Abstract
Eastern hemlock (Tsuga canadensis), a canopy-dominant conifer native to the eastern U.S., is currently threatened with extirpation by the invasive stylet-feeding hemlock woolly adelgid (Adelges tsugae). To understand interactions between eastern hemlock and hemlock woolly adelgid, and their interactions with other forest defoliators, we carried out two experiments. First, in 2018, we evaluated the impact of feeding by hemlock woolly adelgid on gypsy moth (Lymantria dispar) larval preference for, and performance on, eastern hemlock. To assess preference, we surveyed 245 field-grown hemlocks for gypsy moth herbivory damage and conducted laboratory paired-choice bioassays. To assess performance, gypsy moth larvae were reared to pupation on adelgid-infested or uninfested hemlock foliage and pupal weight, proportional weight gain, and larval period were analyzed. Adelgid-infested hemlocks experienced more gypsy moth herbivory than uninfested control trees, and laboratory tests confirmed that gypsy moth larvae preferentially feed on adelgid-infested hemlock foliage. Gypsy moth larvae reared to pupation on adelgid-infested foliage gained more weight than larvae reared on uninfested control foliage. Our results suggest that the synergistic effect of adelgid and gypsy moth poses an additional threat to eastern hemlock that may increase extirpation risk and ecological impact throughout most of its range. Second, we have conducted over a decade of research into rare eastern hemlock (Tsuga canadensis; hemlock) trees that appear resistant to hemlock woolly adelgid (Adelges tsugae; HWA). Following clonal propagation of these rare 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 elongate hemlock scale (Fiorinia externa) and HWA densities 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
Many interactions between co-occurring insect herbivores are mediated by their impact on the shared host plant (Kaplan & Denno, 2007)
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
We found that HWA-resistant eastern hemlocks had significantly higher survival, better growth (Fig. 2A – C), and retained more foliage (Fig. 2D) than HWA-susceptible hemlocks
Summary
Many interactions between co-occurring insect herbivores are mediated by their impact on the shared host plant (Kaplan & Denno, 2007). Plant defense against leaf-chewing herbivores is primarily mediated by JA (Gilbert & Liebhold, 2010; Kroes et al, 2014), and blocking the induction of JA-related defenses may make the foliage of adelgid-infested hemlocks more digestible and/or converted into body mass by gypsy moth larvae. This interpretation is supported by work on other systems where negative cross-talk between these pathways has been shown to improve the performance of a later-arriving herbivore (reviewed in Stam et al, 2014). Preisser1* 1Department of Biological Sciences, The University of Rhode Island, Kingston, RI 02881 2Department of Plant Sciences and Entomology, The University of Rhode Island, Kingston, RI 02881 3Current address: Bartlett Tree Research Laboratories, The Morton Arboretum, Lisle, IL 60532 *Author for correspondence
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
