Hemlock woolly adelgid (adult)

Hemlock woolly adelgid

Hemlock woolly adelgid (crawler)

Hemlock Woolly Adelgid

The Hemlock Woolly Adelgid (HWA) is an aphid-like insect native to Asia. It was first recognized in the Eastern U.S. in Virginia in 1950's, and reached New York in the 1980's, though it has been present in the Western United States since 1924. The insect targets Eastern and Carolina Hemlock trees (Tsuga canadensis and Tsuga caroliniana).  As one study in Southern New England found, "severely damaged sites lost over 95% of understory T. canadensis trees, up to 90% of saplings, and from 50 to 69% of total stand density and basal area" 2.

The HWA is spread by the wind, by birds, or by humans or other animals- infestations do not spread by flying HWA. HWA produce two generations a year, and all are female and reproduce asexually. The HWA is active during the winter, which allows it to avoid predators (which are generally active during the summer) and to take advantage of the hemlock's increased energy intake during the winter.


Although the insect itself is tiny, during most times of the year the HWA can be found and recognized because of the woolly covering it creates to protect itself and it's eggs. They will be found on the underside of hemlock branches near the tips, and at the base of the needles. The wool is not as strong or silky as spider egg sacks, and is drier than spittle bug froth. 

What Happens?

The Hemlock Woolly Adelgid lives and feeds on hemlock trees. After the eggs hatch, the nymphs, or crawlers, move to the new needles of the tree and attach to the base of the needles to feed from the starch reserves and nutrients. This causes the needles to dessicate, dry up, and prevents the growth of buds. The dead needles fall off, and the decrease of buds prevents new growth. 

The hemlocks could also be reacting to the infestation by producing abnormal xylem, which could obstruct water movement and lead to the death of the tree because of water stress 3.

HWA infested trees can take as few as two years after the time of infestation until death, although 10-12 years is more common, and in the North infested trees have survived for as long as 20 years. 

Ecological Importance of Hemlocks

Hemlocks are considered a "keystone" species, meaning that they play a critical ecological role. The dense shade provided by their branches keeps the soil underneath from drying out, creates cool habitat during the summer for plants and foraging animals, and protects the same areas during the winter from snow and wind, again providing habitat and food. They are often found growing along streams, and the shade they provide cools the water and keeps the streams and their aquatic macroinvertibrates happy. Hemlocks are shade-tolerant and long-living (500+ years old), which allows them to come to dominate large stands, creating this distinctive ecosystem. For more details about the ecological role of hemlocks, read The Department of Conservation and Natural Resources' paper on the Value and Importance of Hemlock Ecosystems in the Eastern United States.

Economic Importance

Due to aesthetic preferences, a decline in forest cover or diversity is associated with a decline in property value. The presence of healthy vs unhealthy hemlocks affects the value of the property on which the trees are located, but also on the value of nearby properties 4.


1. The New York Invasive Species Clearinghouse, New York Invasive Species Information. Cornell Cooperative Extension Invasive Species Program.

2. Orwig, David A., Foster, David R. Forest Response to the Introduced Hemlock Woolly Adelgid in Southern New England, USA. "Journal of the Torrey Botanical Society". 125.1 (1998): 60-73. Web.

3. Rivera, Laura N., Jean- Christophe Domec, John Frampton, Fred Hain and John S. King. The Effect of Hemlock Woolly Adelgid (HWA) Infestation on Water Relations od Carolina and Eastern Hemlock: Can Ecophysiological Investigation of Tree Water Relations Improve Silvicultural Management of the HWA? Alliance of Saving Threatened Forests. 2010. 

4. Holmes, T. P., Murphy, E. A., Royle, D. D. The Economic Impacts of Hemlock Woolly Adelgid on Residential Landscape Values: Sparta, New Jersey case study. USDA Forest Service Southern Research Station. 2005. 

Last updated September 19, 2023