Pear rust mite—Epitrimerus pyri
Feeding by this rust mite (family Eriophyidae) causes brownish discoloration on pear fruit and leaves. On apple, only leaves are fed on and discolored.
Adults and nymphs of pear rust mites are wedge shaped with the head and two pairs of legs at the wider end. These tiny mites are 1/120 inch or less in length so you need at least a 20X hand lens to see them. They are more easily seen using a binocular microscope with good lighting. The tiny eggs are spherical, nearly clear when first laid, and become pale-straw colored before hatching.
These mites are often not noticed until their feeding damage appears and the cause is investigated. When overwintering on bark and buds, the mites are brown to reddish. During the growing season when they feed, they are cream-colored to whitish.
Darkened foliage can also be caused by the feeding of spider mites. In comparison with rust mites, spider mites are rounder and larger. Spider mites commonly have two dark spots and often produce webbing. Rust mites leave no webbing on plants.
Eriophyid mites develop through three life stages: egg, nymph, and adult. The mite overwinters as adult females beneath bud scales and on young bark, predominantly on branches 2 to 4 years old. As buds open in spring, the mites become active and adult females soon begin laying eggs. After hatching, immature mites grow rapidly through two instars.
In apples the mites feed primarily on the surface of leaves. In pears they move to developing leaf clusters and feed within buds. As pear leaves mature and harden, pear rust mites move either to young succulent leaf tissue or to fruit.
Soon after petal fall, aesthetically damaging populations may develop around the calyx or stem end or both of pear fruit. The russeting on the fruit becomes visible around late May or June, when pears have increased in size and turned down.
By late summer, only females are present, and they seek overwintering sites under scales of newly developed pear tissues. If tender foliage remains present, the females may remain active and feeding on leaves into November. Several generations occur each growing season.
Pear rust mites feed on the surface of apple and pear foliage and on pear fruit, causing a browning of the tissue. Injury to leaves if abundant may stunt the growth of young trees. If these mites are abundant and uncontrolled, the feeding and russeting may spread over the entire fruit, marring fruit appearance. Internal fruit quality is not affected.
The intensity of russeting is determined by the number of mites and the length of their feeding period. Pear rust mite is commonly not detected until tissues grow and expand, which is after the feeding damage has occurred.
In backyard fruit trees natural enemies can keep pear rust mites under fairly good control. To conserve predators of mites, avoid the application of residual (persistent), broad-spectrum insecticides and miticides for all pests, including carbamates (carbaryl), organophosphates (e.g., malathion) and pyrethroids ("-thrin" insecticides e.g., bifenthrin, permethrin). Control ants and reduce dustiness, such as by hosing off foliage of small trees. See Protecting Natural Enemies and Pollinators for more information.
Aesthetic damage to fruit surfaces generally does not justify treatment in backyard trees. If treatment is desired, best control is achieved with a fall application of micronized sulfur, or during the dormant season (after November 1) sulfur plus horticultural or narrow-range oil. To avoid plant damage, do not spray oil on trees that were drought-stressed during late summer or fall.
For more information see An Illustrated Guide to Plant Abnormalities Caused by Eriophyid Mites in North America (PDF).