Live oak erineum mite—Eriophyes mackiei
This eriophyid mite (family Eriophyidae) causes glossy, green, raised blisters or swellings on the upper surface of oak leaves. The swellings may later turn brown. The mite in California infests coast live oak, canyon live oak, huckleberry oak, and interior live oak.
The mites occur in yellow to orange felty masses (erineum) in depressions on the underside of blistered leaves. They occur mainly in lower, shady portions of the tree canopy.
The mites are whitish or yellowish, slender, and wormlike and resemble the eriophyid mites found on citrus buds. Their 2 pairs of legs protrude from the head (wider end) of the mite. Because they are only about 1/100 inch (0.25 mm) long a hand lens of at least 20× or a dissecting binocular microscope is needed to see them.
Oak leaf blister fungus (Taphrina coerulescens) causes similar swellings on the upper surface of oak leaves. Its masses of pale spores on leaf surfaces resemble the felty masses caused by erineum mites. However, Taphrina spores can occur on either leaf surface while the felty colonies of erineum mites generally occur only on the underside of oak leaves.
Microscopic examination may be required to confidently distinguish between erineum mites and Taphrina spores. One detection method is to place infested plant tissue in a container with 90% ethyl alcohol. Shake this for about 10 seconds so tissue is thoroughly coated and eriophyids are killed and dislodged. At a magnification of at least 20× examine the fluid for pale, elongate eriophyids.
Eriophyids develop through four life stages: egg, protonymph, deutonymph, and adult. The mites overwinter in the erineum pockets on the underside of leaves and may also occur in buds. Mite feeding during spring when new leaves are produced causes the plant to produce the feltlike growths within which the mites feed. Oak leaf erineum mite has multiple generations per year.
Leaves infested with this erineum mite develop swellings on the upper surface and felty masses containing mites on the under surface. Infested leaves may become curled or otherwise distorted. This damage is harmless to oaks.
No control is known or needed for this aesthetic pest. Eriophyids can be difficult to control with pesticides. Where a pesticide will be applied, it may be best to delay application until the next growing season and spray terminals as new growth develops and buds swell. Potentially effective miticides include horticultural oil, neem oil, and wettable sulfur. Do not apply oil within about 3 weeks before or after spraying sulfur and vice versa or plant damage can occur.
For more information see A Field Guide to Insects and Diseases of California Oaks and An Illustrated Guide to Plant Abnormalities Caused by Eriophyid Mites in North America .
Adapted from the publications above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Swellings on leaves from feeding of live oak erineum mites.
Erineum (felty masses) on underside of leaves with live oak erineum mites.
Damage from oak leaf blister fungus, a look-alike malady.