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<i>Castanea crenata</i> Siebold & Zucc. (Fagales: Fagaceae), a species of chestnut native to Korea and Japan, is distributed in Korea, Japan, and northeastern China, where chestnuts are a major economic agroforest product. <i>Curculio</i> spp. is among the main known pests of chestnuts around the world. In Korea, only phosphine (PH₃) is permitted for the fumigation of <i>C. sikkimensis</i> larva-infested chestnuts. However, it is applied for large-scale fumigation, and its use is restricted. Moreover, it requires a long exposure time and an application device; thus, it cannot be used by small-scale farmers. In this study, the activity of ethyl formate (EF) as a fumigant against <i>Curculio sikkimensis</i> in chestnuts was investigated, and its potential for practical use by farmers was evaluated. The sorption of EF according to the filling ratio (FR) and fumigation time was tested, and the results revealed that 2.5% FR was the most effective. For <i>C. sikkimensis</i> in chestnuts, the mortality rate increased proportionately with the dose of EF. After exposure to 160 g/m³ of EF in a 12 L desiccator, the adult <i>C. sikkimensis</i> showed 100% mortality. According to the time–dose mortality data collected over 12 h of fumigation, the LCT₉₀ and LCT₉₉ values were estimated as 1052.0 and 1952.0 g·h/m³. The results revealed that immersion was not an effective method for controlling <i>C. sikkimensis</i>. According to the LCT values, a dose of 180.0 g/m³ and 12 h of fumigation resulted in 100% mortality on a small scale (2 m³). The results of this experiment indicate that EF could be conveniently used as a fumigant by farmers.