Pear Rust Mite

Pear rust mite is a sporadic but serious pest of fresh-market pears. It may become a pest if pyrethroids are used at any time of the year. An increase in pear rust mite populations is seen in orchards that have been under mating disruption for several years where miticide use has been reduced. Rust mites are so small that you need a 14 to 20X hand lens to see them. They are wedge-shaped, with the wider end at the head. Pale brown female mites overwinter in bark crevices or behind loose bud scales usually on 2- to 4-year-old wood. As buds open in spring they move to developing clusters, leaves, and fruit to feed. During the growing season, adults are pale white to cream-colored.

Pear rust mite feeds on the surface of fruit and foliage, causing a bronzing of the tissue. Injury to leaves may stunt the growth of young trees, but on older trees it is of minor importance compared to fruit damage. Soon after petal fall, damaging populations may develop on fruit around the calyx or stem end, giving a localized russetting to those areas. Feeding and russetting may spread over the entire fruit if mites are unchecked. Late-season feeding tends to be scattered more uniformly over the fruit surface, with the intensity of russetting determined by the number of mites and the length of their feeding period. Rust mites are not an economic pest of naturally russetted varieties and in these orchards are even considered beneficial because they serve as a predator food source.

Pear rust mite may increase in orchards where mating disruption is used to control codling moth because broad-spectrum insecticides are not being used. Dormant or delayed-dormant treatments will help suppress populations of this pest but control of pear rust mites is best obtained during the postharvest period. In-season treatments are necessary when monitoring indicates a need.

Rust mites do not come under complete biological control in unsprayed orchards, though they are heavily suppressed in most years. Rust mites become much more of a problem in sprayed orchards where predaceous mites are destroyed by pyrethroids and other materials, especially if high populations of rust mite are allowed to overwinter.

Biological control and sulfur sprays with or without oil are organically acceptable methods.

In February as the buds begin to swell, sample one fruiting spur from the treetop and one from eye level of 50 trees per 20-acre block for a total of 100 spurs. Check under loose bark and in crevices of 2- to 4-year-old wood and at the base of the bud for the presence of this mite. (Rust mites are so small that you need a 14 to 20X hand lens to see them). If two or more spurs are infested treat during the green tip to 1% bloom period. For more information, see DORMANT TO DELAYED-DORMANT SAMPLING.

At bloom, collect one flower cluster from the treetop and one from eye level of 50 trees for a total of 100 clusters. If any pear rust mites are present, treat. For more information about this sample, see SAMPLING AT BLOOM.

During the fruit development period, sample the orchard weekly. Examine 40 fruit clusters (as described in SAMPLING DURING FRUIT DEVELOPMENT). Look at fruit with a hand lens to examine the calyx area for presence of pear rust mite. After turn down, when the fruit calyx turns down toward the ground, pear rust mites may be located anywhere on the surface of the fruit. Treat if two or more pears have rust mites or any pear has more than 30 mites.

At harvest, assess your IPM program by monitoring fruit in the bins for pear rust mite damage. Sample 200 fruit per bin from 5 bins per orchard (or 20-acre block in large orchards) for a total of 1,000 fruit. (See HARVEST FRUIT SAMPLE for more information.)

Following harvest, examine 20 top shoots; treat if two or more shoots are infested. (See POSTHARVEST SURVEY for more details about sampling during this period.)