Damage is usually first noticed on the lower inside branches but spreads upward and outward as the infestation progresses. The mites spin fine silken webbing throughout the twigs and needles. The webbing is most noticeable on the underside of the branches where dust particles, dead needles and dead mites adhere. To check for mite infestations, vigorously shake a branch over a piece of white paper, then rub your hand across the paper or press the paper in half. If red smears appear, mites are probably present. Trees should be checked regularly since mite populations can increase rapidly.
More information: Spruce Spider Mite Facts