Treatment Options for Management of Walnut Scale and Frosted Scale

Figure 1. The covering of a group of walnut scales forms a daisy shape. Removing the cover reveals the yellow adult females.

Walnut scale (See Figure 1) 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 and possibly flatheaded borer damage. Historic UC Pest Management Guidelines emphasize the efficacy of insecticide applications at the crawler stage of insect development (late April to mid-May); however, with the introduction of new pest management tools such as insect growth regulators (IGRs), new studies have been conducted to evaluate the efficacy of these products at earlier times during the insect lifecycle. Both walnut scale and frosted scale overwinter as immature nymphs; therefore, delayed dormant application of IGRs has the potential to inhibit maturation and subsequent reproduction of these pests.

Figure 1. The covering of a group of walnut scales forms a daisy shape. Removing the cover reveals the yellow adult females.

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 IGR products, Esteem® (IRAC Group 7C) and Centaur® (IRAC Group 16), were included in the study, with both delayed dormant (Feb. 8, 2023) and crawler-stage (Apr. 26, 2023) application timings. Centaur® was also investigated at two rates. Crawler-stage applications of Senstar® (a combination of spirotetramat and pyroxifin) and Assail 20SG® (a neonicotinoid) were also included in the study.

Table 1. Nine treatments were established in a Tulare County walnut block in 2023 to evaluate efficacy in management of walnut scale.

Delayed Dormant IGR Applications Inhibit Maturation
Delayed dormant application of Centaur WD® at either 34.5 oz/acre or 46 oz/acre reduced walnut scale survival by 81% of that of untreated control treatment by April. Both Esteem and Centaur reduced populations of mature frosted scale observed in late April by over 85% of that on untreated trees (Figure 2).

Figure 2. 10 weeks after the delayed dormant application of IGRs, frosted scale populations were observed on untreated (A) and IGR-treated (B) trees. Photo B is from an excised branch of a Centaur® (34.5 oz/acre)-treated tree, illustrating the lack of maturation of frosted scale resulting from the delayed dormant IGR treatment.

Crawler Populations Affected by IGRs and Conventional Insecticides
All treatments suppressed the rates of crawler emergence over time in comparison to the untreated control; however, delayed dormant applications of both IGR treatments (Centaur® and Esteem®) resulted in the highest suppression of the first-generation curve (Figure 3).

Figure 3. Crawler emergence was recorded using double-sided sticky tape traps that were changed approximately weekly. Crawler emergence is reported as a rate of the average number of crawlers from two subsample traps per cm2 length of twig per day.

Both rates of Centaur® applied during the delayed dormant period resulted in similar suppression of crawler emergence. Crawler stage application of Centaur® at the higher rate resulted in similar levels of crawler suppression as the delayed dormant IGR treatments (Figure 3). Moderate suppression of first-generation crawlers was observed with crawler stage treatments with the low Centaur® rate, Esteem® and Assail® 30SG (Figure 3). Crawler stage application of Senstar® suppressed first-generation crawler emergence (Figure 3) and resulted in modest suppression (45%) of total crawler populations across the season as compared to the untreated control (Figure 4). All IGR treatments, regardless of the rate or timing, performed similarly regarding total crawler populations across the season (Figure 4). The range in total crawler suppression across treatments of similar statistical significance was 88.2% (Centaur, 46 oz/acre, delayed dormant) to 54% (Assail, crawler stage), illustrating the variability in crawler counts in the orchard system (Figure 4).

Figure 4. The average total number of crawlers emerged over the season was evaluated across treatments.

Delayed dormant applications of Centaur® or Esteem® offer excellent suppression of walnut scale and frosted scale populations. Delayed dormant applications may offer similar efficacy at lower product rates due to the opportunity to achieve better coverage prior to leaf-out. Additionally, delayed dormant applications of these products may inhibit maturation of nymphs into adults, thus limiting sexual reproduction and subsequent laying of eggs.

In prior studies, the efficacy of crawler stage Assail® application became apparent the year following application. Based on this background information, the populations of adult walnut scale will be evaluated across all treatments in April 2024 to fully capture the efficacy of these products over time.

Additionally, future studies are planned to further determine the value of dormant versus delayed dormant applications of IGR treatments for management of walnut scale. The results of the current study, however, do demonstrate a need for updating the current UC IPM guidelines for management of walnut scale. To date, the UC IPM guidelines only recommend crawler-stage applications of IGR products while the current study demonstrates the value of IGR applications earlier in the season.