Walnut Scale

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.

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).

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.