| Summary: | <i>Drosophila suzukii</i> is a quarantine pest that is rapidly spreading in berries. This study evaluated the synergistic effect of combination treatment with ethyl formate (EF) and cold temperature for <i>D. suzukii</i> control on imported grapes. A higher insecticidal effect was observed at 1 °C than at 5 °C at all developmental stages, and the pupal stage showed the strongest tolerance to cold temperature. After EF fumigation alone, eggs showed the highest tolerance at 216.67 mg·h/L (LCT<sub>99</sub> value), and adults showed the highest susceptibility at <27.24 mg·h/L. Among the combination treatment methods, cold temperature after fumigation resulted in the best synergistic effect. The effect of this combination was significant, with 23.3% higher mortality for eggs, 22.4% for larvae, and 23.4% for pupae than observed with EF fumigation alone. Furthermore, the period of complete <i>D. suzukii</i> control in the 12 L desiccator was shorter in the combination treatment group at the LCT<sub>80</sub> value than at the LCT<sub>50</sub> value of the egg stage. EF showed a very high sorption rate (24%) after 4 h of exposure at a grape loading ratio of 15% in a 0.65 m<sup>3</sup> fumigation chamber. As the grape loading ratio for combination treatment decreased, <i>D. suzukii</i> mortality increased, but when EF was administered at the LCT<sub>80</sub> value, there was little difference in the mortalities of the eggs and larvae but not the pupae. All <i>D. suzukii</i> developmental stages were completely controlled within 7 days after combination treatment, and phytotoxicity was not observed in grapes. These results suggest that the combination of cold-temperature treatment and EF fumigation could be used for <i>D. suzukii</i> control.
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