Laurent Bonneau, Research Asst, Dept of Natural Resource Management & Engineering;
Stephen H. Broderick, Extension Forester, Cooperative Extension System;
Dr. Daniel Civco, Dept of Natural Resource Management & Engineering.
University of Connecticut.
The eastern hemlock (Tsuga canadensis) has suffered significant decline in southern Connecticut over the past decade. Much of the decline has been attributed to an introduced insect called the hemlock woolly adelgid (Adelges tsugae)
Early research reports indicated that adelgid infestations usually resulted in widespread hemlock mortality within three to four years. This has led some loggers and professional foresters to recommend harvesting hemlocks at the first sign of the adelgid.
A recent study, completed at The University of Connecticut Department of Natural Resource Management and Engineering in cooperation with the U.S. Forest Service, has found that hemlocks can recover from adelgid infestations. In addition, site characteristics were found to play an important role in the health of the hemlocks and their resistance to long-term damage caused by the adelgid.
Because the hemlock woolly adelgid is an exotic species, it has no natural predators in our region. Researchers have looked for natural predators of the adelgid in Japan and China in an attempt to find a biologic control. Insecticidal soaps and horticultural oils provide effective control if branches are thoroughly soaked, including the undersides of each branch. While this form of spraying is virtually impossible in most forest applications, it can be used to protect infested hemlock hedges or individual ornamental trees.
Site characteristics clearly influenced the degree of decline and recovery of adelgid infested hemlocks. Hemlocks on north and northeast facing slopes appear to have suffered little or no damage from the adelgid. These slopes are cooler and moister than slopes facing other directions, which may result in healthier hemlocks that are more resistant to stresses caused by the adelgid.
Hemlocks located on ridge tops and upper slopes showed a greater decline in health than did hemlocks in adjacent valleys or riparian zones. The hemlocks along the banks of the Salmon River, for example, were healthier in 1995 than at any time in the prior ten years, even though adelgids were first observed there in 1985 and are still present. Ridge top sites are generally drier and hold thinner, more acidic and less fertile soils than sites lower down. Hemlocks on such sites, already stressed from the poorer growing conditions, may be more susceptible to damage from adelgid attack.
Hemlocks that became infested with the adelgid in combination with other insect pests showed the greatest decline in health. In Devil's Hopyard State Park, for example, where virtually all the dominant and co-dominant hemlocks have died, a hemlock looper (Lambdina fiscellaria fiscellaria and Lambdina fiscellaria athusaria) infestation coincided with the woolly adelgid attack. In the Selden Neck Preserve, where the hemlocks were rated in poor health, the hemlock scale (Fiorinia externa) was prevalent along with the woolly adelgid. While the Selden Neck hemlocks showed some recovery in 1995 it is still uncertain whether they will survive this combined attack.
It has been found that hemlock forests can resist and/or recover from infestation by the hemlock woolly adelgid. Hemlocks in valleys and on north and northeast slopes appear to have the greatest resistance to adelgid damage. On such sites, preemptive salvage harvesting at the first sign of adelgid infestation appears to be unwarranted.
Hemlocks located on ridge tops and upper slopes are most susceptible to severe damage from adelgid infestations. These stands should be monitored regularly. If the adelgid or its egg sacs are found, sample trees should be rated each year to measure changes in their health. Salvage harvesting should only be done if overall forest health has declined dramatically.
Other information on Hemlock Woolly Adelgid is available at the US Forest Service Hemlock Woolly Adelgid Newsletter site