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We investigated the efficacy of various insecticides, including diamide, on the oriental tea tortrix, Homona magnanima Diakonoff (Lepidoptera: Tortricidae), a major pest of tea (Camellia cinensis), in Kagoshima Prefecture, Japan. The results showed that diamide insecticides had low efficacy against the oriental tea tortrix in the southern part of the prefecture (Nansatsu region; Makurazaki City, Minamikyushu City). The LC50 values ranged from 28.26 to 267.62 ppm(Resistance Ratio(RR): 41.0–387.9)for flubendiamide, 40.60 to 67.75 ppm(RR: 24.9–41.6)for chlorantraniliprole, and 40.33 to 107.49 ppm (RR: 93.8–250.0) for cyantraniliprole, indicating that the oriental tea tortrix has developed resistance to diamide insecticides. The insecticides other than diamide and benzoylurea IGR were highly effective. However, in other areas of the prefecture, all insecticides(including diamide)tested showed high efficiency. Furthermore, crossbreeding experiments showed that diamide resistance is autosomal, incompletely recessive. In the future, it is necessary to monitor the recovery of susceptibility against diamide insecticides in the Nansatsu region while avoiding the expansion of resistance development. [ABSTRACT FROM AUTHOR]