How to Manage Pests

UC Pest Management Guidelines


Walnut Scale

Scientific Name: Quadraspidiotus juglansregiae

(Reviewed 12/07, updated 4/09)

In this Guideline:


The 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. If the circular scale covering is lifted off the female, the body underneath is yellowish and has indented margins; these two characteristics help distinguish walnut scale from other armored scales on walnuts.

The walnut scale has two generations a year in the Central Valley. It overwinters as second instar females and males. In spring, both sexes resume development and mature at the same time. Adult males emerge from the scale covering as tiny winged insects to mate with the mature females, who remain under the scale covering. After mating, females lay eggs in mid-May; eggs hatch in 2 to 3 days. Female crawlers move around the branches for a short time before they settle down, begin feeding, and secrete the scale cover. Male crawlers move to the margins of a female cover and settle. Initially the scale cover is white (white cap stage), but it changes to gray or brown after about a week. The first generation completes development by mid-July; females lay eggs in mid-August. These eggs hatch and the crawlers settle and molt once before winter.


Armored scales suck plant juices from the inner bark by inserting their mouth parts into twigs and branches. Infested trees look water stressed, and inside fruiting wood on lateral bearing cultivars may die back when encrusted with scale insects. Extremely heavy populations can cause the bark to crack; however, walnut scale rarely causes economic damage.


Natural enemies often can be relied on to keep walnut scale from causing damage. However, insecticide sprays may disrupt natural control and cause scale populations to flare up to damaging levels. If control is needed, sprays may be applied at the delayed dormant period, or, if oil is to be used, in late spring to target the susceptible crawler stage.

Biological Control
Several natural enemies of the walnut scale often hold it in check. Two predators—the twicestabbed lady beetle, Chilocorus orbus, and another small beetle, Cybocephalus californicus —often occur in large numbers and will control low to moderate population levels of the walnut scale. If populations are high, however, the time required for these two predators to bring the population numbers down may be excessive.

Two parasitic wasps, an Aphytis and an Encarsia species, also are prevalent on walnut orchards and help control this pest.

Organically Acceptable Methods
Biological control and sprays of narrow range oils can be used in an organically certified crop.

Monitoring and Treatment Decisions
Start monitoring for walnut scale during the dormant season to determine the need for a treatment. Walnut scale monitoring can be combined with the monitoring of other pests as described in DORMANT MONITORING.If a treatment becomes necessary, make it during the delayed dormant period before shoot growth begins. If a high degree of parasitization is observed, treatments may be delayed until after crawlers emerge in late spring and oils, which are less disruptive to natural enemies, can be used. Put out double-sided sticky tape (in May) to monitor for crawler emergence.

Common name Amount to Use** R.E.I.+ P.H.I.+
(trade name) (conc.) (dilute) (hours) (days)

Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy and impact on natural enemies and honey bees. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
CAUTION: Oils are not recommended for use during the dormant season on walnut trees.
  (Seize) 35WP 4–5 oz 1–1.25 oz 12 21
  COMMENTS: Do not exceed two applications/growing season or apply closer than 14 days apart. A nonionic surfactant may be added to increase efficacy. Apply concentrate in a minimum of 100 gal water/acre.
  (Supracide) 25W 8 lb 2 lb 3 days 7
  COMMENTS: Do not combine with oil or severe phytotoxicity may occur. Do not graze livestock in treated orchard. Do not apply more than once during dormancy or make more than one application during the growing season.
  (Lorsban) 4EC 4 pt 1 pt 24 14
  COMMENTS: Make no more than 2 applications/season. Levels in surface waters of this material that are high enough to be toxic to certain aquatic invertebrates have occurred following rains; avoid runoff into surface waters. Certain formulations emit high amounts of volatile organic compounds (VOCs); use low-VOC formulations (PDF). Regulations affect use for the San Joaquin Valley from May 1 to October 31, 2015 and 2016. Review the Department of Pesticide Regulation's updated fact sheet (PDF).
D. NARROW RANGE OIL# Label rates  
  MODE OF ACTION: Contact including smothering and barrier effects.
  COMMENTS: Oils will suppress low to moderate populations during the summer months, but oils can be destructive to the walnut aphid parasite, Trioxys pallidus. Do not apply if trees have suffered from a lack of adequate soil moisture or other stressing factors (insects, disease damage, etc.) at any time during the year or if temperatures are expected to exceed 90°F at time of application. 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). For additional information, see their Web site at
** For concentrate application, use the amount given in 80–100 gal water/acre, or lower if the label allows; for dilute application, amount is per 100 gal water to be applied in 300–500 gal water/acre, according to label.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Walnut
UC ANR Publication 3471

Insects and Mites

  • C. Pickel, UC IPM Program/UC Cooperative Extension, Sutter/Yuba counties
  • J. A. Grant, UC Cooperative Extension, San Joaquin County
  • W. J. Bentley, UC IPM Program/Kearney Agricultural Center, Parlier
  • J. K. Hasey, UC Cooperative Extension, Sutter/Yuba counties
  • W. W. Coates, UC Cooperative Extension, San Benito County
  • R. A. Van Steenwyk, Insect Biology, UC Berkeley
Acknowledgment for contributions to Insects and Mites:
  • W. H. Olson, UC Cooperative Extension, Butte County
  • L. C. Hendricks, UC Cooperative Extension, Merced County
  • G. S. Sibbett, UC Cooperative Extension, Tulare County

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