A Hemlock in a Haystack: The search for an insect-resistant tree

by | Feb 13, 2013

Dead hemlocks along the Blue Ridge Parkway. Image: Ben Smith, Alliance for Saving Threatened Forests

Across western North Carolina, the landscape is changing.  Mountainous forests once ruled by hemlock trees are left with gray shadows where the evergreen giants previously stood.

The culprit to this massive forest fatality is the tiny hemlock woolly adelgid, an invasive insect from Asia. The adelgid has already infested and killed many of North Carolina’s naturally occurring hemlocks.  Infested trees look like those that are drought stressed, losing needles and, in this case, dying.

Efforts of the N.C. Forest Service and others to manage this insect using pesticides have saved some of our trees already, but scientists agree that a long-term solution is needed to restore and conserve North Carolina’s hemlocks.

Hemlocks naturally resistant to adelgids, such as those in Asia, would certainly be a long-term solution.  Resistance of our native hemlocks to adelgids, however, is rare.  The Alliance for Saving Threatened Forests, an organization based out of N.C. State University, is attempting to seek them out, however rare they may be. Led by Dr. Fred Hain, the Alliance endeavors to locate hemlocks in the forest that are surviving this insect assault without the protection of insecticides. If and when a survivor tree is located, it is tested to confirm it has not been treated with pesticides, then tested for its resistance to the adelgid. If the tree is found to be resistant, the tree would be cloned and bred with other hemlocks to establish resistance throughout our natural hemlock populations.

A young adelgid, settled at the base of a hemlock needle. Image: Kelly Oten, N.C. State University

So, is it possible? Does a resistant hemlock naturally exist?  Researchers at the University of Rhode Island certainly think so. Drs. Evan Preisser and Richard Casagrande have found several small hemlock stands that multiple rounds of testing have shown to be highly adelgid-resistant. This offers hope to the Alliance for Saving Threatened Forests, who expect resistant trees also exist in the south. Finding a resistant stand in the south would allow them to grow hemlocks that are resistant to the adelgid, as well as acclimated to the southern climate.

You can help!
The Alliance for Saving Threatened Forests needs eyes all over the state, looking for and reporting surviving hemlocks.  When found, potential survivors can be reported online. Hain says that the person who successfully finds a surviving hemlock just might have the variety named after them. Your name as a tree variety– has a ring to it, doesn’t it?

But the organization isn’t putting all of their eggs in one basket.  In addition to hunting for the elusive survivors, they are also exploring other options to develop a resistant hemlock. Breeding our native hemlocks with resistant hemlocks from Asia may produce offspring that carry the resistance gene. At the National Arboretum, hemlock breeding has already been successful in doing just that – the Carolina hemlock, which occurs exclusively in the southern Appalachians, has been bred with the highly resistant Chinese hemlock and produced a resistant hybrid.  The current challenge is to breed the Eastern hemlock, which is ubiquitous throughout the Eastern U.S., with the Chinese hemlock.

As with the story of the American chestnut, there is still hope for the hemlock and its return to reign in Western North Carolina, especially if the Alliance for Saving Threatened Forests has anything to say about it!

A healthy hemlock, found surrounded by dead hemlocks. Could this be a resistant hemlock? Only time and extensive testing will tell. Image: Don Roach, N.C. Forest Service