We have been listening to Paul Parent who has a call-in gardening radio show for many years out of Maine. After I read this thread I realized that he would be familiar with hemlock issues. This is from his weekly newsletter that he sends out for free to those who sign up. This is from an August, 2011 issue:
"If you have Canadian hemlock growing on your property, this is the time to examine the foliage of the plant for possible insect problems called wooly adelgids. The insect is most active in the early spring and again in the fall of the year. Look for the insect on the tips of the plant where the new growth was made during the summer months. The adelgids will be on the underside of the foliage in clusters, and they will resemble small dots of cotton attached to the foliage. This insect is sucking the energy from the plant's blood stream and also laying eggs that will hatch in the weeks to come. As the eggs hatch, they will move up the plant and feed on new branches killing them and repeating the process in the spring until the tree has died.
As a home owner it is not always possible to spray your trees, especially if they are tall and large. The best and most effective way to treat this plant is with a "systemic" product that will move up the plant protecting it from top to bottom without spraying the entire tree. Bayer Advanced has developed such a product called "Tree and Shrub." All you have to do is mix the product the product with water and pour it around the base of the tree. In a couple of weeks the tree will move the product out of the ground and take it to the top of the tree and to all of the branches, protecting it from this problem for 12 months. Your local garden center will have this product or a similar product made by Ortho, Bonide, or Ferti-Lome at their store for you to use. Be sure to read the directions before using, so the product will work effectively for you and control this insect pest."
I have used the Bayer Tree & Shrub on a hydrangea that had some sort of borer with great results. Lowes and HomeDepot carry it.
Also, here is the link to Paul Parents website which lists his contact information. His radio show is every Sunday from 6-10 am (stations are listed on his site
http://www.paulparent.com/). You don't have to wait too long to get on air and ask a question. I would urge anyone who has any type of gardening/landscaping question to call in. This guy really knows his stuff.