The mites puncture the epidermal cells on the surface, which then die and turn yellow. The scale-like leaves of thujas appear yellow-mottled from close up and dull-green from a distance when infested.
Heavily infested thujas turn brown and may wither. In the course of mass developments, the mites begin to move slowly from severely infested areas to parts of the tree that are still mite-free.
What remains are parts of the plants with withered needles: often, the last remains of empty egg skins are still visible on such deserted feeding areas. The remains are often the only evidence of spider-mite infestation when diagnosing damage.
The damage is often mistaken for dry-stress damage following a water shortage. A magnifying glass or, even better, a stereomicroscope is required for an exact diagnosis.