Mandibles and genae of adult females of most members in the genus Doubledaya (Coleoptera: Erotylidae: Languriinae) exhibit distinct left–right asymmetry, i.e., the left side of the traits is larger than the right, which may facilitate boring into hard materials for oviposition; females of D. bucculenta and D. sinuata excavate holes or relatively deep depressions with their mandibles on internodes of dead bamboos to deposit the eggs on the inner surface of internode cavities. To determine the relationship between the degrees of mandibular and genal asymmetry and hardness of oviposition substrates, we measured the mandibular length and genal width of D. tonkinensis and D. sinuata, both of which oviposit on Sinobambusa sat bamboos, and examined the structure of their oviposition holes. For D. tonkinensis, we additionally examined the features of host culms. D. tonkinensis females showed a slight degree of left-directional asymmetry in the mandibles compared to males. By contrast, D. sinuata females showed distinctly marked left-directional asymmetry in the mandibles and genae. Females of D. tonkinensis excavated soft walls of internodes of weakened, young bamboo culms, whereas D. sinuata excavated hard internode walls. Cross sections revealed that the holes of D. tonkinensis and D. sinuata were funnel-shaped, indicating that the females first excavate a cone-shaped depression in the internode using mandibles followed by ovipositor's penetrating wall from the bottom of the depression to the inner surface of the internode cavity. The results strongly suggest that the markedly asymmetric mandibles of Doubledaya females facilitate oviposition into hard substrates.