Pearleaf blister mite—Eriophyes (=Phytoptus) pyri
This eriophyid mite (family Eriophyidae) feeds on pears, causing discolored fruit and leaves, distorted fruit, and reduced fruit yield. Note that numerous species of mites can occur on plants.
Because eriophyids are microscopic, plant injury from their feeding as described below under "Damage" is what indicates these mites are present. Although with magnification the mites can be observed wandering across plant surfaces, they mostly occur hidden within buds and blistered tissue on the underside of leaves.
The carrot-shaped or wormlike mites are pale yellow to pinkish or whitish. The body is striated and has a few long hairs. Adults are about 1/100 inch (0.25 mm) long. A dissecting binocular microscope or a hand lens of at least 20× is needed to see them. Deutonymphs (first-stage immatures) are about one-half the length of adults. Protonymphs (second-stage immatures) are about the same length as adults. Adults and nymphs have 4 legs at the head (wider) end of their body. The oval eggs are translucent to white and about one-fourth the length of adults.
Pearleaf blister mite develops through four life stages: egg, protonymph, deutonymph, and adult. The mites overwinter beneath the scales of fruit and leaf buds and are generally most abundant in terminal buds. They can feed throughout winter and cause infested buds to drop. When buds remain viable and open in the spring, the mites feed on leaves and around blossoms.
Infested leaves develop blisters where the mites feed on the underside. Most blister development occurs in spring. Adult females lay eggs in the blisters and the hatching mites feed within the injured tissue. Each female lays about 1 to 2 dozen eggs during her life span. Mites move in and out of a blister through a small hole in its raised center. At maturity the adult mites commonly leave a blister and move to new locations to feed, causing development of more leaf blisters. The mites develop through several generations on leaves. Before leaf drop in fall, the mites leave the blisters and migrate to buds to overwinter.
Development from egg to reproductive adult occurs in 10 to 30 days, the quicker at warmer temperatures. Pearleaf blister mite has several generations per year
Mites feeding during winter can cause infested buds to blacken, become dry, and drop before spring. During the growing season, mite feeding on the underside of leaves causes swollen blisters on the upper leaf surface. Blisters are initially pale green, pinkish, or reddish but may darken to black or dark brown by the end of the growing season.
Feeding in fruit blossoms can kill them, causing a reduction in fruit yield. Fruit that develops can be misshapen and commonly develops brown, scabby scars (russeting) on the skin. Pears with naturally russeted surfaces (e.g., Bosc, Hardy, Nelis, and Winter) generally do not show the scabby damage of pearleaf blister mite.
In residential fruit trees, natural enemies may keep pearleaf blister mites under adequate biological control. The most important natural enemies are predaceous mites, such as Phytoseiulus species and western predatory mite. Various predatory insects are also important, especially the spider mite destroyer (a tiny black lady beetle) and sixspotted thrips.
To improve the effectiveness of biological control, control ants, minimize dust (e.g., periodically hose off small trees), and avoid the application of broad-spectrum, persistent insecticides and miticides for all pear pests. See Protecting Natural Enemies and Pollinators for more suggestions.
If mite damage has been common and intolerable, make an application of azadirachtin, narrow-range or horticultural oil, or neem oil after harvest during late October or November. Make sure soil has adequate moisture for roots before spraying oil; irrigate before applying oil if fall rainfall has been low. Thoroughly drench shoot terminals with spray to obtain adequate control.
Alternatively spray micronized or wettable sulfur on buds and terminals at the delayed-dormant stage, after buds swell but before most have opened. Sulfur can also be applied during November. Do not apply sulfur within 1 month before or after applying oil and vice versa or plant damage may occur. Once fruit is set in spring, the damage has already occurred and will become apparent as fruit and leaves grow. Spraying trees can only prevent or reduce damage the following growing season.
Adapted from An Illustrated Guide to Plant Abnormalities Caused by Eriophyid Mites in North America from the U.S. Department of Agriculture and Integrated Pest Management for Apples and Pears, Pest Management Guidelines: Pear, and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM).
Pale swellings on the underside of leaves due to feeding of pearleaf blister mites.
Brown, scabby discoloration on fruit and blackish discoloration on the underside of leaves from mite feeding.
Adult pearleaf blister mites magnified.