Several species of pit scales attack many of the common deciduous and evergreen oaks that grow in California. The valley oak, Quercus lobata, is attacked more frequently and severely than other species. Damage can be severe in oaks along the Central Coast as well in the Central Valley.

Pit scales suck juices from twigs and cause twig dieback, which first becomes apparent in mid- to late summer. Dead leaves and twigs on affected trees remain throughout the winter, giving the tree an unsightly appearance. A severe scale infestation delays leafing out of deciduous oaks for as long as 3 weeks in spring. Heavy attacks of pit scales year after year may kill young trees.

The pitting effect caused by scales is most noticeable on the bark of younger twigs. Pits are surrounded by a doughnut-shaped swelling with the scale in the center. If there are large numbers of scales, the pits coalesce, making the twig surface appear roughened and dimpled.

The adult scale is a brown or dull green, flattened, circular, immobile insect about the size of the head of a pin. Immature scales, known as crawlers, are difficult to see without a magnifying glass.

Adult scales, all of which are female, produce living young from April through October in northern California; maximum numbers of young are produced in May and June. The immature scale then moves about for several days before settling on a twig where it remains for the rest of its life. The crawlers enlarge by late fall, and the cycle begins again the following spring.

Natural enemies of the pit scale are uncommon in California although parasite exit holes may sometimes be observed. Insecticide application is the only tool currently available for managing these pests. Because heavy infestations over several years can kill young trees and weaken older ones, consider management actions as soon as problems are detected.

Soil injections of the insecticide imidacloprid (Merit) will give the best control with least environmental impact. These applications must be made by a professional pest control applicator.

Residents wishing to treat deciduous trees in their landscapes can apply narrow-range insecticidal oil to trees in spring just before the buds open. Be sure to cover all bark and branch tips thoroughly. Thorough coverage can be difficult with very large trees unless high-pressure equipment is used. However, infestations on small to medium-sized trees can be adequately managed with one annual application over several consecutive years. Insecticidal oil should be diluted to a solution of 1.5 to 2% (1.5 to 2 parts oil to 100 parts water). Once leaves are on the tree, it is too difficult to get the degree of coverage needed for oil treatments to be effective.

Mixtures of oil and organophosphate insecticides applied in spring to kill hatching crawlers are not recommended because of the environmental hazards associated with applying these insecticides to large trees in residential areas, the difficulty of getting adequate coverage with available application equipment, and the availability of safer, more effective alternatives.

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