Decline of the Hemlock, Part 2

The last column discussed the decline of the hemlock and the implications for streams and aquatic wildlife. For anybody who missed it, hemlocks play a critical role in regulating stream flow and temperature, and maintain a unique and diverse assemblage of terrestrial and aquatic life. They are the only evergreen tree in the region that tends to grow in the areas around streams, and their loss will leave a hole in the forest ecosystem that will be difficult to fill with any other species. Because of their importance, researchers have been working to determine how they can stop or slow the path of the hemlock woolly adelgid — the rapidly spreading invasive pest that causes hemlock death within 5-10 years of infestation.

Insecticides and predator beetles are the two main strategies used to control the hemlock wooly adelgid (HWA). National Park biologists first tried an organic pesticide sprayed on by hand, though this turned out to be quite impractical and washed off in the rain. Strategies shifted toward inoculating trees with a pesticide injected into the soil or into the trees themselves.

This strategy seems to work better, though is still impractical in that it requires injecting every single tree and only lasts a few years. It does, however, give trees critical temporary protection as researchers attempt to slow the pests by other means; the most promising of which thus far comes in the form of introduced predator beetles that prey only on the adelgid. Though the technique shows potential and researchers have experimented with a number of different strategies, such as releasing eggs instead of grown beetles and using multiple species of beetles that are most active at different times during the year, the hemlocks have yet to achieve any significant relief. For this reason, effective pesticide injections in trees are still critical, and Coweeta scientists have been doing research to define the most efficient and effective ways to do this.

Researchers have found that the effectiveness of these insecticide injections vary based on injection method, timing of application and individual tree characteristics. Injected pesticides work by traveling up the xylem of the trees. Xylem is found in the sapwood and carries minerals and water absorbed from the soil by the roots up through the tree and to the leaves where photosynthesis occurs. Xylem also carries the pesticide from the injection site to the base of the needles where the adelgid feed on stored carbon that is food for the tree. Injected pesticides, therefore, effectively mix with the water and minerals that are flowing through the tree’s xylem.

Coweeta researchers suspected that using the same amount of insecticide on different hemlock trees could lead to different dosages if the chemical was diluted to differing degrees depending on the amount of water that the tree was taking in through its roots. To test their suspicion, they monitored hemlocks of varying sizes at different times of the year and in different locations. They were able to measure how much water use increased with tree size and during different weather conditions. Then, using their results, they created a model that specifies how much insecticide to use depending on tree size and current weather conditions.

Based on their study, they found that using the manufacturer’s recommended dosages resulted in small hemlocks receiving significantly higher concentrations of insecticide than larger trees. They also discovered that peak water uptake in hemlocks occurs in April, which happens to coincide with peak feeding time for the adelgid, and that injections are most effective when they do not penetrate more than 2 cm into the stem. Equipped with this information, managers can treat trees more effectively- ensuring that large trees get enough insecticide and preventing waste from overdosing on smaller trees.

The pesticides used to control HWA are non-selective, meaning they kill all insects, including beneficial insects. The pesticides are not to be used around streams or ponds. Your local cooperative extension agent can provide landowners with more details on how to properly and safely treat hemlock trees infested with the hemlock woolly adelgid.


This column is produced by members of the Coweeta Listening Project (CLP), a branch of the Coweeta Long Term Ecological Research Program. Views expressed here are not representative of the USDA Forest Service or the Coweeta Hydrologic Lab. Please share questions, comments, or suggestions for future topics at or Coweeta Listening Project, UGA, 210 Field St., Room 204, Athens, Georgia 30602.


Original Citation: The Coweeta Listening Project. Franklin Press. Column on "Science, Public Policy, Community." Page B6. Sept 2, 2011.