Efficacy of insecticides on mixed populations of citricola scale and California red scale, 2023

A field trial was conducted in a 26-year-old mixed navel strains orchard at the Lindcove Research and Extension Center, Exeter, California to compare the efficacies of 7 insecticide treatments to control interspersed populations of citricola scale and California red scale (CRS). Treatment details, rates, application method, and timings are provided in Table 1. The treatment design was an RCB, each plot consisted of 17 trees in a row planted at a spacing of 24 × 22 ft and was replicated 3 times. Treatments were allocated based on pretreatment counts of citricola scale adults/twig on 4 May 2022. The number of citricola scale adults on 5 twigs (~30–40 cm length) randomly selected from the NE quadrant of each tree were counted on 5 trees/row. Insecticide treatments were applied on 18 May, targeting adult citricola scale early in the season, or on 4 Aug, targeting citricola scale first-instar nymphs (Table 1). Treatments were diluted in 750 GPA except for Movento (250 GPA). Soil treatments were applied via injection through irrigation line and foliar treatments were made using a 100-gal high-pressure D30 diaphragm pump sprayer with mechanical agitation at 250 psi. Posttreatment counts for first- and second-instar citricola scale nymphs were conducted on 15 Aug 2022 and 23 Jan 2023, respectively, by collecting 5 leaves/tree from the NE quadrant of 5 sample trees within each plot. The leaves were evaluated for live insects under the stereoscope. Posttreatment count of adults was done on 15 May 2023 by counting adult scales per twig on five ~30–40 cm twigs/tree from 5 sample trees in each plot completing a season-long evaluation of citricola scale that has 1 life cycle per year. Pretreatment evaluations for the California red scale were done by counting the number of females on 5 twigs per tree on 4 May 2022. Posttreatment CRS densities were evaluated on 29 Nov 2022 on twigs and fruits. Five twigs and 10 fruits/tree from 5 sample trees/plot were collected and evaluated for live scales under the stereoscope. On 12 Dec, all fruits within reach up to 1.5 m height (821–2,774 per treatment) were classified as having 0, 1–10, or > 10 scales per fruit to determine the percentage infestation and percent of fruit with > 10 scales (downgraded at the packinghouse due to visible CRS patches). Data were analyzed using ANOVA after Log₁₀ (x + 1) of numbers or arcsine square root (x) transformation of percentages and means were separated according to Fisher’s LSD test (P < 0.05).

All treatments except Movento significantly reduced the number of live citricola scale first-instar/per leaf on 15 Aug compared to the untreated control (Table 2). Movento alone lowered the population of immatures and adults compared to untreated control on 23 Jan and 15 May 2023 but was significantly higher than all treatments with Pridixor or Plenexos. Interestingly, Admire Pro reduced the population of citricola scale on 15 Aug, but the population rebounded and was similar to the control on 23 Jan, and higher than the control on 15 May. Early-season soil application of Pridixor lowered the population of scales on all 3 evaluations. Plenexos (foliar application) lowered the populations compared to Admire Pro and Movento alone. On the final 15 May evaluation date, Plenexos and Pridixor + Movento were the most effective treatments and provided season-long control 1 year after treatment.

There were no significant differences in the number of CRS per twig prior to treatment on 4 May or posttreatment count on 29 Nov (Table 3). The number of live scales/fruit on 29 Nov was significantly higher in Admire Pro and was different from Movento, Plenexos, and Prixidor + Movento. On 12 Dec, all treatments except Admire Pro reduced the percentage scale-infested fruit. Similarly, the percentage of fruit infested with > 10 scales was significantly lower for all treatments except Admire Pro, compared to the untreated control.¹

Footnotes