Several Asterolecanium species of pit scales (family Asterolecaniidae) attack many of the common deciduous and evergreen oaks.
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 can 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.
MANAGEMENT Natural enemies of the pit scale include ladybugs and braconid wasps.
Deciduous trees in their landscapes can be treated with insecticidal oil in spring just before 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. Once leaves are on the tree, it is difficult to get the degree of coverage needed for oil treatments to be effective.
Photos: Tyson Woods