Density-dependent survival: Hemlock woolly adelgid life stages

Since its initial introduction to Virginia in the 1950’s, Hemlock Woolly Adelgid, (Adelges tsugae Annand; HWA) has become a notorious forest pest of eastern and Carolina hemlock (Tsuga spp.) in the Eastern forest landscape of the U.S.  Irreversible damage to hemlock stands from HWA in the southeastern states and central New England created the expected hypothesis that widespread hemlock mortality would also occur when HWA eventually reached Massachusetts forests in 1989.  However, more than twenty years later HWA populations in Massachusetts have remained largely stable and hemlocks seem to be surviving.  To further investigate the key  density-dependent factors of HWA population dynamics that have been shown by previous experiments to be possible drivers of this observed stability, we infested sixty-four hemlock trees with varying densities of HWA sistens ovisacs in a typical hemlock forest in Massachusetts and documented subsequent HWA density, fecundity, and the amount of new growth on experimental trees.  We used a 2 x 2 randomized block design with previously and newly infested hemlocks divided into 1 m tall saplings and branches of mature trees.  We compare our results from HWA population trends in a natural forest setting with findings from a nearly identical experiment in a uniform atypical hemlock habitat—a hemlock plantation. Our findings are consistent with similar experiments and show significant patterns of density-dependent survival and fecundity decline in the summertime generation. We show that production of sexuparae in this life stage plays a key role in the density-dependent survival of HWA, and find that this response is likely to be a function of previous infestations of HWA to the tree and the current year’s crawler density of the summertime generation of HWA on each tree.