Spatial and temporal associations between hemlock woolly adelgids (Hemiptera: Adelgidae) and surrounding environment

Eastern hemlock, Tsuga canadensis (Pinales: Pinaceae) is an important tree species in eastern North America ecosystem, but now threatened by the hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae). Understanding spatial associations between A. tsugae and surrounding environment can be helpful for management of A. tsugae populations. This study was conducted to determine the distribution pattern of A. tsugae and its spatial associations with surrounding environment. A total 100 grids (each grid being 10 m by 10 m) were laid out inside eastern hemlock stands in Blackwater Fall State Park (WV), Cathedral State Park (WV), and Buchanan State Forest (PA). In each grid, two 50-cm branches from a selected tree were cut at peak ovisac occurrence from June 2012 to May 2014.  In addition, surrounding trees (i.e. species, diameter at breast height, and height) and geographic factors (i.e. altitude and slope) were sampled. Spatial distribution patterns of A. tsugae and its spatial associations with the surrounding environments were analyzed by using Spatial Analysis by Distance Indices (SADIE). In most of cases, A. tsugae populations were spatially aggregated (P < 0.05) regardless of sites and seasons. Overall, tree species diversity, diameter at breast height, and hemlock tree height showed spatial associations with A. tsugae populations (P < 0.05) even though such relationships were not consistent among sites and time. These results can be used to improve sampling and management of A. tsugae populations.