Scale infestation in walnut orchards has increased in recent years. The abundance and distribution of the various scale species in different regions and on different cultivars is not well understood. Walnut scale appears to be the primary scale that colonizes walnut, but frosted scale, European fruit lecanium, San Jose scale, and Italian pear scale are also found in walnut orchards. Each species has a different life cycle, and previous studies in walnuts have shown that scales have different susceptibilities to insecticides depending on species. Proper identification of scales and a better understanding of their seasonal phenology is important for effective management.
Multiple factors may be contributing to the recent increase in scales. Possibilities include decreased parasitism, increased hyperparasitism (parasitism of the parasites), changes in cultural practices, and reductions in broad-spectrum insecticide use for other pests. Information is needed to develop a more detailed management approach.
To further complicate matters, scale feeding is associated with an increased incidence of Botryosphaeria infection, which causes canker development and can kill major branches in older trees. The increase in Botryosphaeria cankers has lowered the tolerance levels of scale in orchards.
Area IPM Advisor Emily Symmes, Area IPM Advisor Kris Tollerup and others in UCCE—Richard Buchner, Janet Caprile, Elizabeth Fitchner, Joseph Grant, Janine Hasey, Dani Lightle, Nick Mills and Katherine Pope—are working to improve understanding of the scale species present in walnut orchards in several key walnut-producing counties in California (Butte, Contra Costa, San Joaquin, Tehama, Tulare, Yolo and Yuba-Sutter). Their research thus far has involved identifying the predominant scale species, tracking the life cycles, and evaluating parasitism rates of walnut scale as a possible cause for the recent increase in scale numbers.
Results of this research will enable growers and pest control advisers to manage scales more effectively using the appropriate thresholds, techniques, and timings. Effective scale management may also reduce the incidence of Botryosphaeria dieback, reducing the need for fungicides. Only applying insecticides or fungicides when needed could reduce the amount of pesticides applied to the orchard and increase protection of the environment and human health.