Purple Scale

Purple scale is one of the armored scales. The cover of the adult female purple scale resembles a mussel shell in shape. The immature male cover is shorter and much narrower than the female cover. Mature males are winged insects that search out the immobile females. After mating, females lay 40 to 80 eggs under the cover. After egg hatch, crawlers emerge from under the cover and settle on branches, twigs, leaves, or fruit and begin to form their covers. They are covered with a mass of waxy threads until about half grown; at that time a brown cover with a purplish tinge forms. Purple scales prefer the cooler, shady parts of trees; temperatures above 80°F (27°C) greatly reduce scale numbers. Two generations occur between May and October and a third may be partially completed before cold weather starts.

Purple scale is an occasional pest in certain coastal areas where the mild climate and humid conditions favor its buildup. It attacks all parts of the tree. Its feeding causes yellowish halos to develop on leaves; on young fruit, the feeding sites remain green. When there are high numbers of scale, defoliation and twig dieback can occur; this usually takes place in limited patches on the lower north side of trees.

Parasites usually provide good control of purple scale. Biological control may require supplementary insecticide applications at times, especially on dusty trees next to dirt roads. Controlling Argentine ants will also assist with purple scale biological control.

The most effective purple scale parasite is Aphytis lepidosaphes, a parasitic wasp that is generally distributed in areas where purple scale occurs. This parasite develops externally on the body of immature scales under the scale cover. Because this parasite is not commercially available, conserve naturally occurring populations of this beneficial in the grove. If insecticide applications are necessary, during August and September either spot treat (i.e., spray only those trees with high numbers of purple scale) or spray every fourth to sixth row at 4- to 6-week intervals if the entire grove is infested. This will assist in preserving natural enemies.

Several predators including the twicestabbed lady beetle, Chilocorus spp., and the Australian lady beetle, Rhyzobius (Lindorus) lophanthae, are important.

Use biological control and organically acceptable oil sprays on organically certified citrus.

If a spray is needed, it may be sufficient to spot treat (i.e., spray only those trees with high numbers of purple scale) with an oil spray or wash dusty trees with water. Oil sprays for the California red scale also control the purple scale.