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 others 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 hemlock. So all hemlock-dependent ecosystems would be threatened by the hemlock’s demise.

The threat of hemlock loss is real, but it is not inevitable. USDA-approved biological control agents, primarily predator beetles, exist 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. But it requires human involvement to introduce the predator beetles. Public hemlocks have received some help from USDA, but protecting hemlocks on private lands requires your help.

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 has identified the origin point of our HWA import in 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.

The purpose of this site is to emphasize the important role for 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 and to contribute to their long-term return to health and survival. And you can do this without resorting to short-term chemical treatments involving environmentally dangerous insecticides.

To learn more about biological control of our Hemlock Woolly Adelgid , go to Bio Control

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

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. This analysis of mitochondrial  DNA from all known HWA populations identified the 5 genetically distinct HWA populations depicted below.


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. So the rest is up to you!

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 susceptible to the HWA in China, but not to the HWA we have brought into eastern US (from Japan). While western hemlocks (Tsuga heterophylla) growing in the wild are not generally attacked by the western HWA. Only those hemlocks stressed by other factors are subject to HWA infestation – so again a very different situation than we are facing here in the eastern US.

World Origins of Hemlocks

A similar 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: