Range expansion and population dynamics of co-occurring invasive herbivores

The hemlock woolly adelgid Adelges tsugae (HWA) and elongate hemlock scale Fiorinia externa (EHS) are invasive herbivores that are responsible for widespread mortality of eastern hemlock (Tsuga canadensis) on the East Coast of the US. We assessed the spread and community-level impact of these species on hemlock forests by resurveying 141 hemlock stands across a 7500 km² latitudinal transect running from coastal CT to northern MA. All of these stands had been previously identified via aerial photography and surveyed in either 1997-98 (CT) or 2002-04 (MA) for HWA and EHS density as well as hemlock stand vigor. We rated two branches on each of 50 trees per stand for HWA and EHS density. Overall stand canopy loss was also assessed. HWA density decreased with increasing latitude, possibly due to climatic limitations. EHS density decreased with increasing latitude and longitude, potentially indicating the front wave of EHS’s northeasterly range expansion. HWA density has decreased at almost all sites since the previous survey, but the total number of infested sites has increased. EHS distribution and density has increased dramatically since 1997-98 and is now present in every CT stand and many MA stands. Although canopy loss increased with increasing HWA density, it was not correlated with EHS density.