Tsuga - Threats

Threats

The two eastern North American species, T. canadensis and T. caroliniana are under serious threat by the sap-sucking insect Adelges tsugae (Hemlock Woolly Adelgid). This adelgid, related to the aphids, was introduced accidentally from eastern Asia, where it is only a minor pest. Extensive mortality has occurred, particularly east of the Appalachian Mountains. The Asian species are resistant to this pest, and the two western American hemlocks are moderately resistant. Tsuga species are also used as food plants by the larvae of some Lepidoptera species including the Autumnal Moth and the Engrailed, and older caterpillars of the Gypsy Moth. Once these infest a tree, they can do more than simply kill one tree. Larger hemlocks that are infected have large relatively high root systems that can bring other trees down if one falls. The foliage of young trees is often browsed by deer, and the seeds are eaten by finches and small rodents.

Old trees are commonly attacked by various fungal disease and decay species, notably Heterobasidion annosum and Armillaria species, which rot the heartwood and eventually leave the tree liable to windthrow, and Rhizina undulata, which may kill groups of trees following minor grass fires that activate growth of the Rhizina spores.