How to Manage Pests

UC Pest Management Guidelines


Frosted Scale and European Fruit Lecanium

Scientific names:
Frosted scale: Parthenolecanium pruinosum
European fruit lecanium: Parthenolecanium corni

(Reviewed 12/07, updated 12/07)

In this Guideline:


The frosted scale is the most important soft scale pest of walnuts. Its frostlike cover and copius honeydew secretion in spring make it easy to recognize. This scale has one generation a year. It overwinters as a nymph on twigs and small branches. In spring it grows rapidly, becomes convex, forms a frostlike waxy cover, and secretes large amounts of honeydew. In late spring females lay many eggs, which fill the entire space beneath their cover, and die after egg production. The white waxy substance weathers away, leaving oval, dark brown covers that may be present for a year or more.

Newly hatched nymphs or crawlers emerge from beneath the scale cover from late May through June and settle mostly on the underside of leaves. Here they feed for the rest of the summer. In fall, the nymphs molt and move back to twigs.

The European fruit lecanium has essentially the same life cycle. The immature stages closely resemble those of the frosted scale, but the adults do not form the thick, frostlike cover in spring. Instead, the cover is domed, shiny brown, and about 0.25 inch (6 mm) in diameter with several ridges along the back.


Soft scales suck plant juices from leaves and twigs. Low to moderate populations apparently are not damaging, but heavy populations reduce terminal growth and vigor, resulting in smaller nuts and poor kernel quality. The secreted honeydew may cover nuts and favor the growth of sooty mold, increasing the chances for sunburn damage.


Parasites play an important role in controlling these soft scales. If this natural control is disrupted by adverse weather or by insecticides applied for other pests, a treatment may be required.

Biological Control
The most important of the parasitic wasps that attack these soft scales are Coccophagus, Encyrtus, and Metaphycus spp. The Metaphycus wasps produce several generations a year, compared with one generation of the scale, and parasitize all stages of the frosted scale except the eggs. Parasitized nymphs are almost black and have convex covers; unparasitized nymphs are flat and opaque. Several parasites commonly emerge from a single parasitized adult scale, leaving a perforated cover.

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

Monitoring and Treatment Decisions
Monitor scales during the dormant period. For details on how to monitor these scales with other pests see DORMANT MONITORING. Examine the previous season's growth on randomly selected trees throughout the orchard. If you find more than five nymphs per foot of last year's wood throughout the orchard and parasitism is not significant, a treatment is warranted. If a treatment is necessary, apply it during the delayed dormant period before rapid scale growth begins in early spring. Treatments applied to first instar scales on leaves are also effective. High populations of soft scales often result from the use of chemicals that are disruptive to parasites and predators. If a high degree of parasitization is observed, treatments may be delayed until after crawlers emerge in late spring. Set out double-sided sticky traps in early spring to monitor for crawlers.

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.
  (Supracide) 25WP 8 lb 2 lb see comments 7
  COMMENTS: Do not combine with oil or severe phytotoxicity may occur. Do not graze livestock in treated orchard. Make no more than one application during the dormant period or more than 2 cover sprays each season. Take mitigating measures to prevent this pesticide from entering surface water with orchard runoff. Restricted entry interval is 48 hours when 8 or less lb per acre used and 14 days when more than 8 lb per acre applied.
  (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).
C. NARROW RANGE OIL# Label rates 4 HS
  MODE OF ACTION: Contact including smothering and barrier effects.
  COMMENTS: Moderately effective on these scales during the delayed dormant period. Apply only after buds begin to swell as a dilute application in at least 300 gal/acre. An application in summer will suppress low-to-moderate populations. In most areas, oils can be applied to walnuts during the delayed dormant period (as buds begin to swell) and 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. 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). 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.
HS  Do not apply after husk split.
# 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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