6 Harper and Cowles: Susceptibility of Chinese Hemlock Arboriculture & Urban Forestry 2013. 39(1): 6–10 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 east- ern hemlocks (T. canadensis), which are being lost due to infestations of hemlock woolly adelgid (Adelges tsugae). Chinese hem- lock is highly resistant to hemlock woolly adelgid, but is a host for elongate hemlock scale (Fiorinia externa). Horticultur- al 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. Key Words. Adelgid; Chinese Hemlock; Fiorinia externa; Horticultural Oil; Tsuga chinensis. Eastern hemlock [Tsuga canadensis (L.) Carrière)] is highly valued for planting in landscapes due to its evergreen, dense foliage, graceful, pyramidal growth form, and tolerance of moist soils. T. canadensis is very shade tolerant, which allows it to retain needles on several years of growth and makes it amena- ble to shearing for use in landscape hedges. The threat of losing eastern hemlock in landscape plantings due to hemlock woolly adelgid (HWA) (Adelges tsugae Annand), has led to investiga- tions of replacement species of hemlocks. The relationship between hemlock species and their associated strains of adelgids is complex. Eastern and Carolina hemlocks (Tsuga caroliniana Engelmann) quickly decline in health and die when fed upon by adelgids, to which they are not adapted. Other species of hemlock found in western North America and Asia have coevolved with or exhibit forms of resistance to hemlock woolly adelgids, and are rarely damaged by their feeding (Montgomery et al. 2009). The strain of HWA found in the eastern United States is believed to have originated from Japan (Havill and Montgomery 2008), and several studies point to Chinese hemlock [Tsuga chinensis (Franch.) E. Pritz] as highly resistant to this strain of HWA (Del Tredici and Kitajima 2004; Hoover et al. 2009; Montgomery et al. 2009; Weston and Harper 2009). Additionally, T. chinensis exhibits many of the same growing characteristics as T. canadensis —it is fast growing, shade tolerant, has a graceful form, and is fully hardy in USDA Zone 6 (Del Tredici and kitajima 2004). Besides being highly resistant to A. tsugae, T. chinensis may also be somewhat less susceptible to spider mites, presumably O. unun- guis (Jacobi), than T. canadensis (Del Tredici and Kitajima 2004). It would be premature to recommend replacing T. canaden- sis with T. chinensis if Chinese hemlocks are too susceptible to other pests. Based on observations of a planting of eastern and Chinese hemlocks established at Lasdon Park and Arbore- ©2013 International Society of Arboriculture tum (Katonah, New York) since 2003, both species appear to be suitable hosts for the elongate hemlock scale (EHS) (Fiorinia externa Ferris). In 2008, populations of EHS were found on 73% and 35% of specimens of T. canadensis and T. chinensis, respectively (Weston and Harper, 2009). Elongate hemlock scale is an armored scale (Family: Diaspididae) first discov- ered in Queens, New York, in 1908 and formally described in 1942. It is known to occur on several native and commonly planted ornamental trees and shrubs, including yew (Taxus spp.), Douglas-fir (Pseudotsuga menziesii), true firs (Abies spp.), spruce (Picea spp.) and native eastern and Carolina hem- locks (McClure and Fergione 1977). Johnson and Lyon (1994) described EHS as a “destructive pest of established landscape trees and hedges,” but in contrast noted it infesting “very few” stands of native hemlocks; the range was reported to include southern New England (Massachusetts, Connecticut, U.S.), other northeastern states such as New Jersey, Maryland, and Pennsylvania, and as far west as Ohio. Elongate hemlock scale has more recently been recognized as a pest of forest as well as landscape trees (McClure 2002) and within the past five years has caused significant decline of forested stands in New England—sufficient to warrant salvage harvest of the dying trees (Kyle Lombard, NH Forest Health Program Coordinator; David Irvin, Connecticut, U.S., DEEP Forester, pers. comm.). After a newly hatched scale crawls to, or is transported by wind to, a suitable host plant, it inserts a stylet into the under- side of the needle, removing plant fluids from the mesophyll cells. As scale populations build—especially on hemlocks already stressed by drought or HWA infestation—foliage may exhibit chlorosis, followed by needle drop, branch dieback, and whole-plant death within 10 years of initial infestation (McClure 2002). Although potentially serious, EHS infesta-
January 2013
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