Farm advisors are not seeing a need to treat for walnut scale on a yearly basis. Area Integrated Pest Management Advisor Emily Symmes says that makes monitoring and sampling ever so important. She added that growers will see an additional benefit if they are able to stretch out those applications as populations of natural defenses should rise.
More About Management
From UC IPM website: Many species are usually well controlled by beneficial predators and parasites (natural enemies). Exceptions are when natural enemies are disrupted by ants, dust, or the application of persistent broad-spectrum insecticides. Preserving (conserving) parasites and predators (such as by controlling pest-tending ants) may be enough to bring about gradual control of certain scales as natural enemies become more abundant.
A well-timed and thorough spray of horticultural (narrow-range) oil during the dormant season, or soon after scale crawlers are active in late winter to early summer, can provide good control of most species of scale. Certain scale problems on large plants and hosts especially sensitive to scale damage may warrant the application of a systemic insecticide. If plants perform poorly or are repeatedly damaged by pests, the best course of action may be to replace the plant with a pest-resistant species or cultivar that is better adapted to the site conditions. Read more from the UC IPM website.