Mouthparts and genae of the adult females of Doubledaya bucculenta Lewis (Coleoptera: Erotylidae: Languriinae) exhibit distinct directional asymmetry. Females excavate holes with their mandibles on hard internodes of bamboos to deposit their eggs on the inner surface of internodes. To determine the relationship between the spatial distribution pattern of completed oviposition marks (COMs) and the degree of mandibular asymmetry, COMs were counted on the culms and internodes of the bamboo Pleioblastus simonii (Carrière) Nakai (Poaceae), and the survival rate of immature stages was investigated. Another languriine beetle, Anadastus pulchelloides Nakane, exhibits a smaller degree of mandibular asymmetry. For this species, COMs were also investigated on culms and internodes but of the perennial grass Arundinella hirta (Thunb.) Tanaka. Females of the two beetle species selected large basilar internodes of host plant culms for oviposition in the field. The mean number ±SD of COMs per COM-containing internode was 1.1 ± 0.4 for both D. bucculenta and A. pulchelloides, suggesting that COMs deterred females from ovipositing on COM-containing internodes. Larvae of D. bucculenta completed development within an internode. The survival rate of D. bucculenta immature stages increased as the internodes increased in size. Given the positive correlation between adult body size and internode size, it is possible that the markedly asymmetric mandibles of D. bucculenta females help with oviposition in internodes that give higher fitness to the offspring.