In 1997, the hemlock woolly adelgid (HWA; Adelges tsugae) was discovered on the Arnold Arboretum’s Hemlock Hill. This tiny insect, a relative of the aphid, feeds with lethal effect on the hemlock species of eastern North America. Native to eastern Asia, HWA was first detected in Virginia in the early 1950s and has since spread throughout the mid-Atlantic and southern New England. Sadly, researchers studying the insect have observed very high mortality rates among infested forests.
Beginning in 1997, Arboretum staff monitored the condition and rates of decline of roughly 1,900 eastern hemlocks (Tsuga canadensis) on Hemlock Hill. Presently, it is believed that 100 percent of the population is infested with HWA. To date, over 500 trees have been lost. As has been observed on other sites, those trees located in more marginal habitat conditions—thin soils or south- and west-facing exposures—have often been the first to succumb.
Because of the absence of host resistance and limited cultural control options, chemical treatments are the only reliable means of protecting hemlocks. Clearly any chemical treatment brings concern for the larger environment. At the same time, Hemlock Hill is an important resource for a large urban population that for over 150 years has enjoyed the singular educational and aesthetic experiences of a majestic hemlock-dominated forest.
Finding balance among stewardship, education, and public service goals, we are presently protecting hemlocks that are of sufficient vigor to recover and that grow in conditions that are favorable for treatment and do not present risk of water contamination. We control HWA on selected trees with applications of horticultural oil and, more recently, soil-injections of Imidacloprid (®Merit), a treatment now provided to over 40,000 trees at Great Smoky Mountains National Park. While using this method, we pay close attention to ongoing research that monitors for non-target effects and persistence in the environment. Ultimately, it is hoped that these treatments will “buy time” until biocontrols or other non-chemical options can offer reliable protection.
Other management measures include:
Research Programs on Hemlock Hill. The severe consequences of HWA infestation pose compelling questions about the ecological changes associated with the loss of eastern hemlock. Beginning in 2004, the Arboretum collaborated with the Harvard Forest, a research institute, to examine changes on Hemlock Hill. Harvard Forest scientists established six 15 x 15 meter research plots in order to measure the changes that occur when hemlock is removed from the forest system. Measurements established baseline data for soil temperature, available nitrogen, organic soil mass and understory vegetation. Analysis compared nitrogen cycling, decomposition rates and regeneration across the six plots. Completed in summer 2008, the study is part of a longer-term Harvard Forest effort to assess ecosystem impacts of HWA in southern New England.
A second research project determined that Chinese hemlock (T. chinensis), a close relative of our native species, is fully resistant to HWA. Ongoing field observations assess the suitability of Chinese hemlock as a landscape replacement for the eastern hemlock.