FAQ about Hemlocks and Hemlock Woolly Adelgids

Where Did the Hemlock Woolly Adelgid Come From?

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

How, When and 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 Japanese garden sites constructed between 1910 & 1915 as possible introduction sites. And these sites, ranging from Virginia to Long Island, are consistent with USDA records on early HWA discoveries.

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.

But resistance levels to particular HWA strains differ across hemlock species. For example, the hemlock species from China (T chinensis) and northern Japan (T diversifolia) have shown high levels of resistance to the HWA strain introduced to the US – from Southern Japan. However, each hemlock is  a food source for their respective native HWA strains in their homelands.

The best documented case of biological resistance to HWA in a natural environment is in the Pacific Northwest of North America. There, the Western hemlock (T 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 the western HWA strain is not considered a “forest pest”.