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Science Notes on Hemlock Woolly Adelgid (2 Replies)

Posted February 3, 2018 at 10:08 am

Hemlock Woolly Adelgid (HWA) is an important threat to North American hemlocks. Recent scientific research provides important insights on the origin and timing of this introduction, as well as on possible control strategies for this invasive insect that threatens our native hemlocks.
The most important scientific breakthrough on the hemlock woolly adelgid is provided by analyses of DNA data for all known world HWA populations (by Nathan Havill and associates: 2006, 2016). This research identifies multiple different genetic strains of HWA, each with different locations, biological characteristics, and hosts. It also identifies the HWA strain found in eastern North America as identical to that in southern Japan. So that is the origin of our current HWA infestation!

Knowledge of the Japanese origin of this introduced organism has also stimulated historical research on the timing of its introduction. The oft-cited 1951 date for HWA introduction denotes the first detection of HWA in Richmond, Virginia. However, it appears that the actual introduction of HWA occurred much earlier (in the 1910-1915 period), during the construction of Japanese gardens at Gilded Age Garden sites in the eastern US. Three of these garden sites (two in Virginia and one in Pennsylvania) have been tentatively identified using historical documents, including horticultural transfers from Japan. But the location of a 4th Gilded Age Japanese garden in Long Island, New York is currently unknown. For more information on this historical research see …

The HWA present in the Pacific Northwest has been suggested as a possible source for our eastern HWA import. But Havill’s DNA analyses identify the Pacific HWA as a different organism, one which has been present in western North America for thousands of years. Fortunately, western hemlock species (T heterophylla, T mertensiana) possess sufficient biological resistance to the Pacific HWA strain to protect healthy trees from HWA damage. And as a result, western hemlocks are not dependent on biocontrol agents for their survival, and the Pacific HWA is not considered a forest pest.

In contrast, the Southern Japanese Hemlock (Tsuga seiboldii) is potentially vulnerable to its native HWA, exhibiting only slightly more biological resistance to this HWA strain than do our native hemlocks (T canadensis, T caroliniana). These hemlock species can benefit from the native Japanese HWA predator beetle (Sasajiscymnus tsugae), that protects hemlocks in Japan. And after careful research, this Japanese adelgid-specialist predator was approved by USDA/APHIS for biocontrol release in the eastern US.
Wild hemlocks in the central and southern US have been badly damaged by the “biological avalanche”, produced by a century of uncontrolled HWA population growth.

Sasajiscymnus tsugae feeding on Hemlock Woolly Adelgid

But some northern hemlock areas, protected by HWA cold-sensitivity, are just now experiencing HWA infestation. And recent, horticultural introductions of HWA to western Michigan appear to be escaping chemical containment efforts and moving northward into wild hemlock areas. So both northern and midwestern native hemlock areas will offer important opportunities for conifer lovers to get involved and contribute to hemlock protection and restoration.
My educational website offers a resource for scientific information and materials for the hemlock woolly adelgid.

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Posted February 8, 2018 at 7:45 am

Patrick … this is a very interesting and timely article. With your premission, we’d like to expand it a bit and add links to it in the various Tsuga records in the ConiferBase. Thank you.

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Posted February 8, 2018 at 12:22 pm

Thank you for your interest. I would be happy to work with you and others in expanding this discussion – in response to issues of interest.

One area where Havill’s research “discovery” of multiple strains of Hemlock Woolly Adelgid makes a difference, is in the discussion of HWA resistance for different hemlock species. For example, it is commonly reported that the western hemlock species (T heterophylla, T mertensiana) are resistant to HWA, while our eastern hemlock species (T canadensis, T caroliniana) are not resistant to HWA. But the problem with drawing an inference from this comparison is that the Pacific HWA and the Eastern HWA are different biological organisms. So this represents an “apples & oranges” comparison.

If we compare all North American hemlocks’ response to the same HWA organisms, we find that Western hemlocks have a high level of resistance to the Pacific HWA, but so do our eastern hemlocks – have a high level of resistance to the Pacific HWA. Likewise, both our eastern hemlock species are highly susceptible to our HWA “import” to eastern North America. But both western hemlocks appear to also be very susceptible to this HWA “import”.

Similarly, some Asian hemlock species (T chinensis, T diversifolia) have been shown in US trials to be highly resistant to our HWA from southern Japan. But in their homelands, both these “resistant” hemlock species support the feeding and reproductive activities of their native HWA strains (see below). So again, resistance is HWA-specific. (Note that there are two strains of HWA in Japan – one in the south which feeds on Tsuga seiboldii, another in the north which feeds on Tsuga diversifolia.)

Photo Credit: Dr Carole Cheah, CAES

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