Field and laboratory observations were carried out to understand the significance of prolonged copulation and the male substance transferred from males to females in the white grub beetle Dasylepida ishigakiensis Niijima & Kinoshita. Adults emerged from the soil in the evening and formed mating pairs on sugarcane plants, where copulation lasted for 1–3 h. Mating pairs (30%) observed in the field were accompanied by one or two additional males. Some females mated with one such male after ending copulation with the first male, showing multiple mating. After mating, males dropped to the ground to burrow into the soil earlier than females. Females received a large amount of colloidal secretion from males during mating. The amount and rate of transfer of this secretion from males to females were investigated in this study. Transfer of the secretion from males to females was completed within 30 min, and females that had mated with two males in the laboratory stored a larger amount of male secretion in the bursa copulatrix than those that had mated once. The results of both field and laboratory observations might indicate a possibility that prolonged copulation serves as a post-insemination female guarding behaviour to prevent mating by other conspecific males.