I also have woolly adelgid on my hemlocks. It helps to look at the life cycle of the insect because their form is different as they progress through their life cycle, and you can't always see them. Cornell University publication Hemlock Woolly Adelgid: FAQ's is also helpful.

Not only do the insects slowly kill the host tree. They also weaken it and make it more susceptible to other pathogens and pests. The quote below is the answer given by Master Gardener Rick Bentz to a similar question about Eastern Hemlock

"The basal trunk treatment involves injection or drenching with imidacloprid and or dinotefuran. This treatment provides long term protection against wooly adelgid. The problem you describe may be either Fabrella needle blight or elongate hemlock scale. I have included links to both to help better identify the problem. This might require up close inspection of the affected parts of the tree. If you are uncertain after reviewing the attached links then I would recommend taking a sample in a plastic bag to your nearest Penn State extension office for more detailed diagnosis."
 http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/hemlock-diseases
http://ento.psu.edu/extension/factsheets/elongate-hemlock-scale