Susceptibility of Chinese Hemlock (Tsuga chinensis) to
Injury from Autumn Horticultural Oil Applications
Richard W. Harper and Richard S. Cowles
Abstract: Chinese hemlock (Tsuga chinensis) has been suggested as a possible replacement in landscape plantings for eastern hemlocks (T. canadensis), which are being lost due to infestations of hemlock woolly adelgid (Adelges tsugae). Chinese hemlock is highly resistant to hemlock woolly adelgid, but is a host for elongate hemlock scale (Fiorinia externa). Horticultural oil is a popular insecticide commonly used by professionals to manage various insect and mite pests of hemlock in landscapes, including hemlock woolly adelgid, spruce spider mite, and armored scales. In tests, horticultural oil applied in two separate autumn sprays was not phytotoxic to Chinese hemlocks. Therefore, 1% or 2% horticultural oil may be used to maintain Chinese hemlocks in landscape plantings.
Keywords: Adelgid; Chinese Hemlock; Fiorinia externa; Horticultural Oil; Tsuga chinensis.