What will be the Fate of Hemlocks
in the Eastern US?
The imported Hemlock Woolly Adelgid (HWA) has brought widespread destruction to our native hemlocks in the eastern US. The loss of our hemlocks would be an ecological disaster for our mountain 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”, that can control Hemlock Woolly Adelgid on our native hemlocks and allow the long-term survival and recovery of our native hemlock ecosystems. 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.
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.”
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.
Here is what predator-beetle-assisted Hemlock Restoration looks like
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 affecting other, non-adelgid insect species. 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 (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 a report from the Smokies on the effectiveness of the Japan-origin predator beetle, Sasajiscymnus tsugae (Sasi/St) for 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 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 HWA around the world! Havill’s analysis of mitochondrial DNA 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.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 HWA control agents for the eastern US! (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.)
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 Sasi/St predator beetles for private landowners.
Each of these five known hemlock woolly adelgids 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. For more on the implications of HWA ecosystems for biocontrol of HWA in the eastern US, see my 2016 NENHC poster.
How was HWA introduced to the Eastern US?
Combining Havil’s transformative DNA research on HWA origins with USDA monitoring of HWA in the late 20th century provides a scientific foundation for historical inquiries into HWA introduction 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 the eastern US. One historical inquiry is exploring the role of Gilded Age Gardens involved in international horticulture in such HWA introductions. (Note that the dates in this USDA map refer to the Year(s) of USDA First Discovery of HWA, not to the Year of First Infestation of HWA, as indicated on map.)
For more information on HWA historical introduction, go to Gilded Age Garden Hypothesis.
What would you like to learn more about?
For information on purchasing Sasajiscymnus tsugae predator beetles, go to Predator Beetle source.
To learn more about biological control of the Hemlock Woolly Adelgid, go to Biological Control .
To learn how you and your community can contribute to HWA biological control, go to Community.
To learn more about assessing the effectiveness of predator beetle releases, go to Hemlock Restoration.
For chemical & biological management of HWA infestations, go to Integrated Pest Management of HWA.
To learn more about how genetic mutations can contribute to hemlock diversity go to Witch’s Brooms.