Pear rust mite—Epitrimerus pyri
Adult pear rust mites are tiny whitish brown, wedge-shaped mites.
Pear rust mites are very small – only about 0.2 mm long–so you need at least a 10X lens to see them. Pear rust mite is often not detected until after damage had occurred. The russetting on the fruit becomes visible around late May or June, when pears have increased in size and turned down. On occasion, late-season populations of pear rust mites may be present but feeding only on tender foliage.
Don't confuse rust mites with spider mites, which are rounder and larger, usually have two spots, and often produce webbing. Predatory mites are clear and move rapidly.
Female mites overwinter beneath bud scales and bark predominantly on 1- to 2-year-old wood. As buds open in spring, mites emerge, move to developing clusters, and feed within buds. As clusters open and leaves expand, some pear rust mites move to the leaf tissue and feed until leaves mature and harden, while large numbers remain on the fruit. As leaves mature, pear rust mites move either to young succulent leaf tissue or to fruit.
Eggs are produced shortly after adults become active in spring. Eggs are spherical and nearly clear when first laid. Immature mites grow rapidly through two instars.
Several generations occur each growing season; each takes only 10 to 14 days under ideal conditions. During the growing season, adults are pale white to cream colored. By late summer, only females are present, and they seek overwintering sites under scales of newly developed buds or in protected areas on 1- and 2-year-old twigs, unless tender new foliage is present, in which case they may remain active into November.
Pear rust mites feed on the surface of fruit and foliage, causing a bronzing of the tissue. Injury to leaves may stunt the growth of young trees. After petal fall, populations may develop on fruit around the calyx or stem end, giving a localized russetting to those areas; russetting may spread over the entire fruit surface.
In backyard situations, natural enemies usually keep pear rust mites under fairly good control. Damage does not generally justify treatment in backyard trees. If treatment is necessary, best control is achieved with a fall application of sulfur and oil in November.