I'm often asked if the HWA is still a problem for these trees in Highlands. There is no question that the Hemlock Woolly Adelgid (HWA), an insect, is alive in well in the Highlands area. As long as there are Hemlock trees to feed on, the insect will stay with us for a long time. If you look at a branch of a Hemlock tree and you see this cottony white material present this time of year, you will confirm your trees are infested with the insect. There are several treatments that will save your Hemlock trees.
The two most common insecticides are Merit (Imidacloprid) and Safari or Zylam (both comprised of Dinotefuran). Merit is a popular insecticide developed by Bayer in the 1970's and available in big box stores in a number of formulations. Both work very well. Merit will protect the tree for a number of years (some research indicates up to 7 years or more) but it takes a significant amount of time to translocate through the tree (6 months or more). For heavy infestations, the latter two insecticides are more soluble and therefore move through the tree very quickly before the insect causes extensive damage. However, the residual is not as long, which means applications of this product are needed annually.
The white cottony material seen on the trees are a wax secretion that protects egg sacks and are only present once or twice during the year. Once the eggs hatch, the insects move into the needles, feeding on the nutrients in the tree. This is the best time to tell if your trees are infected with the insect.
At Highlands CC, I've taken the approach to use Zylam to clean up any infected trees and then follow up with Merit in year two.
If you have a small hedge or shorter trees that allow you the ability to reach the top of the trees with a sprayer, and you don't like the thought of using insecticides, there are horticultural soaps and oils that can work to control population numbers. In this case, timing of the application is critical to ensure you are killing a majority of the adult insects. I personally like the guaranteed control that insecticides provide, so I have no experience using the various horticultural soaps on the market.
Again, now is the best time of year to look at your Hemlock trees and determine if your trees are infested. Treatments can occur any time of the year that sap is flowing in the tree. This means March-November are good times to treat in our climate. You'd want to avoid treating in the middle of winter, when the trees are essentially dormant.