For oligophagous insects, larval performance is very important to evaluate host suitability in host use. However, key indices among all performance traits including survival, development, and fecundity of the adult insect to predict host suitability are not clear. To investigate the key performance indices, we observed the performance of an oligophagous moth Grapholita molesta (Busck) (Lepidoptera: Tortricidae), a pest mainly damaging rosaceous plants, on three apple varieties, three peach varieties, and three pear varieties in the laboratory by placing its eggs on fruits. The moth's fitness was evaluated by the intrinsic rate of increase at fruit variety- and species-level variability. Our results showed that the preovipositional period of female moths and longevity of male moths were the indices primarily predicting intrinsic rates of increase for fruit variety and for fruit species. However, the preovipositional period of female moths was different at species-level variability but not at variety-level variability, whereas the longevity of male moths was not different at variety- or species-level variability. These results revealed that preovipositional period of female moths was a key predictor for host suitability in G. molesta. This study highlighted that gravid females may be a principal indicator in host resource optimization for oligophagous insects.