The effects of mid to late-season insecticide treatments on Gill’s mealybug, Ferrisia gilli, in pistachios, 2024

Standard practice in pistachio orchards in California that are infested with Gill’s mealybug is to apply an insecticide in late May that targets a relatively synchronized hatch of first-instar nymphs. When adequate control is not achieved, a second application may be needed 1 generation later as first-instar nymphs of the second in-season generation appear in late July. This trial evaluated the effects of Movento or MBI-306 applied in late May, with or without the addition of a second insecticide in late July, as well as stand-alone treatments in July, for their impact on mealybug density. The trial was located in a commercial mature pistachio orchard near Corcoran, Kings Co., California using an RCB design with 6 blocks of single-tree plots. Treatments were applied on 29 May (the start of the first in-season generation) and 26 Jul (the start of the second in-season generation) 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 on 23 May and every 2 to 4 wk thereafter through harvest. This included 3 evaluations during the first in-season generation (18 Jun, 2 Jul, and 17 Jul) and 2 during the second in-season generation (6 Aug and 22 Aug), with the last evaluation date occurring 1 week prior to harvest. On each evaluation date, the number of mealybugs was counted in each of 10 clusters per tree. Data of the average mealybugs per cluster on each evaluation date, and as averages across dates within each generation, were analyzed by ANOVA using transformed data (sqrtx + 0.5) with treatment means separated by Fisher’s Protected LSD (P = 0.05).

There were no significant differences in precounts that ranged from 0.5 to 0.9 mealybugs per cluster (Table 1). Following treatment, mealybug densities were reduced on all evaluation dates for all 6 treatment programs that received an application of Movento or MBI-306 (Onira EP) on 29 May. Average mealybug density in these treatments across the first in-season generation averaged 0.2 to 0.4 mealybugs per cluster compared to 2.2 in the untreated check. Insecticides applied as a stand-alone treatment on 26 Jul resulted in significant reductions in mealybug density on both post-treatment evaluation dates for Assail, on 1 of the 2 dates for Fujimite and Beleaf, and none for Aza-Direct. When a 26 Jul application of Assail, Fujimite, or Aza-Direct was made to plots that already received Movento on 29 May, there were numerical but not statistically significant reductions in mealybug density throughout the second in-season generation compared to the stand-alone Movento treatment. Application of Assail on 26 July to plots previously treated with MBI-306 resulted in significant reductions in average mealybug density during the second in-season generation compared to when a second application of MBI-306 was made. The lowest overall mealybug density was consistently reported in plots treated with Movento in May followed by Assail in July.¹

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