Hemlock Wooly Adelgid - Identification and Control

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Introduction to the Hemlock Wooly Adelgid

An infested hemlock bough
An infested hemlock bough. Kim Nix

The Eastern Hemlock is not a tree of commercial importance, but rather, one of the most beautiful trees in the forest, extremely beneficial to wildlife, and improves our quality of water. 

Eastern hemlock and Carolina hemlock are shade tolerant and long-lived tree species found in eastern North America. Both survive well in the shade of an overstory, although eastern hemlock has adapted to a variety of soil types. The species natural range extends from Nova Scotia to northeastern Minnesota, southward into northern Georgia and Alabama, and east up the Appalachian Mountains.

The eastern and Carolina hemlock is now under attack and in the early stages of being decimated by the hemlock wooly adelgid (HWA) or Adelges tsugae. Adelgids are small, soft-bodied aphids that feed exclusively on coniferous plants using piercing-sucking mouth parts. They are an invasive insect and thought to be of Asian origin.

The cottony-covered insect hides in its own fluffy secretions and can only live on hemlock. The hemlock wooly adelgid was first found on ornamental eastern hemlock in 1954 in Richmond, Virginia, but was not considered a serious pest because it was easily controlled with pesticides. HWA became a pest of concern in the late 1980s as it spread into natural stands. It now threatens the entire hemlock population of the eastern United States.

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Where are You Most Likely to Find a Hemlock Wooly Aphid?

Map of HWA Infestations
Map of HWA Infestations. USFS

Take a look at this latest USFS infestation map for hemlock wooly aphid as presented at the latest third Symposium on Hemlock Woolly Adelgid in the Eastern United States. Insect infestations (red) generally follow the range of the eastern hemlock but are mainly confined to the Appalachian Mountains in the south and continue north to the mid-Hudson River Valley and southern New England.

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How Do I Identify a Hemlock Wooly Aphid?

HWA "Sac"
HWA "Sac". Kim Nix

The presence of white cottony masses on twigs and at the base of the hemlock needles is the most obvious indicator and good evidence of a hemlock woolly adelgid infestation. These masses or "sacs" resemble the tips of cotton swabs. They are present throughout the year but are most prominent in early spring.

The actual insect is not plainly visible as it protects itself and its eggs with their mass of fluffy white secretion. This "cover" actually makes it hard to control the aphid with chemicals.

HWA display several different forms during their life cycle, including winged and wingless adults. The females are oval, blackish-gray, and about 1mm in length. Newly hatched nymphs (crawlers) are approximately the same size, reddish-brown, and produce white/waxy tufts that cover their bodies throughout their life. The white-cottony masses are 3mm or more in diameter.

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What Does the Hemlock Wooly Aphid Do to a Tree?

Infested Hemlock
Infested Hemlock. Kim Nix

Hemlock wooly adelgids use piercing-sucking mouth parts and feed only on hemlock tree sap. Immature nymphs and adults damage trees by sucking sap from the twigs and at the base of the needles. The tree loses vigor and prematurely drops needles. This loss of vigor and loss of foliage can eventually cause the tree to die. If left uncontrolled, the adelgid can kill a tree in a single year.

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Is There Any Way to Control the Hemlock Wooly Adelgid?

Hemlock with Woolly Adelgid
Kim Nix

Hemlock wooly adelgid are difficult to control because the fluffy secretions protect it from pesticides. Late October is a good time to attempt control as the second generation begins to develop. Insecticidal soaps and horticultural oils are effective for HWA control with minimal harm to natural predators. Horticultural oil can be applied during winter and before new growth emerges in the spring. Oil sprays can damage hemlock during the growing season.

Two predatory beetles, Sasajiscymnus tsugae and Laricobius nigrinus, are being mass produced and released into HWA infested hemlock forests. These beetles feed exclusively on HWA. Although they will not prevent or eradicate HWA infestation, they are good management tools. The use of chemical control can maintain hemlock stands until S. tsugae and L. nigrinus can become established or until more effective biological control agents are discovered and introduced.

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Your Citation
Nix, Steve. "Hemlock Wooly Adelgid - Identification and Control." ThoughtCo, Sep. 3, 2021, thoughtco.com/hemlock-wooly-adelgid-identification-and-control-1342968. Nix, Steve. (2021, September 3). Hemlock Wooly Adelgid - Identification and Control. Retrieved from https://www.thoughtco.com/hemlock-wooly-adelgid-identification-and-control-1342968 Nix, Steve. "Hemlock Wooly Adelgid - Identification and Control." ThoughtCo. https://www.thoughtco.com/hemlock-wooly-adelgid-identification-and-control-1342968 (accessed April 1, 2023).