They are highly specialized plant feeders. Damage appears to be slight. Heavy fine stippling induces a blemishing or staining of leaf surfaces and fruit. May result in a great loss of water. Thus, the modified leaf provides shelter as well as food.
Most galls are on the leaves but some are on flowers, petioles, green stems, and even roots.
Some erinea on the undersurface are domed up and distort the upper leaf surface. The pear leaf blister mite is an exception. Its feeding on the undersurfaces of young leaves in loosening buds results in raised blisters that later in the summer dry out, leaving dead areas on the expanded leaf blades. The pear leaf blister mite may kill the buds of pear trees in the spring. The buds die after the mites leave the cavities.