San Jose Scale

Female San Jose scale give birth to living young that emerge from under the edge of the scale covering. These tiny yellow crawlers wander in a random fashion until they find a suitable place to settle. Immediately upon settling, the crawlers insert their mouthparts into the host plant and begin feeding and secreting a white waxy material (white cap stage); eventually the waxy covering turns black and is known as the black cap stage.

San Jose scales overwinter predominantly in the black cap stage, although in mild years some adult mated females may also survive. In late January, these nymphs resume their growth. Immature male and female scales are indistinguishable until the first molt. At this time, the male scale covering begins to elongate, while the female's remains circular. Males molt a total of four times. Following the final molt, adult male scales emerge from the scale covering as tiny, yellow winged insects. They mate with the females, who remain under the scale covering. After about two months, crawlers begin to emerge from the females, usually in April; peak emergence is generally in early May.

There are usually four generations a year. Summer generations overlap and crawlers are present throughout summer and fall.

Infested trees look water stressed, and fruiting wood encrusted with scale insects may die back. The infested bark often cracks and dies, and heavily-infested scaffold limbs and branches die within 1 to 2 years.

In many orchards, San Jose scale is kept below damaging levels by natural enemies. High numbers of this scale often result from the use of chemicals that are disruptive to parasites and predators; generally San Jose scale is sporadically seen in Northern California. Where damaging populations do develop, the preferred method of control is to target the sensitive crawler stage with an oil spray during the growing season.

Many parasites and predators have been observed feeding on San Jose scale, including most of those listed for walnut scale. In undisturbed situations, these beneficials play a significant role in keeping San Jose scale numbers below economic levels. However, in situations where heavy numbers exist, these parasites and predators may not respond before severe damage occurs, so sprays may be needed.

Biological control and sprays of certain narrow range oils can be used in an organically certified crop. Caution should be taken when applying oils in walnuts to avoid injuring trees (see narrow range oil comments in the table below).

Monitor scales in the dormant period. For details on how to monitor San Jose scale with other pests, see DORMANT MONITORING. Examine scaffold limbs, branches, and prunings for the characteristic black caps and old scale bodies.

Apply an insecticide when there are more than an average of 5 black caps per foot of last year's wood and less than 90% parasitism. If numbers surpass the threshold, consider spraying during delayed dormancy to achieve the best coverage and control and to avoid killing natural enemies.

Insect growth regulators are good choices in an IPM program because they do not cause water quality problems. Pyriproxyfen (Seize) and buprofezin (Centaur) may be applied during dormancy (without oil) and during delayed dormancy.

• Photos of scale