Sibling cannibalism has rarely been documented in the resource-based context of sibling rivalry. The solitary wasp Isodontia harmandi (Pérez) (Hymenoptera: Sphecidae) has an exceptional style of nursery ‘communal brood cells’, in which multiple larvae gregariously develop while relying on maternal prey provisioning. In this species there are several distinct features; e.g., cannibalism-driven brood reduction, single-sex broods, and mass provisioning. Rearing of the wasp larvae in artificial nest chambers provides the novel opportunity to study larval interactions under controlled conditions.To determine the factors that influence the occurrence of cannibalism and its timing, we conducted a rearing experiment using sib and non-sib pairs under two prey-availability treatments, and recorded the occurrence of cannibalism and the processes of larval growth and prey consumption over a 96-hr period. Cannibalism occurred more frequently in the low-prey treatment than in the high-prey treatment, and in female pairs than in male pairs. Cannibalism tended to occur earlier when the amount of prey remaining decreased rapidly. However, cannibalism did not occur only when all prey had been completely consumed, implying that it was not triggered directly by prey exhaustion. Cannibals were always larger than their victims, whose growth rates had often slowed before they were eaten.These results strongly imply that wasp larvae commit sibling cannibalism in the context of resource-based sibling rivalry, ensuring the double benefit of nutritional gains and reduced resource competition. We also investigated the possibility that wasp larvae monitor the size difference and/or body condition of brood-mates as an indicator of prey availability.