How do we know that the survival of HWA-infested hemlocks in Japan isnt due to genetic resistance, rather than biological control?
There is at least one Asian hemlock (Tsuga chinensis) that exhibits high levels of resistance to our HWA “import” from Japan. However, researchers at the National Arboretum (see technical paper Resistance of Hemlock Species to HWA) have shown that our adelgid’s native hemlock host in Japan (Tsuga seiboldii) has little more resistance to the adelgid than our two native eastern hemlocks, Tsuga canadensis and Tsuga caroliniana. The Sasi beetle is the major HWA predator on these nonresistant hemlocks in Japan. So biological control involving Sasi is the most likely explanation for the healthy coexistence of hemlocks and HWA in Japan.
