Efficacy of Aerially Applied Chlorantraniliprole for Control of Two Sweetpotato Weevil Species, 2019

This study was performed to evaluate the efficacy of aerial-applied chlorantraniliprole with a drone for the control of serious sweetpotato weevil pests, sweetpotato weevils (SWC) and West Indian sweetpotato weevil (SWE), in a farm of a sweetpotato grower in Yaese on the Okinawa Island, southern Japan in 2019. A total of 18 plots were established and consisted of a ridge of 20.0 m and 0.8 m wide. Three treatments were replicated six times. About 70 slips of sweetpotato ‘Churakoibeni’, which had been collected from sweetpotato farms, were planted on each ridge with 0.3 m spacing by the grower of the farm on 9 May. Chlorantraniliprole of 5.0% (FMC Chemicals, Tokyo, Japan) applied with a drone (Agras MG-1, DJI Co. Ltd., Shenzhen, China) was compared for its efficacy on the reduction of infesting weevils and their damage on plants both with the application of this insecticide applied with a sprayer (WJ423, Honda Co. Ltd., Kumamoto, Japan) as a conventional insecticide application and with the untreated check. The insecticide was applied on 14 Aug, equivalent to 250 ml/ha diluted 16 times for the drone application and 250 ml/ha diluted 4,000 times for the sprayer application. The drone flew at 2.0 m high from the plant crown at a velocity of 2 m/s. When the insecticide was applied, all plants of the untreated check were covered with plastic sheets for the avoidance of insecticide-drifting. The root system of three plants randomly selected in each plot were harvested on 5 Oct, and tubers ≥ 100 g and the rest of the root system of each plant were weighted at a precision to 0.1 g individually. The roots were dissected, and infesting weevils were counted for each weevil species. Holes made by weevils on the root surface when they had emerged were also counted, although the species that made the hole could not be identified. Thus, the total number of weevils in some plants could be larger than the sum of SWC and SWE found in the plant. The efficacy of insecticide treatments was evaluated by ANOVA and Tukey’s HSD tests (P = 0.05) on the number of weevils infesting the root and on the proportion of the weight weevil-injured roots to that of the entire root system of the plant. In these analyses, the weevil numbers, root weight, and injury proportion were passed through square-root, natural logarithmic, and the root-square arcsine transformed, respectively. All means in this report were calculated on non-transformed data. Insecticide compounds, formulations, and application rates are provided in Table 1.

Fewer SWE were detected than SWC in all treatments, and no SWE in the drone application. The occurrences of SWC were more in the untreated check, showing no significant differences from the insecticide treatments, although the difference was marginally significant at P = 0.07. In contrast, both SWE and the sum of both weevils were significantly lower in the insecticide treatments than in the untreated check, and no significant differences were detected between the two application modes. Root yield was significantly less in the sprayer treatment. Mean root injury in the untreated check was 19.7, 18.7, and 29.8% by SWC, SWE, and both weevils, respectively. Both insecticides significantly reduced mean root injury compared to the untreated check.¹

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