Spatial distribution of hemlock woolly adelgid (Hemiptera: Adelgidae) ovisacs within eastern hemlock

Understanding within-tree distribution of an insect pest can help determining where to sample and target to apply control measures. This study was conducted to characterize the within-tree distribution pattern of ovisacs and determine suitable sample units for hemlock woolly adelgid, Adelges tsugae (Hemiptera: Adelgidae), on eastern hemlock. A total of 46 eastern hemlock trees infested with A. tsugae were selected from hemlock stands in Blackwater Fall State Park (WV), Cathedral State Park (WV), and Buchanan State Forest (PA).  In each tree, 24 branches were cut with regard to six vertical heights and four cardinal directions. The cut branches were brought to the lab to count the number and map the location of ovisacs within a branch using geostatistical procedure. The results of this study showed that A. tsugae ovisacs were more frequently found and aggregated at the tip of a branch, and no significant differences in A. tsugae density (F-test; d.f. = 3; p value > 0.05) were found among the four directions. However, the vertical distribution of A. tsugae ovisacs showed significant differences (F-test; d.f. = 5; p value < 0.05). Upper branches had more ovisacs than middle and lower branches. This study also showed that a 50-cm sample from lower branches would be the suitable sample unit for A. tsugae sampling.