Save and Restore Hemlocks – a natural, science-based strategy

What will be the Fate of Hemlocks in Eastern North America?

Healthy Hemlock


Or this?

Or this?

The imported Hemlock Woolly Adelgid (HWA) has brought widespread destruction to native hemlocks in the eastern US. The loss of our hemlocks would be an ecological disaster for our forest coves and waterways, as well as for 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 Hemlock Recovery!

The threat of hemlock loss is real, but it is not inevitable. There is a USDA-approved predator beetle, a native predator to our HWA “import” from Southern Japan, that can control Hemlock Woolly Adelgid on our native hemlocks and support the long-term survival and recovery of our native hemlock ecosystems in the Eastern US.

This natural restoration and recovery process is already underway in both wild and residential hemlock areas, where these HWA predator beetles have been released. See hemlock restoration. But human action is required to introduce these predator beetles to control HWA! Hemlocks on public lands have received help from USDA, but protecting and restoring hemlocks on private lands requires your help.

It is widely recognized that biological control of hemlock woolly adelgid with HWA predator beetles is the key to restoration and recovery of hemlocks in the eastern US. Once established, an HWA predator will protect all surviving hemlocks in an area and will allow HWA-damaged hemlocks to restore their foliage and resume normal growth. And that hemlock restoration process is currently underway in eastern US hemlock areas where Sasjiscymnus tsugae (Sasi) predator beetles have been released.

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.

                         Here is what predator-beetle-assisted Hemlock Restoration looks like  

 (Note old foliage damage on lower branches and new foliage growth on upper branches)                                                                                                                 


The Sasjiscymnus tsugae predator beetles approved for release in the eastern US are so highly specialized that they require adelgids to survive and reproduce. So there is not a threat of these beetles affecting other, non-adelgid insect species. USDA-sponsored genetic research (below) has identified the origin point of our Hemlock Woolly Adelgid import as Southern Japan, which is also the origin for the predator beetle – Sasajiscymnus tsugae  (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.

Sasi Predator Beetles Feeding at GSMNP

       Sasajiscymnus tsugae Predator Beetles Feeding at GSMNP

Wherever these voracious little black ladybug predators go, their first objective is finding Hemlock Woolly Adelgid to feed and reproduce on. So releasing them on HWA-infested hemlocks will provide maximum benefit – to you and your hemlocks! And the Sasajiscymnus tsugae beetle larvae are even more voracious than the adults, as they feed on HWA eggs, larvae and adults to extend the HWA control process. Here is a link to the HWA predator-rearing Lab in PA, which offers predator beetles and HWA consulting to private landowners: Tree-Savers.

Where Did Our Hemlock Woolly Adelgid Come From ?

USDA-sponsored research by Nathan Havill and associates (2006) has transformed our understanding of the relationship between Hemlocks and Hemlock Woolly Adelgids around the world! Havill’s analysis of mitochondrial DNA collected from all known HWA populations identified the five genetically distinct HWA strains depicted below. So now we know that there are (at least) five different Hemlock Woolly Adelgids, not just one.     (More recent analysis by Havill et al (2016) has split the HWA populations of China and Taiwan into two lineages each and has added an eighth HWA lineage on Ullung Island.)

Note that 4 of the 5 Hemlock Woolly Adelgid 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 genetic 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! It is from the HWA originating in Southern Japan.  So it is to Japan that we must look for HWA control agents for the eastern US!

But 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 lab-reared and available to private landowners during the Spring release season. Here is the commercial lab that rears these predator beetles for private landowners.

How, When & Where was Hemlock Woolly Adelgid Introduced to Eastern North America?

We can combine Havill’s DNA research on HWA origins with USDA monitoring of HWA in the late 20th century to provide a scientific foundation for historical inquiries into the introduction of HWA  in the Eastern US. Such historical inquiries strongly suggest the existence of multiple introduction sites and an early 20th century timetable (1910-1915) for such HWA introductions from Southern Japan to Eastern North America.

Historical inquiry is exploring the role of Gilded Age Gardens in Hemlock Woolly Adelgid introductions to the eastern US. And the map below shows hypothesized Gilded Age Garden HWA introduction sites and their proximity to early HWA infestation areas identified by USDA research. (Note that the dates in the USDA historical map utilized below refer to the “Year(s) of USDA First Discovery” of HWA, not to the “Year of First Infestation” of HWA, as indicated on map.)

The Gilded Age Garden Hypothesis

What would you like to learn more about?

For information on purchasing Sasajiscymnus tsugae predator beetles, see Predator Beetle source.
To learn more about biological control of the Hemlock Woolly Adelgid, see Biological Control .
To learn how you and your community can contribute to HWA biological control, see Community.
To learn  how HWA was introduced to eastern North America, go to Gilded Age Garden Hypothesis.
To learn more about assessing the effectiveness of predator beetle releases, see Hemlock Restoration.
To combine chemical & biological control strategies for HWA, see Integrated Pest Management of HWA.
To learn more about how genetic mutations can contribute to hemlock diversity, see Witch’s Brooms.