA predators. For example, the Chinese hemlock (Tsuga chinensis) is fed upon by the native HWA in China, but not by the HWA that was brought into eastern US (from Japan). While western hemlocks (Tsuga heterophylla) growing in the wild are not generally attacked by the HWA found in the Pacific NW. Only those western hemlocks stressed by other (usually human) factors are subject to HWA infestation – so this is a very different ecological situation than we are facing here in the eastern US!

World Origins of Hemlocks

A similar, DNA-based research strategy was used by Havill and associates to examine world hemlock populations. This research used data from hemlock mitochondrial DNA to identify 9 genetically distinct hemlock populations around the world, as pictured on the map below:


For more information on hemlocks and HWA around the world, see world hemlocks.