Pest description and crop damage Hemlock adelgids are aphid-like insects. They appear as white, woolly tufts on the bark and needles. Adults are black beneath the woolly material. Needles drop prematurely, weakening the tree and sometimes leading to death. Trees with severe infestations may be stressed, predisposing them to other insect and disease problems. The hemlock adelgid is especially a problem on hemlock hedges.
Biology and life history The adelgid overwinters as woolly adults, with reddish-brown crawlers similar to scale crawlers appearing in spring and early summer. Hemlock adelgids are sometimes known as "hemlock chermes."
Western hemlock (Tsuga heterophylla), mountain hemlock (T. mertensifolia), and Northern Japanese hemlock (T. diversifolia) are reported to be resistant to infestation. Eastern or Canadian hemlock is very susceptible. Wipe off minor infestations and prune out larger infestations if possible.
Apply to control crawlers after eggs hatch, usually at the time when new tip growth is expanding.
For more information
Johnson, W.T. and H.H. Lyon (1991), Insects That Feed on Trees and Shrubs, 2nd ed., Cornell University Press (p. 78).