Eastern Forest Threat Center - Hemlock Woolly Adelgid

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Hemlock woolly adelgid infestation

Hemlock woolly adelgid infestation

Connecticut Agricultural Experiment Station Archive, Bugwood.org

Hemlock Woolly Adelgid
Adelges tsugae

Hemlock woolly adelgid, native to Asia, was first reported in 1951 near Richmond, Virginia. By 2005, it was established in portions of 16 states from Maine to Georgia, where infestations covered about half of the range of hemlock.

Keywords: eastern hemlock, Tsuga canadensis, Carolina hemlock, Tsuga caroliniana, aphidlike, Hemiptera, HWA, wool-like wax filaments, stored starches

Distribution Map Distribution Source Image

Threat Description

Hemlock woolly adelgid (HWA) develops and reproduces on all hemlock species, but only eastern and Carolina hemlock are vulnerable when attacked. HWA is a small, aphidlike insect that varies from dark reddish-brown to purplish-black in color. As it matures, it produces a covering of wool-like wax filaments to protect itself and its eggs, which can be readily observed from late fall to early summer on the underside of the outermost branch tips of hemlocks. Ovisacs of the winter generation contain up to 300 eggs; spring ovisacs contain between 20 and 75 eggs. When hatched, nymphs search for feeding sites on twigs at the base of hemlock needles, and once settled, feed on young twig tissue and remain at that location throughout their development. Unlike closely related insects that feed on nutrients in sap, HWA feeds on stored starches critical to the tree’s growth and long-term survival. Hemlock decline and mortality typically occur within 4 to 10 years of infestation in HWA’s northern range, but can occur in as little as 3 to 6 years in its southern range. Other hemlock stressors, including drought, poor site conditions, and disease accelerate the rate and extent of mortality.