The walnut blister mite occasionally occurs in walnut orchards. Adult mites are very small and cannot be seen without a 14 to 20X hand lens. They have a white, slender, striated body with a few long hairs. Immature forms resemble adults but are smaller. Eggs are spherical and pearly white.
Blister mite feeds on the lower surface of leaflets, causing characteristic blister-like swellings on the upper surface of leaflets. It occurs in yellow to orange felty masses in depressions on the underside of blistered leaves. Later in the season, these areas turn brown. The blisters can be large and unsightly, but damage caused by the walnut blister mite is primarily aesthetic.
Blister mites do not cause economic damage to walnut trees and no control is necessary.