The effects of spring insecticide treatments on Gill’s mealybug in pistachios, 2023

Ferrisia gilli is an introduced species of mealybug that feeds on pistachio trees in California. At harvest, it aggregates in the fruit clusters where it reduces the yields, quality, and harvestability. During 2023, we conducted a trial in a commercial pistachio orchard near Lost Hills, Kern Co., CA, to evaluate the effects of insecticide treatments on mealybug density from late Mar until harvest. The trial evaluated single- and 2-spray programs that included treatments in late Mar that targeted small overwintering nymphs as they moved to buds around budbreak, and in late May during peak first-instar (crawler) emergence of the first in-season mealybug generation. The trial was organized as an RCB design with 6 blocks of 10 treatments and an untreated check using single-tree plots (Table 1). Treatments were applied on 30 Mar and 30 May using a handgun at 150 PSI at a water volume of 200 gpa. All treatments were made with the addition of the surfactant Dyne-Amic at a rate of 16 fl oz per 100 gallons of water (0.125%).

Mealybug density was determined 1 d prior to the spring treatments on 29 Mar and every 2–4 wk thereafter through harvest by counting the total number of mealybugs on each of 10 clusters per plot. Data of the average mealybugs per cluster during evaluations within the same generation (Overwintering generation: 14 Apr, 3 May, 22 May; first in-season generation: 28 Jun, 11 Jul, 27 Jul; second in-season generation: 22 Aug, 6 Sep, 12 Sep) were analyzed by ANOVA using transformed data (sqrtx + 0.5) with treatment means separated by Fisher’s Protected LSD (P = 0.05).

Prior to treatment, there were no significant differences in mealybug densities that averaged 0.25 mealybugs per cluster across treatments. Following treatments on 30 Mar, there were no significant differences in mealybug density for the remainder of the overwintering generation (Table 1). Among plots treated only once on 30 Mar, Centaur was the only insecticide to significantly reduce mealybug density during the first and second in-season generations. When spring treatments were combined with Movento in May, combinations including Centaur reduced mealybug density during both generations, whereas Sequoia reduced it during the first and Sivanto reduced it during the second. The application of Movento as a stand-alone treatment on 30 May did not cause significant reductions in mealybug density during the first or second in-season generations. These results run contrary to previous studies where Movento applied in late May (an industry standard practice) routinely provided >90% reduction in mealybug density. Investigations into the potential for spirotetramat resistance within the test orchard are ongoing.¹

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