In this project, mathematical modeling and genetic analysis are combined with experiments and field surveys to better predict the spread of the invasive hemlock woolly adelgid, Adelges tsugae (HWA), across the eastern United States. This forest pest kills eastern hemlock trees and has caused both economic and ecological damage. Genetic markers will be used to identify distinct HWA clonal lines that will each have its spread rate, population growth rate, and cold tolerance measured. This information will be used in combination with survey data reporting the genetic structure of the invasive populations to develop a mathematical model examining the role of dispersal and selection for surviving cold winter conditions in driving the range expansion of HWA.
This research will substantially increase the understanding of how invasive species spread across landscapes. Hemlocks have both economic and cultural importance, and their loss concerns groups ranging from forestry officials to local citizens. Understanding how and why HWA spreads throughout the landscape will allow the identification of locations most at risk for infestation for targeted control efforts. This work also increases the ability of scientists and resource managers to anticipate the movement of geographic range boundaries in response to the changing climatic regimes likely to occur as a consequence of global warming.
Our project considerably advanced our understanding of how invasive species move across native landscapes. We developed several different mathematical models for assessing range expansion, and the papers we have published on this topic have attracted considerable interest from other researchers. We have also increased our understanding of how the hemlock woolly adelgid and elongate hemlock scale, two invasive herbivores that share a common native host plant (eastern hemlock), interact to affect each other and the health of their host. Our findings suggest that management efforts focus on hemlock adelgid, since the elongate hemlock has a relatively minor effect on plant growth and survival. An increased understanding of this three-species interaction will also serve as a guide for other researchers faced with managing multiple invasive pests on a common host. This line of inquiry also led us to become interested in how the adelgid and scale affect their host, resulting in research assessing changes in tree physiology that result from each species’ feeding. In one research project, we found that while scale feeding only induced localized plant defenses, feeding by the adelgid caused both a local and a systemic response that was present in uninfested foliage. In another research project, we found that feeding by the adelgid actually increases nutrient levels in foliage, a counter-intuitive result that suggests the adelgid may be ‘manipulating’ the plant to bring food to it at the cost of whole-tree health. These findings and other may help lead to better management strategies for the adelgid, an invasive pest that has killed hundres of thousands of hemlocks in the eastern US.
