A Four-Year, Seven-State Reforestation Trial with Eastern Hemlocks (Tsuga canadensis) Resistant to Hemlock Woolly Adelgid (Adelges tsugae)

Ian G Kinahan,Chad M Rigsby,Evan L Preisser,Alana Russell,Richard A Casagrande,Gabrielle Grandstaff

doi:10.3390/f11030312

Abstract

We conducted over a decade of research into individual eastern hemlock (Tsuga canadensis; hemlock) trees that are potentially resistant to hemlock woolly adelgid (Adelges tsugae; HWA), an invasive xylem-feeding insect that is capable of rapidly killing even mature trees. Following clonal propagation of these individuals, in 2015 we planted size- and age-matched HWA-resistant and HWA-susceptible hemlocks in HWA-infested forest plots in seven states. In 2019, we re-surveyed the plots; 96% of HWA-resistant hemlocks survived compared to 48% of susceptible trees. The surviving HWA-resistant trees were also taller, produced more lateral growth, retained more foliage, and supported lower densities of the elongate hemlock scale Fiorinia externa, another invasive hemlock pest, than the surviving HWA-susceptible trees. Our results suggest that HWA management may benefit from additional research exploring the identification, characterization, and use of HWA-resistant eastern hemlocks in future reforestation efforts.

Highlights

Chemical suppression and biological control efforts are often the primary tools for managing non-native forest insects and pathogens, research that identifies the potential for pest resistance in host plant populations has played an integral role in many forest pest management programs [1]
While biological control has proven relatively unsuccessful in population-level management of this pest [7], research on surviving individual American chestnuts has identified some degree of blight resistance in these trees [8]
We found that HWA-resistant eastern hemlocks outperform HWA-susceptible eastern hemlocks when trees of both types are planted in HWA-infested forests, a result that may prove critical in HWA management

Summary

Introduction

Chemical suppression and biological control efforts are often the primary tools for managing non-native forest insects and pathogens (hereafter, ‘pests’), research that identifies the potential for pest resistance in host plant populations has played an integral role in many forest pest management programs [1]. ‘DED’), for instance, is the most destructive pest of shade-trees in the United States [2], having wiped out nearly 100 million American elms (Ulmus americana L.) in both urban and forest populations [3]. Mature individual American elms persist, in DED-devastated forests throughout the eastern USA [2], and research suggests that these trees are less susceptible to the disease [4]. While biological control has proven relatively unsuccessful in population-level management of this pest [7], research on surviving individual American chestnuts has identified some degree of blight resistance in these trees [8]. In addition to research on genetically-modified individuals, several ongoing breeding programs utilizing backcrossing of lingering American chestnuts with

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Discussion

Conclusion