Agriculture: Walnut Pest Management Guidelines

San Jose Scale

  • Diaspidiotus (=Quadraspidiotus) perniciosus
  • Description of the Pest

    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.

    Biological Control

    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.

    Organically Acceptable Methods

    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).

    Monitoring and Treatment Decisions

    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.

    Common name Amount to use REI‡ PHI‡
    (Example trade name) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.
    Note: Oils are not recommended for use during the dormant season on walnut trees and should be applied with caution during the delayed dormant period. Do not apply between bud break and shoot elongation because they can injure the tree.
    (Seize 35WP) 4–5oz 12 21
    COMMENTS: Apply concentrate applications in a minimum of 100 gal water/acre. Timing of this product can be adjusted to provide some early-season control of codling moth.
    (Centaur WDG) 34.5–46 oz 12 60
    C. NARROW RANGE OIL# Label rates 4 0
    MODE OF ACTION: Contact including smothering and barrier effects.
    COMMENTS: An option primarily for organic growers; oil is destructive to the walnut aphid parasite, Trioxys pallidus. Apply as a dilute application in at least 300 gal/acre if applied in the delayed-dormant period. An application in summer will suppress low to moderate numbers. In most areas, oils can be applied in summer. However to avoid injury, the trees must not have suffered from a lack of adequate soil moisture or other stressing factor (insects, disease damage, etc.) at any time during the year and the temperature must not exceed 90°F at or shortly after time of application. Do not apply after husk split. If in doubt, check with your farm advisor. In any case, do not apply oils to walnuts during the dormant season or between bud break and shoot elongation. Check with certifier to determine which products are organically acceptable.
    1 Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest may occur.
    * Permit required from county agricultural commissioner for purchase or use.
    # Acceptable for use on organically grown crops.

    Important Links

    Text Updated: 07/17
    Treatment Table Updated: 07/17