Adelges tsugae, the hemlock woolly adelgid (/əˈdɛl.dʒɪd/[1]) or HWA, is an insect of the order Hemiptera (true bugs) native to East Asia.

[2] Accidentally introduced to North America from Japan, HWA was first found in the eastern United States near Richmond, Virginia, in 1951.

Three times the length of its body, the stylet bundle pierces the host plant's parenchymatic ray tissue to derive nutrition from stored reserves.

[8] The presence of HWA can be identified by its egg sacs, which resemble small tufts of cotton clinging to the underside of hemlock branches.

[5] In its native Asian habitat, a third winged generation called sexupera occurs; although this generation's sexual reproduction requires a species of spruce not found in the Eastern United States, and therefore dies, Between 100 and 300 eggs are laid by each individual in the woolly egg sacs beneath the branches.

[11] S. tsugae is a black lady beetle that is relatively host-specific, feeding only on three known aldegid species, including HWA.

Since 1995, the Pennsylvania Department of Conservation and Natural Resources's Bureau of Forestry has released hundreds of thousands of adult S. tsugae beetles into affected hemlock forests of the eastern United States to determine its effectiveness at controlling the spread of the adelgid.

[12] From 1995 to 1997, experiments in Connecticut and Virginia found that releasing adult Sasajiscymnus tsugae beetles into infested hemlock stands resulted in a 47 to 88% reduction in adelgid densities within 5 months of introduction.

[14] The environmentally safest chemical control methods for treating individual trees are nontoxic insecticidal soap and horticultural oil.

[15] Toxic systemic insecticides may be applied to the foliage and bark of a tree and can persist in killing the adelgid for up to four years after application.

The chemical is injected directly into the tree and transported to the twigs and needles where the hemlock woolly adelgids are feeding.

Where hemlocks occur in pure stands in that region, the most commonly observed tree species to succeed it is black (sweet) birch.

[17] In the southern extreme of its range, hemlock typically occurs not in pure stands, but in linear riparian areas and other moist sites.

Succession in these areas is affected by the presence of Rhododendron maximum, which often coexists with hemlock, because a combination of influences restricts regeneration to shade and otherwise understory-tolerant plant species.

Brook trout is a native fish species to the eastern United States and is known to prefer the cool, shaded streams of hemlock forests during spawning events.