Effectiveness of incidence-based acaricide application for resistance management of the citrus red mite, Panonychus citri (Acari: Tetranychidae): a case study from Japanese citrus orchards.

Control of the citrus red mite, Panonychus citri (McGregor), is essential in citrus cultivation to prevent fruit value loss. In Shizuoka Prefecture, Japan, although synthetic acaricides have been applied to P. citri only once per year since the late 2000s, acaricide resistance remains a serious problem. Our aim was to determine the state of susceptibility of P. citri to acaricides, and confirm the effectiveness of current control strategies in citrus-producing areas. First, we clarified the ovicidal effect of 10 acaricides at field-recommended doses on 26 strains of P. citri collected from fruit-producing orchards, a scion-producing orchard, and nursery trees introduced from inside or outside the prefecture. Next, LC50 values for four acaricides were determined for the eggs and adult females of five strains collected either from orchards adopting control methods recommended by their respective agricultural cooperatives or from nursery trees. We confirmed milbemectin resistance in a strain collected from nursery trees and pyflubumide resistance in three strains from orchards where this acaricide had been applied annually for four or five consecutive years. These findings suggest that nursery trees represent a potential pathway for the introduction of milbemectin resistance from external regions, and that pyflubumide resistance can be established rapidly despite low-frequency (once-yearly) application. Notably, severe resistance was not observed in areas where producers decided on acaricide application based on mite density. These findings suggest that decision-making for acaricide application based on the incidence of P. citri may contribute to delaying or mitigating resistance development in citrus-producing areas.