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”.