This is a good close up photo from UGA of a moderate infestion.
HWA attaches at the base of the new needle growth QUICKLY, stopping new growth and causing needle drop and eventually killing the host tree. Mature HWA are a dark colored bug covered with a white wool like, waxy material which creates a safe environment for itself and its eggs. This white wool makes them easy to see and when crushed HWA leaves a yellow/orange colored stain.
Even though infestation WILL become extreme, new growth is stopped with only a very few of the HWA. The best success comes with early treatment. It is important to check your hemlock trees for infestation on a regular basis.