When invasive herbivores compete, who wins? Lessons from a forest invasion

The invasive hemlock woolly adelgid Adelges tsugae (‘HWA’) poses a major threat to eastern hemlock (Tsuga canadensis) in eastern North America. High-density HWA infestations can quickly kill even mature trees, and substantial hemlock mortality has occurred throughout the eastern seaboard. Despite fears that HWA would remove hemlocks from southern New England, however, stand-level mortality has occurred much more slowly than predicted. This period also saw the rapid range expansion of a second introduced insect, the elongate hemlock scale Fiorinia externa (‘EHS’). While EHS can reduce hemlock growth and may be capable of killing stressed trees, its impact on tree health is minimal compared to that of HWA. Neither HWA nor EHS possess natural enemies capable of substantially limiting their population growth in the invaded range, and both are now abundant in southern New England. I discuss the results of my lab’s research into both direct and plant-mediated competition between these two species and their individual and joint impact on hemlock physiology. Understanding interactions between HWA and EHS, and why they have such disparate impacts on hemlock health, provide information important to controlling both threats.