What can be done to save large diseased Hemlock trees from pests?


I have some large diseased hemlocks that I’m hoping can be saved. It was suggested that I contact you regarding the possibility of using beetles to try to combat the pests. Can you please provide guidance? 


Jonathan Foster, Special Projects Assistant

As you may have told, Hemlock woolly adelgid is a bit of a scourge for our hemlocks and is somewhat inexorably encroaching into Maine. It’s difficult to treat and the state is currently pursuing a quarantine strategy to slow the spread. You can find out all you need to know about the pest and how to manage it on the State of Maine’s Hemlock Wooly Adelgid page.

Essentially, you have a multi-pronged approach available to you:

1) Cultural control–the HWA spreads by contact, so pruning back hemlock branches likely to come into contact with vehicles, pets, pedestrians, etc., as well as removing nearby bird feeders (birds are a primary vector for spread) will reduce transmission. Also maintaining hemlock health, including watering during droughts, is a crucial component to helping your trees fight off infection.

2) Mechanical control–remove heavily infested branches if they are reachable. Because the adelgid is an aphid-like organism that doesn’t easily reinvade a host if it’s removed from the plant, sometimes a good strong jet of water can dislodge and kill a great number of the insects. This may or may not be feasible, depending on the size of your tree.

3) Chemical intervention–you can read above about whole tree drenches with horticultural oil and/or insecticidal soap, as well as systemic soil drenches, as pesticide options. You will probably need to contact a local arborist or landscape operation with a pesticide applicator license if you choose to go this route, and keep in mind that the oil and soap are contact poisons, so they must be thoroughly applied and then reapplied after rainfall.