A local Canadian Hemlock in a front yard.

Hemlock Woolly Adelgid

Background – The Forest

In the eastern portion of North America there is a tree biologists know as Tsuga canadensis. The common name for this tree is the Canadian hemlock. You can read more about this tree and other  plant species in A Field Guide to Eastern Trees: Eastern United States and Canada, Including the Midwest or Guide To The Trees,Shrubs & Woody Vines of Tennessee. It is a unique tree in the forest and provides specialized habitat for other species. Typically in the Appalachian mountains the tree can be found inside sloping valleys above mountain streams.

A local Canadian Hemlock in a front yard.
A Canadian Hemlock in a front yard down the street from my home. In some forests that can be a very prolific species.

The Problem – Hemlock Woolly Adelgid

Hemlock Leaves
Healthy Hemlock leaves  Sorry it is blurry my camera doesn’t like close-ups.

Unfortunately, there is an aphid (a small sap sucking insect) which was introduced to North America from Asia. This aphid is called the Hemlock Woolly Adelgid (Adelges tsugae). This is an example of an exotic pest like many exotic species it has become problematic. The aphid kills hemlock trees and has been doing so for approximately 60 years in the Appalachian mountains. Initially the tree looks healthy then it develops white spots underneath the foliage. These white spots are actually the egg sacs of the aphids. The aphids bite the leaves at their base in order to get to the sap. Because of the damage to the base the leaves fall off. Within five to ten years the tree will die.

woolly adelgid on hemlock leaves
Hemlock leaves with woolly adelgid, see the white spots on the twig adjacent to the leaves. The white blobs are the egg sacs.

The Solution – Combating the Hemlock Woolly Adelgid

Currently there are two different approaches for controlling the aphid population. First, a chemical treatment can be applied to the tree. This will travel through the vascular system of the tree and when aphids eat the sap they will die. Using this technique trees can be protected for 5-10 years at a time. The major downside to chemical treatment is that harsh chemicals are introduced to the environment and may cause additional problems down the road (especially if used repeatedly and in large quantities).

A second major approach to combating the aphid is biological control. Biological control is utilizing another species to control a problematic species. I must admit anytime biological control is mentioned I shudder and I did not disappoint in this instance. When biological control works it works with great effectiveness. Consider, if one species is allowed to eat another species and multiply then it will continue to attack the species for a long time.

The problem with biological control systems is that when they fail they often do so by creating an entirely new set of problems. Often attacking other native species or out competing them. The problem is that once the species is loose and breeding in the wild it is virtually impossible to eliminate without severe destruction in the environment.

Once I learned that biological control was being used with hemlocks I was immediately concerned about this approach. I then read to learn what sorts of biological control were being utilized. Fortunately it appears that current approaches to biological control require a species to get approval. In order to get approved the species undergoes rigorous testing to attempt to determine any possible problems that may arise. Currently desirable traits for candidates for biological are that they are very specific in the species that they attack. This presumably limits the potential for problems.

A better solution in my opinion is genetic modification of the hemlock species. This can be achieved by two different methodologies. One process involves a breeding program where trees which can breed with the hemlock and have demonstrated resistance to the pest are combined in order to produce resistant children. A second process involves lab based genetic modification using sophisticated equipment and techniques. Both approaches are expensive and require a lot of work by scientists.

Conclusion – The Cat is Out of the Bag

In the case of the Canadian hemlock biological control is already underway and that means we can only wait and see how it ends. Hopefully it ends well and without problems. In my humble opinion biological control should be a last resort.  The species genetics can be protected with chemical treatment in isolated areas in controlled conservation areas. This allows time to approach the problem

Another Canadian hemlock in the neighbors front yard
Another Canadian hemlock in the front yard of a neighbors house.

genetically though there could be problems to other species which depend on the hemlock. Provided a plan with the chemical reagent can be implemented to protect the species genetics and other dependent species. Then genetic modification is a superior approach but it may take decades to accomplish.