Citrus Rust Mite (Silver Mite)

This pest is known as the rust mite on oranges and the silver mite on lemons. It is an occasional pest in coastal areas of Southern California and is a problem in some years in inland Southern California growing areas. Citrus rust mite is about the same size as a bud mite and requires a hand lens to view; it is deeper yellow than the bud mite and wedge shaped. A generation may be completed in 1 to 2 weeks in summer, but development slows or stops in winter, depending on temperature.

The rust mite feeds on the outside exposed surface of fruit that is 0.5 inch (1.3 cm) or larger. Feeding destroys rind cells and the surface becomes silvery on lemons, rust brown on mature oranges, or black on green oranges. Rust mite damage is similar to broad mite damage, except that somewhat larger fruit are affected. Most rust mite damage occurs from late spring to late summer.

Citrus rust mite tends to occur together with broad mite but usually in greater numbers. Both species thrive in warm, humid conditions. Monitor rust mite from early spring through summer. On orange trees, look for rust mites on young foliage in early spring; by late spring, most of the population will be on fruit. On lemon, rust mites are mostly on fruit throughout the season. To identify previous infestations, check outside fruit for scarred rind tissue. To assess current season levels, examine small green fruit on the inside of the canopy. A 10X to 15X hand lens is necessary to identify these minute mites. They usually feed in protected places, such as the stylar end of the fruit. When mite numbers are high, the mites move over the entire fruit. No effective natural enemies are known, but general mite predators feed on rust mites at times.

Once you find one or more infested fruit and if rust mites were a problem the previous year, watch the orchard closely. Threshold levels depend on last year's rust mite problems and current market conditions. If the mite numbers increase quickly or if scarring appears, a pesticide application is generally required. In some cases, the infestation is localized and a spot treatment may be sufficient for control.