Hemlock trees are very important to north Georgia. They shade our rivers and streams, allowing trout and other fish to survive. They firm up the soil around streams, preventing erosion and help to filter runoff water. They are majestic trees that add beauty and value to our properties.
Unfortunately, Georgia's hemlocks are facing a severe threat from a tiny insect called the Hemlock Wooly Adelgid.
The hemlock wooly adelgid was accidentally introduced to the eastern U.S. in the 1950s. It is native to China, Japan and the Pacific Northwest region of the US. It was first discovered in GA in 2003 in Rabun County. Wooly adelgids are now found almost anywhere in GA where hemlocks grow.
Adelgids are small aphid-like insects that feed on hemlocks by sucking the sap out at the base of the needles. They are usually identified by the white, cotton-like waxy coating they produce on the underside of hemlock stems. They are capable of killing a mature hemlock tree within four years of initial infestation. Adelgids are spread by birds, wind, and human activities.
Adelgids feed on all ages and sizes of hemlock trees. They use their straw-like mouthparts to suck starches out of a tree, reducing the tree's ability to put out new growth.
Infested hemlocks will show signs of decline after a few years. Branch dieback, heavy needle loss, and a grey color are all symptoms of hemlock wooly adelgid infestation.
There are a few options for chemical control of hemlock wooly adelgid. The easiest method is to perform a soil drench with a systemic chemical. For hemlocks with light to moderate adelgid damage, chemicals with the active ingredient ‘imidacloprid' are suitable. There are several brand names available that can be purchased at garden centers and feed supply stores. For hemlocks heavily infested, it is preferable to use a chemical containing "dinotefuran."
To apply these chemicals, mix the chemicals with water and pour the mixture into the soil around the base of the tree. During regular water uptake, the trees will also take up the chemical, which will distribute through the tree and kill the feeding adelgids. Always read the chemical label carefully for proper mixing instructions and safety precautions. Most of the available chemicals have treatment instructions for HWA on the labels. The label will give the proper dosage rate as a certain number of ounces per inch of tree diameter. It is best to remove any mulch or debris from around the base of the tree before applying the soil drench.
Soil applications are best made from March-June and from September-November when there is enough soil moisture for the hemlocks to actively take up the chemical.
Clark MacAllister is the Dawson County extension agent. For more information, call (706) 265-2442.