- (Public Value) UCANR: Promoting economic prosperity in California
- Author: Elizabeth J Fichtner
- Author: Charlotte Burks
- Author: Mel Thayer
- Author: Robert Van Steenwyk
Walnut scale (Figure 1A) is an important economic pest of walnuts in California. High populations of walnut scale may affect tree vigor as well as predispose trees to diseases caused by several plant pathogenic fungi. Historic UC Pest Management Guidelines emphasize the efficacy of insecticide applications at the delayed dormant (March) stage of plant development or at the early crawler stage (late April-early May) (Figure 1B) of insect development. Both walnut scale and frosted scale overwinter as immature nymphs; therefore dormant application of insect growth regulators (IGRs) has the potential to inhibit maturation and subsequent reproduction of these pests. New research is needed to evaluate the efficacy of dormant IGR treatments for management of walnut and frosted scale.
In 2023, UC ANR and UC Berkeley researchers initiated a new collaborative study investigating the efficacy of four products applied at various rates and times (Table 1). Two insect growth regulator products, Esteem® (IRAC Group 7C) and Centaur® (IRAC Group 16), were included in the study, with both dormant (February 8, 2023) and early crawler-stage (April 26, 2023) application timings. Centaur® was also investigated at two rates. Early crawler-stage applications of Senstar (a combination of spirotetramat and pyroxifin) and Assail 20SG (a neonicitinoid) were also included in the study.
The study will address the impact of dormant IGR applications on populations of viable adult frosted scale and walnut scale in late April, just prior to anticipated walnut scale crawler emergence. Additionally, the influence of all treatments on walnut scale crawler populations will be evaluated. The results from the study will be shared on the UCCE San Joaquin Valley Trees and Vines website (https://www.sjvtandv.com/) and in the UCCE Tulare County's “In a Nutshell newsletter”.
- Author: Elizabeth J Fichtner
As California's nut industry has grown, so too have the opportunities for UC ANR farm advisors and specialists to present the latest innovative research topics to the state's nut growers, crop consultants and farm managers. UCCE farm advisors utilize large industry-sponsored trade show venues and magazines to extend unbiased, third-party generated data to our primary clientele groups who produce California's almonds, pistachios, walnuts, and pecans. The partnerships between our applied research community and agricultural marketing companies such as Malcolm Media and Jason Scott Marketing, publishers of Pacific Nut Producer (PNP) and West Coast Nut (WCN), respectively, magnifies the reach of UCCE's extension efforts beyond county lines, thus fostering regional and statewide impacts of otherwise local programs.
Annual trade shows such as the California Tree Nut Conference, the Tree and Vine Expo, and the California Walnut Conference attract hundreds of growers to centralized locations in the Central Valley. The educational content at these conferences, including presentations and oversight of panel discussions, is contributed by UC ANR academics with wide-ranging research expertise. The value of this partnership is underscored by the unbiased role of UC scientists in the research arena, particularly at venues designed primarily as marketing events. These trade show events generally engage over 1000 growers per event, allowing UC-generated information to reach greater numbers of contacts than that of our local meetings or newsletter publications. The added benefit is that the costs of the venue and associated luncheon are covered by the corporate sponsors presenting their products in the trade areas.
The farm advisor community similarly capitalizes on the high subscriber numbers of serial publications to extend written extension materials to our clientele base through the state. PNP, established in 1995, reaches nearly 10,000 subscribers and has relied on the UC ANR farm advisor community for relevant, science-based editorial content. For example, Brent Holtz, PhD, UC ANR has authored the monthly Almond Tasks editorial for 15 years. The promulgation of information is heightened by both the circulation volume and the availability of an online archive of articles.
The rich editorial provided by trade magazines includes articles written by UC academics and associated collaborators, as well as articles composed by journalists based upon the work or presentations conducted by UC academics. Subscribers should always take note of article authorship in trade magazines. Articles written by journalists are not proofread or edited by the UC research community; consequently, small errors may be unintentionally propagated in the trade literature. In the past decade, the demand for editorial on nut crops has increased so dramatically that the research community cannot meet the demand. Consequently, trade magazines have had to rely on technical articles composed by freelance journalists. Researchers appreciate the work of the ag- journalism community and do their best to provide information, quotes, and figures for such articles. Regardless, there should be a distinction between articles written by the research and extension community and those authored by our peers within the journalist community.
The written and spoken content presented for industry functions may vary considerably from the content farm advisors print or post in UC forums, such as UC newsletters and grower meetings. Trade shows are funded by marketing; therefore, research that demonstrates a lack of product efficacy (or perhaps even a product detriment to a crop or commodity) will not be welcome at trade shows. As a result, certain topics may be excluded from the agendas at trade events. Similarly, higher emphasis may be placed on agenda items qualifying for continuing education credits than on horticultural practices or techniques. Rest assured, UC ANR academics continue to provide third party, unbiased extension materials by maintaining UC-sponsored meetings for growers and publishing our newsletter editorials both in print and online. UC extension resources include our website, UCCE San Joaquin Valley Trees and Vines (https://www.sjvtandv.com/about-the-authors), individual farm advisor websites and blogs, and as well as the UCCE Growing the Valley podcast (https://www.growingthevalleypodcast.com/).
- Author: Elizabeth J Fichtner
- Author: Dani Lightle
- Author: Themis Michailides
- Author: Emily Symmes
- View More...
Walnut scale (Quadraspidiotus juglansregiae) is a common pest of walnut throughout California's Central Valley. Populations of the pest in commercial walnut orchards (Figure 1 A and B) appear to have increased over the past decade, inciting a recent surge of research on the biology and management of walnut scale. University of California researchers have recently monitored crawler emergence patterns in multiple walnut growing regions, thus improving the phenological models utilized in timing insecticide applications. Other studies have focused on evaluation of insecticide chemistries for management of the pest, including softer insecticide chemistries and insect growth regulators (IGRs). Additionally, walnut scale has been found to interact with canker-causing, plant pathogenic fungi in the canopy, thus exacerbating diseases that may further compromise orchard productivity (Figure 1 C, D and E).
Despite its name, walnut scale is not unique to walnut. It is native to North America and has a wide host range including woody shrubs, deciduous plants, and conifers. In California, walnut scale populations sampled from commercial orchards all represent a single species; however, the population composition of walnut scale across other hosts and regions has not been assessed at a genetic level. In California walnut orchards, walnut scale has historically been considered a minor pest. In the past decade, the insect has transitioned from an orchard inhabitant to a pest as the effective insecticides for walnut scale control have been cancelled.
The causes of walnut scale's recent emergence as a pest are not fully understood. It is speculated that populations of walnut scale are higher than historic levels. The interaction of scale with canker fungi, such as Botryosphaeria spp., may also be a new relationship for walnut scale. These pathogenic fungi have recently emerged as economic pathogens of walnut despite having been previously endemic in California. Generally, disturbances in natural or agricultural ecosystems may induce changes in the roles of ecosystem inhabitants, such as insects, pathogens, weeds, and other microbes. For example, prior to the introduction of DDT and other similar chemical insecticides for the management of codling moth, scale insects were not considered economic pests of walnut orchards in California. Utilization of these insecticides disrupted natural enemies (Figure 1F) of several scale species, resulting in a disruption in the orchard ecosystem and necessitating management of scale populations.
Walnut Scale and Botryosphaeriaceae. Walnut scale predisposes trees to infection by Botryosphaeriaceae fungi both directly and indirectly. A direct association between scale and fungal infection is observed when the scale body is lifted from the shoot to reveal a developing canker (Figure 1D). In this direct interaction between the scale and the pathogen, the feeding activity of the insect likely created an infection court (mode of entry) for the pathogen to infect and colonize the branch (Figure 1E). Infestation of twigs by walnut scale also predisposes new growth to infection and disease development. In this indirect interaction between the insect and the pathogen, the canker develops on plant tissue that remains uncolonized by the insect (Figure 1C). Research studies with three different pathogens in the Botryosphaeriaceae family suggest that disease levels are 60-70% higher on branches colonized by scale than on uncolonized branches.
Walnut Scale Identification. Walnut scale is an armored scale with a scalloped waxy coating (Figure 2). The adult female scale is revealed by lifting the waxy coating. The body of the adult female walnut scale can be differentiated from that of the San Jose Scale (a non-native armored scale) by the ridges on the body. Conversely, the adult female San Jose Scale has a smooth body and a pronounced nipple on the coating (Figure 2).
Walnut Scale Lifecycle. Walnut scale nymphs overwinter in orchards and metamorphose to the adult stage in spring. The females remain non-motile but the adult male is winged and able to fly. After mating, the females lay eggs and the eggs hatch in two to three days and then the crawlers emerge. Crawler emergence is in late April to mid-May in California, depending on the climate and location. For example, initial crawler emergence has been observed as early as mid-late April in Contra Costa andTulare County orchards, and as late as
early May in Tehama County orchards (Table 1).
Female crawlers move around, allowing the scale to colonize the current season's shoots. Once a female crawler finds a place to settle and begin feeding, she secretes the scale cover. Male crawlers migrate to the margins of the female cover and settle. Initially the scale cover is white, but it changes to gray or brown in about a week. The female then goes through two instar stages and the male goes through four instars before maturing to the adult and a second generation is initiated in the same season. Second generation crawlers emerge in late July to early August with crawler populations peaking in late August to early September. Second generation crawler activity is generally complete by the end of October; however, at some sites, crawlers may be active into mid-November. These second-generation crawlers will settle and molt before winter.
Detection of Crawler Emergence. To target insecticide applications to the crawler stage, double-sided sticky tape can be wrapped around walnut scale-infested branches in mid-April (Figure 3A). Emerging crawlers stick to the tape as they navigate to find feeding sites. Tapes should be changed weekly and checked under a magnifying glass to observe crawlers (Figure 3B).
Chemical Control. Walnut scale can be managed with either broad-spectrum insecticides or IGRs. IGRs are a preferable tool for use in an integrated pest management program because they are less disruptive to natural enemies than contact insecticides. IGRs work by disrupting the molting process; therefore, their activity is not realized immediately but rather over time as the insect develops. When applied at the delayed dormant stage (ie. March), IGRs would have the opportunity to inhibit the maturation of the overwintering scale to the adult stage. When applied in late spring (ie. late April-early May), IGRs may impact egg hatch as well as the development of the first-generation nymphs.
IGRs including buprofezin and pyriproxyfen are effective for management of walnut scale. In UC trials, pyriproxyfen was only tested as a delayed dormant application, whereas buprofezin was tested at both at the delayed dormant and the crawler stage timings. In season, the delayed dormant buprofezin application was superior to the crawler application for reducing crawler populations; however, applications of the product at either timing yielded similar suppression of scale populations in the subsequent season. Only one IGR application per year should be adequate for management of the pest. Effective monitoring and decision-support can help determine the need to treat on a yearly basis. For more information, on monitoring visit the Sac Valley Orchards website (https://www.sacvalleyorchards.com/walnuts/insects-mites-walnuts/walnut-dormant-monitoring-and-treatment-decisions/).
Products other than IGRs also exhibit efficacy in walnut scale management. In UC trials, acetamiprid, spirotetramat, and bifenthrin/imidacloprid, when applied at the crawler stage, were effective for scale management. The full impact of both spirotetramat and bifenthrin/imidacloprid were not fully realized until the following season.
Summary. Walnut scale had become an insidious pest in walnut orchards. Orchards with high disease pressure caused by fungi in the Botryosphaeriaceae family will benefit from both management of walnut scale and pathogens. If populations of the scale are high, or appear to have increased recently, it may be time to apply an IGR for scale management. IGRs may offer scale suppression over multiple years and often the efficacy of products may not manifest until successive seasons due to the mode of action.
For more information on walnut scale and other pests and diseases of walnut, visit the UC IPM website (ipm.ucdavis.edu). Research reports from studies conducted with support from the California Walnut Board are posted online (walnutresearch.ucdavis.edu). Mention of any particular chemistries or trade names does not constitute a recommendation and are for informational purposes only. Always consult with your licensed crop consultant and adhere to the pesticide label and local and state regulations. Additionally, check with certifier to determine which products are organically acceptable.