AbstractThe reproduction process of the ambrosia beetle, Xylosandrus mutilatus (BLANDFORD) (Coleoptera: Scolytidae), was examined by successive censuses of field populations and by artificial rearing experiments. The reproductive strategy is discussed in relation to utilization of fungal resources, the amount of which was evaluated by total gallery length (main gallery and side galleries) for the field populations or fungal area in the laboratory. Fecundity of mother adults increased with the expansion of gallery systems for cultivating their associated fungi. The number of offspring also depended on the amount of fungal resource, whereas the variation of female body size within a brood was not affected by the fungal amount. The body size apparently declined with delay in larval feeding period expressed as the order of pupation and eclosion in the field and with decreasing fungal area per larva in the laboratory. These results suggested that the combination of resource‐dependent oviposition by a mother beetle and dominant resource utilization by earlier‐hatched individuals in a brood can contribute to the efficient production of larger females when resources are limited.