Prioritizing eastern hemlock (Tsuga canadensis) for secondary imidacloprid treatment against the invasive hemlock woolly adelgid (Adelges tsugae) in Shenandoah National Park

Walters, Katelyn

ABSTRACT: Shenandoah National Park (SNP) contains over 20,000 eastern hemlocks (Tsuga canadensis); a foundation species in the southern Appalachian Mountains. Hemlock is shade-tolerant and retains a dense needle canopy year-round, creating a unique microclimate providing habitat for many species. The decline in eastern hemlock from hemlock woolly adelgid (Adelges tsugae; HWA) infestation has negative implications for the overall function of forest ecosystems. The effect of slope aspect and time since imidacloprid insecticide treatment on crown health change and diameter growth of eastern hemlock in SNP was quantified. Data was compared from hemlock trees located on southeastern (SE) and northwestern (NW) aspects at sites that were either treated two, five or eight years ago. Trees at control sites received no treatment. Change in crown health was significantly affected by aspect and time since treatment (p<0.001) but change in diameter growth rate was not affected by either variable. Crown health decreased in trees treated eight years ago on both aspects and remained the same as initial treatment health in trees treated five years ago. The SE aspect trees treated two years ago showed a decline in crown health while the NW aspect trees experienced a significant increase in crown health. Increased precipitation on the SE aspect may be causing the insecticide to break down faster resulting in insufficient absorption of treatment. This hypothesis is supported by significantly lower imidacloprid residue detected in branch samples from SE aspect trees (p=0.04) and could explain the continued decline of SE aspect hemlock crown health after treatment. Aspect may have a role in treatment effectiveness initially, but within five years, hemlock trees will exhibit similar health decline. A concerning observational finding was the lack of cones on either aspect, despite evidence of treatment effectiveness. Treatment may result in short-term hemlock recovery and survival, but not reduce HWA stress long enough to allow for reproduction. It is recommended that SNP management select a subset of the eastern hemlock population for future imidacloprid treatment due to the costs of time, money and ecological degradation. Based on this research, the selected trees should be adult hemlocks located in hemlock dominated stands on higher elevations of NW aspects.