Oak Pit Scales

Pest Notes: Introduction

Several Asterodiapsis species of pit scales (family Asterolecaniidae) attack many of the common deciduous and evergreen oaks that grow in California, with the valley oak, Quercus lobata, affected more frequently and severely than other species. Damage can be serious in oaks along the Central Coast as well as in the Central Valley.

Damage

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 may remain throughout the winter, giving the tree an unsightly appearance; however, don’t confuse this with the natural retention of dead leaves characteristic of a few oak species such as pin oak and Turkish oak. A severe scale infestation delays the leafing out of deciduous oaks for as long as three weeks in spring. Heavy attacks of pit scales year after year may kill young trees.

The pitting effect scales cause is most noticeable on the bark of younger twigs. Surrounding the pit is 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.

Life Cycle

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 are uncommon in California, although parasite exit holes may sometimes be observed. Insecticide application is the main tool currently available for managing these pests; however, it may be possible to prune out isolated areas of infestation if detected very early. Because heavy infestations over several years can kill young trees and weaken older ones, consider management actions as soon as problems are detected.

Pit scales can be managed using narrow range oil applied to trees in spring just before buds open. Be sure to cover all branch tips and bark 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% (1.5-2 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.

Although mixtures of oil and persistent insecticides such as pyrethroids applied in spring to kill hatching crawlers can be effective, these sprays aren’t generally recommended for large trees in residential areas because of the environmental hazards associated with applying these insecticides and the difficulty of getting adequate coverage with available application equipment.

References

Dreistadt SH, JK Clark, Flint ML. 2016. Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide. UC ANR Publication 3359. Oakland, CA.

Koehler CS, Brown LR, Eads CO. 1980. Pit Scales on Oak. UC ANR Publication 2543. Oakland, CA.

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