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Hemlock Woolly Adelgid General Information

WHAT IS THE ADELGID AND HOW DOES IT HARM HEMLOCKS?

The hemlock woolly adelgid (adelges tsugae), an aphid like insect introduced into this country from Japan, threatens to kill all unprotected hemlocks in western North Carolina and throughout the Appalachians.  The hemlock woolly adelgid (HWA) literally sucks the life out of hemlock trees; an adelgid, only a fraction of a millimeter long, attaches to the base of each needle, and begins feeding on the sugars and nutrients passing to and from the needle.  When HWA populations on a tree become high enough, the tree can not obtain sufficient energy and begins to die.  Without intervention, death of the tree occurs four to ten years after initial infestation depending on the health of the tree and other factors.

WHY IS THE ADELGID SO DEVASTATING?

Since our hemlocks did not evolve with the HWA, they have no known resistance to the insect, and HWA has no significant predators in this region.  HWA also reproduces at extremely high rates.  An adelgid can lay over 100 viable eggs at a time, and they go through two or three life cycles per year depending on winter temperatures.  And since they reproduce asexually, a single individual could grow to a population of 1,000,000 (100 x 100 x 100) within one year.  If unchecked, that prodigious reproductive capacity combined with the lack of environmental pressure allows a hemlock to go from lightly infested with only a few scattered adelgids to heavily infested and covered with adelgids in only a year.  HWA affects both native species of hemlock (eastern and Carolina), but fortunately affects no other species in the area.

WHAT ARE THE SYMPTOMS OF ADELGID INFESTATION?                    

Adelgids are most easily detected by their white, cottony, egg masses.  Adelgids produce egg masses in fall and spring on the underside of the twigs at the base each needle, but not on the needle itself.  The egg masses usually persist from October through June.  During late summer the adelgid may be much more difficult to detect directly since they appear tiny black specks attached at the base of needles.  The infestation usually begins on just a few isolated branches, which may be anywhere in the crown, and gradually spreads to encompass the entire tree and all needles.  Once the latter stage is reached, the foliage takes on a gray cast, the tips of twigs die, followed by needles beginning to fall off throughout the crown.  At that time, the adelgid population actually declines since the tree is producing so little energy.  The tree responds by drawing on stored energy reserves and beginning to grow again.  HWA then infests the new growth and the tree dies shortly afterwards.  The entire process takes four to ten years.

HOW CAN HEMLOCKS BE SAVED?

Fortunately, hemlocks can be protected from HWA, even after heavy infestation has occurred.  Appalachian Arborists has been treating hemlocks in western North Carolina since 2002, and we have not lost a tree yet!  Soil injection, stem injections and soap/dormant oil sprays are the primary means of controlling HWA.  For more information on methods available to treat hemlocks click here.

WHY TREAT HEMLOCKS?

Hemlocks provide aesthetic, monetary, and ecological benefits to homeowners.  Many people regard hemlock as one of our most beautiful native trees for the graceful sweep of their branches, rich dark green color, and overall pleasing form.  Their evergreen foliage and thick crowns make them ideal for screens and hedges, and make them an asset throughout the year.  Not surprisingly, studies have shown* healthy hemlocks increase property values while unhealthy hemlocks decrease home values.  Hemlocks can also help the bottom line by shading homes and reducing summer energy bills.  The costs for having a hemlock tree removed or replacing a hedge with a fence typically far exceed the costs of treatment.  From the perspective of wildlife, hemlocks cannot be replaced.  The shelter they provide attracts birds and other animals, and their dense shade plays an important role in regulating stream temperatures.

*Holmes, Thomas P.; Elizabeth A. Murphy, and Denise D. Royle.  “The Economic Impacts of Hemlock Woolly Adelgid on Residential Landscape Values: Sparta, New Jersey Case Study.”  Third Symposium on Hemlock Woolly Adelgid in the Eastern United States.

HOW DOES THE ADELGID SPREAD?

Most long distance transport of HWA is thought to occur on birds.  Once established on a property wind spreads HWA very effectively.  Consequently, when HWA is found on one tree in a property it is most likely on all of the trees.

HWA IN WESTERN NORTH CAROLINA

In 2001, HWA was first found in Cashiers and Montreat, but was likely present at low levels at least a few years earlier, especially around Linville and farther north.  Although the intensity of the infestation, and consequently the health of the hemlocks, varies greatly from area to area, by the spring of 2006, HWA was ubiquitous in the western North Carolina.  The adelgid has already killed some hemlocks along the Blue Ridge Parkway, around Linville Falls, and in Great Smoky Mountains National Park.

OTHER HEMLOCK PESTS

While HWA poses by far the greatest threat to hemlocks in western North Carolina, two other pests commonly infest hemlocks in region: elongate hemlock scale and spruce spider mite.  The scale, another small insect introduced from Asia, shows up as oblong grayish patches on the undersides of the needles themselves.  Spider mites are a native arachnid whose presence is identified by stippled yellowing of the half of needles closest to the twig.  Both pests primarily affect hedges and yard trees rather than forest trees, and do not kill otherwise healthy trees.  However, elongate scale can become damaging when hemlocks are weakened by HWA or other trees, and spider mite populations sometimes surge after hemlocks have been systemically treated for HWA.  Appalachian Arborists has experience controlling both of these pests.