Black Scale

Black scale adult females are about 0.2 inch (5 mm) in diameter and dark brown or black with a prominent H-shaped ridge on the back. Young scales are yellow to orange crawlers and are found on the leaves and twigs of trees. A hand lens is usually needed to detect the crawlers. Black scales are in the soft scale family (Coccidae) and usually have one generation per year in interior valley olive-growing districts. In cooler, coastal regions multiple generations occur. Black scale prefers dense, unpruned portions of trees. Open, airy trees rarely support populations of black scale.

Young black scales excrete sticky, shiny honeydew on leaves of infested trees. At first, affected trees and leaves glisten and then become sooty and black in appearance as sooty mold fungus grows on the honeydew. Infestations reduce vigor and productivity of the tree. Continued feeding reduces bloom the following year. Olive pickers are reluctant to pick olive fruits covered with honeydew and sooty mold. Cultivars with dense canopies are more likely to be affected.

Biological control and pruning to open up closed canopies are key to managing black scale. Monitor to detect the presence of honeydew on leaves and track black scale population levels in olive groves. Control ants in the orchard because they disrupt biological control (for more information, see ANTS).

A number of parasites attack black scale; the most common are Metaphycus helvolus, M. bartletti, and Scutellista caerulea (=S. cyanea). These parasites, combined with proper pruning, provide sufficient control in northern and coastal orchards. In other regions, biological control is often ineffective because black scale's development pattern hampers parasite establishment. In addition, where insecticides such as carbamates (IRAC Group 1A), organophosphates (1B), and pyrethroids (3) are used, biological control will be disrupted. Be sure to monitor scale populations if disruptive insecticides are used.

Pruning to provide open, airy trees discourages black scale infestation and is preferred to chemical treatment.

Biological and cultural control and certain oil sprays are acceptable to use in an organically certified crop.

The first indication of black scale is usually the presence of honeydew on the leaves. Check 25 to 30 trees during April and October, the two periods of greatest honeydew accumulation, to get an idea of population levels.

Sample for adult scales in May, focusing on two to three areas in each block, particularly those that have had scale problems in the past.

1. First, select 10 trees in each area.
2. Count the number of adults on the terminal 18 inches of 10 branches on each tree; be sure to include the lower, inner, and outer sections of the tree.
3. Sum the numbers of black scale in each 10-branch sample and divide by the number of branches sampled to determine the infestation level.

Infestations fall into one of four levels: light (0 to fewer than an average of 1 per branch sampled), moderate (1 to 4), heavy (4 to 10), and severe (more than 10).

Light Infestations

These typically do not require treatment in open-canopy orchards. Closed-canopy orchards should be pruned and an application of a dormant oil considered.

Moderate Infestations

These may occur following a cool summer or within a closed orchard canopy. This level of scale infestation typically does not cause damage; however, it presents the potential for substantial damage and economic losses the next year. In trees with open canopies, the scale population should decrease or remain stable, depending on summer temperatures. If the summer is mild, apply a narrow range oil. If trees in the orchards have closed canopies, prune them and apply oil or an oil + insecticide combination treatment.

Heavy Infestations

These can cause economic damage; if left untreated, the next generation will inflict substantial crop loss. Heavy infestations are rare in open canopies, but orchards with closed canopies must be pruned, chemically treated, or both.

Severe Infestations

These occur in closed-canopy orchards in which treatment of moderate or heavy scale infestations is delayed. Economic loss can be extensive. Prune the orchard, removing severely damaged branches, and treat with an insecticide. The best application timing is after egg hatch to treat the crawlers (mid-July) but before August to avoid damage to the following year's crop.