Efficacy of Organically Acceptable Insecticides Against Drosophila suzukii (Diptera: Drosophilidae) in Blueberries, 2014 * Open Access

The efficacy of two unregistered organically acceptable insecticides on SWD was assessed in June and July, 2014. The experiment was conducted at the Horticultural Crops Research Station near Castle Hayne, NC in a planting of rabbiteye blueberries, var. Premier. Treatments were replicated four times and arranged in an RCB design, blocked by rows, with a one-row buffer in between blocks. Plots consisted of five plants, 0.00415 acres, each.

The following four treatments were compared: untreated control (UTC), Veratran D, Ecotec + TriTek, and Entrust SC ( Table 1 ). Following label recommendations, Entrust was tank mixed with a nonionic surfactant, Dyne-Amic. Treatments were applied on 23 June using a CO ₂ pressurized backpack sprayer fitted with three solid cone nozzles (TeeJet TG 5 with medium size filter) calibrated to deliver a spray volume of 50 gpa at 45 psi pressure. Due to issues with the filter clogging during the Veratran D application, the following adjustments were made for the 30 June application: a 16-gauge slotted strainer replaced the medium filter, and pressure was reduced to 30 psi. On both dates, treatments were applied to both sides of the row to ensure full plant coverage.

SWD infestation in field grown fruit was measured by collecting 30 berries per plot on the following dates: 23 June (pretreatment) and 30 June (7 d after treatment [DAT] 1). A sample of 50 fruit, instead of 30, were collected on 7 July (7 DAT treatment 2). Apparently undamaged fruit were randomly collected from the center three plants in each plot and placed in plastic SWD rearing containers. Samples were brought back to the lab and held for 7 d after which fruit were examined for infestation under a stereomicroscope. Adult SWD monitoring traps (yeast, sugar, and water solution) were placed near the trial plots to determine local presence of SWD.

Because field infestation rates can be highly variable, semifield bioassays were also conducted to assess the performance immediately after treatment (0 DAT, acute activity) and 7 DAT (residual activity). Standard bioassay arenas, constructed from 32 oz. deli containers fitted with a floral water wick in the bottom, were used. An approximately 10-inch long stem and 5 sound appearing berries from each plot were collected from the middle bush in each plot and placed into each arena. In the laboratory, the berries were placed into a small plastic cup then placed back into the arena to prevent movement of the berries during the experiment. Five female and five male SWD, from a laboratory maintained research colony, were placed in each arena and mortality was assessed at three time points: 1, 3, and 5 d after infestation (DAI). Fruit from bioassay arenas were dissected to measure fly reproduction 7 DAI and the total number of live offspring (larvae plus pupae) were counted.

Data were subjected to ANOVA via SAS Proc GLM (v. 9.4), and means were separated via the Tukey-Kramer procedure. Treatment and replicate were considered independent variables, and proportion data were arc sine square root transformed to meet the assumption of normality and to correct heteroscedasticity.

Presence of SWD was low in the field throughout the experiment, with an average of two flies collected per monitoring trap per week. Similarly, field infestation was low with only one-third instar larva found in any fruit (data not presented).

Female SWD mortality was higher when exposed to Entrust SC treated plant material in bioassays 3 DAI than for flies exposed to untreated plants and at 5 DAI both male and female mortality was higher ( Table 2 ). Male SWD mortality was also higher on Veratran D treated plants than untreated plants 5 DAI. None of the treatments significantly reduced in either the number of offspring (larvae and pupae) in bioassays, but data across all treatments were highly variable ( Table 3 ).

While semifield bioassays were also conducted on plant material and fruit collected 7 d after initial treatment (7 DAT) to test for residual effects of treatment, there was no impact on either the proportion of adult male or adult female mortality or the number of offspring produced ( Tables 4 and 5 ). Throughout the trial period, there were several rain events, which may have impacted the residual activity of the applied materials.

Author notes