Hemlock woolly adelgid is a tiny, sap-feeding insect originating from Asia (McClure, 1987, McClure et al., 2001). It was first confirmed in Virginia in the 1950s, likely arriving on plant material imported for a private plant collection (Havill and Montgomery, 2008). Since that time, it has been recognized as a serious pest, killing entire populations of eastern hemlock (Del Tredici and Kitajima, 2004). Currently, HWA is established in much of the eastern hemlock range (Hessl and Pederson, 2012), which includes portions of the eastern United States, north to Maine, south to Georgia, and westward. It has been conjectured that HWA poses a major threat to the long-term survival of vulnerable host species such as eastern hemlock (Foster, 2014). The insect’s success is a result in part of its ability to expand its range readily via vectors such as wind, birds, and mammals. HWA’s negative impact on eastern hemlock is also a result of its ability to feed year-round (McClure, 1987; Ward et al., 2004). By withdrawing sap from young twigs, the insect impedes tree growth, causing needle discoloration (i.e., turning grayish green) and premature needle drop. The loss of new shoots and needles weakens trees critically, eventually leading to death (McClure et al., 2001). This insect is capable of killing untreated susceptible hemlock in less than a decade in horticultural settings (Harper and Weston, 2016; Hoover et al., 2009; Weston and Harper, 2009), and in some cases in less than 5 years (McClure, 1987; Radville et al., 2011). Although HWA is arguably the most important eastern hemlock pest, other insects not considered in this study may also play an important role in overall plant health, including elongate hemlock scale (Fiorinia externa).
Eastern hemlock is functionally important in horticultural settings because it tolerates shade, functions well as a screen, and withstands pruning and shearing. With eastern hemlock’s potential loss as a viable horticultural planting, horticulturists will have one less shade-tolerant evergreen screen available. According to one extension horticulturist (G.G. Giordano, personal communication): “As hemlock wooly adelgid became an increasingly serious problem over the last several decades, the difficulty of suggesting a replacement that would perform the functions that eastern hemlock provides in the landscape became more and more apparent. Even available species that could partially perform some of these functions lacked the critical aesthetic found only in eastern hemlock.”
Eastern hemlock’s susceptibility to HWA is impacting nursery growers as well (K. MacIndoe, personal communication): “We have averaged $10,000 in sales of [eastern] hemlock over the past 10 years. We have sold $0 worth of [eastern] hemlocks the past 2 years.”
Although new plantings of eastern hemlock are likely in decline, the control of HWA on susceptible hemlock species can be achieved through recurring insecticide applications (Harper and Cowles, 2013; McClure, 1987) and is usually most efficacious when insect populations are low and tree health has not been seriously compromised (Ward et al., 2004). Effective chemical HWA control includes the use of contact insecticides such as horticultural oil or systemic insecticides like imidacloprid (McClure, 1987; Webb et al., 2003). In addition to pesticide applications, eastern hemlock health can be maintained with cultural practices such as mulching to help maintain adequate soil moisture and to avoid nitrogen fertilizers that are linked to enhanced HWA survival and reproduction (McClure et al., 2001). Insecticide-free control of HWA is continuing to garner interest and involves the strategy of replacing pest-prone species with pest-resistant ones. This strategy, known as host plant resistance, or HPR, is an important strategy in agricultural and forested settings (Beck, 1965; Herms, 2002). With mounting social and regulatory pressures demanding the use of less pesticides in urban and suburban environments, HPR may offer a long-term, cost-effective strategy (Herms, 2002) for the management of HWA.
Research suggests that chinese hemlock may serve as a suitable HPR replacement for eastern hemlock (Harper and Weston, 2016; Weston and Harper, 2009), especially when viewed through the lens of consumer preference (Dampier et al., 2015). However, there is a dearth of studies comparing economic benefits against the costs of establishment and of insect control required to maintain healthy hemlock species in HWA-infested areas. While building upon previous hemlock research, this work addresses the knowledge gap by providing a comparative 25-year economic benefit–cost modeling and analysis, tracking estimated purchase, establishment, and maintenance costs of the two tree species. A time horizon of 25-years was chosen as a suitable “project” duration, despite both tree species having a lifespan that greatly exceeds this period.
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