Role of Wind, Birds, Deer, and Humans in the Dispersal of Hemlock Woolly Adelgid (Homoptera: Adelgidae)

Abstract

Studies in Connecticut revealed that wind, birds, deer, and humans can play an important role in the dispersal of hemlock woolly adelgid, Adelges tsugae Annand. Eggs, crawlers (mobile first instars), and winged adult sexuparae were captured on sticky traps hung in the forest canopy and placed on the forest floor up to 1,350 m (farthest distance tested) leeward of an infested hemlock stand. Greatest numbers of airborne adelgids were trapped at the center and edge of the stand; few were captured 600 m beyond the infestation. Numbers of airborne adelgids trapped at three heights in the forest (5, 10, and 15 m aboveground) did not differ and reflected the uniform vertical distribution of the adelgid source population on the hemlock trees themselves. Examination of birds captured in mist nets revealed that 19 of 22 birds (representing 10 species) trapped in an infested hemlock forest, and 3 of 14 birds (representing three species) captured in an old field 2 km from the nearest known hemlock, were each carrying 1–15 eggs and crawlers of A. tsugae on their bodies. Among these birds were several ground-dwelling species that spend little or no time in hemlock trees. Among seedlings that had been planted at various distances from an infested hemlock stand, significantly more adelgids occurred on hemlock seedlings that had been browsed by deer than on seedlings that had not. The interaction between distance and browsing accounted for a significant amount of variation in adelgid density. Within the heavily infested stand, browsing by deer was inconsequential. However, browsing was increasingly important as a means of adelgid dispersal as distance of seedlings from the infested stand increased. The trunks of all 40 mature hemlock and northern hardwood trees examined within the infested forest had ovisacs of A. tsugae affixed to their bark. These ovisacs, which contained on average> 100 eggs, had been dislodged from the hemlock canopy above. Ovisac density on these trunks at breast height ranged from 0.5 to 3.6 per 616 cm2. Studies revealed that eggs and crawlers removed from hemlock could survive under ambient conditions without sustenance up to 2 wk in the laboratory. These data suggest that logging operations could be important in the long-range dispersal of A. tsugae .