FAQ about Hemlocks and Hemlock Woolly Adelgids

Where Did Hemlock Woolly Adelgid Come From?

DNA analysis has shown that Hemlock Woolly Adelgid (HWA) introduced to Eastern North America is identical to that found in Southern Japan and different from the HWA strains in all other parts of the world – including Western  North America. This research (2006, 2016) by Nathan Havill, USFS and associates, confirms that Southern Japan was the origin for our HWA “import” to Eastern North America.

How, When & Where was Hemlock Woolly Adelgid Introduced to the Eastern US?

Scientific research on the origins of HWA has identified Japanese gardens at US Gilded Age Estates in the early 20th century as the likely points of introduction for the hemlock woolly adelgid from Southern Japan. Current research has identified multiple Gilded Age Japanese garden sites constructed in the early 20th century (between 1910 & 1915) as likely HWA introduction sites. And these sites, ranging from Virginia to Long Island, are consistent with USDA historical data on early HWA discovery sites.

How important is “biological resistance” of Hemlocks to Hemlock Woolly Adelgids?

Every hemlock species has a different level of susceptibility or resistance to each strain of hemlock wool adelgid. Total biological resistance, in which a hemlock is impervious to HWA, is not a property found in natural hemlock ecosystems, as HWA could not survive without a food source.

In southern Japan, our HWA “import” feeds on a native hemlock (Tsuga seiboldii) that does not have a high level of resistance to this HWA. So this hemlock depends on the HWA predator Sasajiscymnus tsugae to control HWA numbers and allow hemlocks (including T canadensis imports) to grow and prosper.

But resistance levels to particular Hemlock Woolly Adelgid strains differ across hemlock species. For example, the hemlock species from China (Tsuga chinensis) shows high levels of resistance to the HWA strain introduced to the US – from Southern Japan. However, in China T chinensis supports feeding and reproduction by their native HWA strains.

The best documented case of biological resistance to Hemlock Wooly Adelgid is in North America’s Pacific Northwest. There, the Western hemlock (Tsuga heterophylla) is so resistant to its native HWA strain, that only hemlocks that have been stressed or damaged are susceptible to HWA attack. At this level of biological resistance, HWA predators play no role in the successful existence of wild hemlocks in the Pacific NW. And both our eastern hemlocks are also highly resistant to the HWA strain found in the Pacific NW