Soft Scales

Brown soft scale is a mottled, yellow-brown when young, becoming darker at maturity. It has three to five generations per year and is often found on the nut. Adult scales are flattened and elongated, resembling a football sliced in half and about 1/8 inch (3.3 mm) in length.

Black scale is similar in size to the brown scale, dark brown to black, later becoming more mottled black and brown. It has an H-shaped ridge on the dorsum. There is generally only one generation per year in the Central Valley. However, in some years two generations have been detected.

Frosted scale is elongated, slightly humped, and has a waxy, white frostlike coating in spring. It is about 1/4 inch (6.4 mm) at maturity and has one generation per year. It is found in the warmer interior growing areas.

European fruit lecanium is identical to the frosted scale but does not have the frosted coating. High temperatures early in summer will increase mortality of immature stages of European fruit lecanium.

Immature stages of these scale species are very small (about 1/64 inch) and difficult to distinguish from each other; look for the adults nearby to determine which species is present. Soft scales molt twice before they reach maturity. The stage just before the adult is frequently referred to as the rubber stage. In this stage, the scales remain soft, somewhat translucent and are still susceptible to parasitism. Upon molting to adults, the shell hardens and becomes opaque.

In spring, soft scales produce heavy amounts of honeydew, providing a substrate on which sooty mold grows. Sooty mold growth covers leaves and can affect photosynthesis. Moderate-to-high scale populations can also retard shoot growth and shell splitting. Scales are most common on vigorously growing trees.

These scales are normally kept under control by native parasites. Because of the increase in the use of permethrin treatments for true bugs in pistachio, natural enemies of scales are more frequently killed, and soft scales are becoming more prevalent.

There are a number of effective parasites of these soft scales. Most are tiny parasitic wasps in the genus Metaphycus. Metaphycus luteolus destroys the scale in its early instars before it can reproduce or cause substantial injury. This parasite produces several generations during each scale generation. In addition, predators, including the predaceous lady beetle, Rhyzobius (Lindorus) lophanthae, and green lacewings, Chrysopa and Chrysoperla species, feed on these scales.

There is a strong relationship between phytocoris bugs and soft scale. Where phytocoris bugs are plentiful, scale density is commonly low. During March and April, young adult soft scales (prior to forming their crustlike covering and laying eggs) and soft scale eggs are a primary food source for phytocoris bugs. In fall, phytocoris bugs feed on second-instar scales that are migrating from the leaves to the woody shoots.

Biological control and oil sprays in the dormant period are acceptable for use in an organically certified crop.

In early to mid-January, examine one-year-old fruiting wood for live and parasitized scale paying special attention to previously infested areas. Look at 12-inch dormant shoots that are randomly selected from throughout the orchard.

• A light-to-moderate population in early February would be an average of one to five live scale per inch of fruiting wood.
• A heavy population would be an average of 10 or more live scale per inch.

Other factors to consider when making treatment decisions are the age, vigor, and split nut percentages.

Apply a dormant treatment in mid-February before the scales reach the rubber stage of development. For optimal control, treat frosted scale before its waxy coating develops, and treat European fruit lecanium before it reaches the rubber stage in late February.

Depending upon weather conditions and scale pressure, re-treatment may not be necessary for 3 to 5 years.