San Jose scale—Diaspidiotus (=Quadraspidiotus) perniciosus
This armored scale (family Diaspididae) can be a serious pest of nut and stone fruit trees and has numerous other hosts.
Identification
Crawlers (mobile first instars) are bright yellow, about the size of the sharp end of a pin, and have well developed antennae, eyes, and legs. After settling to feed, crawlers lose their appendages and become immobile. Settled first instars soon secrete a white cover (the white-cap stage), which later darkens (the black cap).
The cover of adults and older nymphs is gray or brown and gray with a raised, gold-colored nipple (skin of earlier nymph) near the center. As they grow the cover of males becomes elongate whereas the cover of females remains round. Female covers are 1/25 to 1/12 inch in diameter at maturity. The female body remains beneath the cover and is roundish or deltoid shaped, smooth, and yellow.
At maturity the elongated male covers are smaller than covers of females. The adult male emerges from beneath its cover and is a delicate, orange insect with long antennae and one pair of wings.
The cover of female San Jose scales resembles that of olive scale (Parlatoria oleae), except that the female olive scale’s body beneath its cover is purple. Both species can occur on many of the same hosts, but olive scale is under excellent biological control and is rarely abundant unless its natural enemies are disrupted.
Walnut scale (Diaspidiotus juglansregiae) also resembles San Jose scale, except that under its cover, the margin of the female’s yellow body has distinct indentations. Walnut scale is often found in daisy-shaped groups that develop when male crawlers settle under the margin of the circular female cover and begin forming their elongated covers adjacent to their mother. Walnut scale is usually abundant only on ash, birch, and walnut.
Life cycle
San Jose scale overwinters on bark mostly as the black cap (older first instars). In the spring, the scales grow and the roundish females become distinguishable from the elongated, immature males. Males molt four times, whereas females molt twice.
Winged adult males begin emerging in March and April and females produce a sex pheromone that attracts the adult males. About a month after mating, the first generation of crawlers appears in about May. San Jose scale has 3 to 4 generations per growing season, taking about 7 to 8 weeks per generation during warm weather.
Damage
San Jose scale generally is a serious pest only on nut trees and stone fruits. It sucks juices from limbs and twigs and injects a toxic saliva. When abundant, its feeding causes a loss of tree vigor, growth, and productivity and the death of limbs.
If scales are abundant and not controlled, they can kill fruit spurs and scaffold limbs within 1 to 3 years and cause trees to decline. After deciduous plants drop their leaves in the fall, brown dead foliage often remains on heavily infested branches that were killed by the scale. A red halo is produced around a feeding site on 1-year-old green wood.
San Jose scale can also be a pest on roses, generally only when the roses grow near infested fruit or nut trees. Hosts on which San Jose scale is generally not abundant enough to cause damage include acacia, aspen, cottonwood, maple, mulberry, poplar, pyracantha, and willow. See ScaleNet from the U.S. Department of Agriculture for a complete list of hosts and references for more information.
Solutions
Numerous parasitic wasps and predatory insects feed on San Jose scale as listed at ScaleNet. The twicestabbed lady beetle, a dark, pinhead-sized nitidulid beetle (Cybocephalus californicus), and tiny parasitic wasps, including Aphytis and Encarsia species, are important natural enemies of San Jose scale. To conserve (preserve) natural enemies and improve biological control of scales, control ants, reduce dustiness (e.g., periodically hose off shrubs), and avoid the use of persistent, broad-spectrum insecticides and miticides. See Protecting Natural Enemies and Pollinators for more suggestions.
If spraying insecticide is warranted, use horticultural or narrow-range oil or another insecticide least disruptive to biological control. Horticultural oil can be sprayed during the dormant season, except not on plum and walnut which may be damaged by dormant-season oil. If spraying during the foliage season, monitor scale crawlers beginning in April with double-sided sticky tape and spray 2 to 3 weeks after the bright yellow, pin-point-sized scale crawlers are first observed in the tape traps. Alternatively, on high-value trees, use pheromone traps and degree-day monitoring to time applications as described in the UC IPM Pest Management Guidelines: Almonds. See the section "Monitoring" in Pest Notes: Scales for how to effectively time applications using sticky tape traps.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM) and The Scale Insects of California Part 3: The Armored Scales (Homoptera: Diaspididae), California Department of Food and Agriculture.
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San Jose scales on old bark.
Nymphs, immature male (the elongated cover), and mature female (center) San Jose scales.
The bodies of female San Jose scale with covers removed.
Yellow crawlers and the white covers of settled, first instars.
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