Small insect, big problem: Understanding the hemlock woolly adelgid

The hemlock woolly adelgid, Adelges tsugae (hereafter ‘adelgid’), is an invasive insect in the eastern United States that has devastated populations of the eastern hemlock Tsuga canadensis. For nearly 20 years, Dr. Elkinton and his lab have been researching the adelgid’s population ecology and its interactions with its host plant and potential biological control agents. Some early achievements were a three-year study of adelgid demography in southern New England that provided the first detailed look at its population dynamics and research exploring the adelgid’s interactions with another invasive hemlock-feeding insect, the elongate hemlock scale Fiorinia externa. The lab later moved into chemical ecology for work addressing differences in plant chemical defense between adelgid-susceptible eastern hemlocks and some rare individual trees that appear largely resistant to adelgid attack. Dr. Elkinton and his group later conducted pioneering work on comparing adelgid demography in the eastern US and in the Pacific Northwest (where the adelgid occurs naturally). They documented that native predators control adelgid densities in this portion of its native range and used their demographic data to develop a population model identifying the stages of the adelgid life cycle most vulnerable to biological control agents. This model predicted that some widely used biological control agents had little impact on adelgid densities, a finding that was later confirmed experimentally. This work led to a rethinking of the adelgid biological control program and a current focus on a biological control agent specifically targeting the most vulnerable portion of the adelgid life cycle.