Description of the Pest
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 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 mouthparts 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 numbers can cause the bark to crack. What is of greater economic concern is that scale insects can increase Botryosphaeria infection and canker development.
Natural enemies parasitize some scale but cannot be relied on to keep walnut scale from causing damage. If scale insects are present, apply pesticides to reduce both scale and Botryosphaeria in orchards. Depending on the insecticide, apply either at delayed dormant (typically March in the Sacramento Valley or February in the San Joaquin Valley) or after crawlers emerge (typically May).
Several natural enemies can reduce the numbers of walnut scale. Two predators—the twicestabbed lady beetle, Chilocorus orbus, and another small beetle, Cybocephalus californicus —often occur in large numbers and may control low numbers of the walnut scale. Two parasitic wasps, an Aphytis and an Encarsia species, may be present in walnut orchards and parasitize this pest.
Organically Acceptable Methods
Use biological control and sprays of narrow range oils 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
Start monitoring for walnut scale during the dormant season to determine the need for an insecticide application. Walnut scale monitoring can be combined with the monitoring of other pests as described in DORMANT MONITORING.
If an insecticide application becomes necessary, make it during the delayed-dormant period before shoot growth begins, especially if using certain insect growth regulators (IGRs). If a high degree of parasitization is observed or when using an insecticide that must have foliage present, consider delaying applications until after crawlers emerge in late spring. Place double-sided sticky tape around limbs near adult scales in early spring (mid- to late April) to monitor for crawler emergence and time treatments.
|Common name||Amount per acre||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.|
|(Centaur WDG)||46 oz||12||60|
|MODE-OF-ACTION GROUP NUMBER1: 16|
|(Seize 35WP)||4–5 oz||12||21|
|MODE-OF-ACTION GROUP NUMBER1: 7C|
|COMMENTS: A nonionic surfactant may be added to increase efficacy. Apply concentrate in a minimum of 100 gal water/acre.|
|(Centaur WDG)||46 oz||12||60|
|MODE-OF-ACTION GROUP NUMBER1:16|
|(Movento)||9 fl oz||24||7|
|MODE-OF-ACTION GROUP NUMBER1: 23|
|(Brigadier)*||12.8 fl oz||12||7|
|MODE-OF-ACTION GROUP NUMBER1: 3A/4A|
|D.||NARROW RANGE OIL#|
|(various products)||Label rates||See lable||See lable|
|MODE OF ACTION: Contact including smothering and barrier effects.|
|COMMENTS: Oils will suppress low to moderate numbers 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.|
|(Assail 70WP)||2.7–3.4 oz
|MODE-OF-ACTION GROUP NUMBER1: 4A|
|COMMENTS: Toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.|
|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.|