Nest-building Hymenoptera have been a major testing ground for theories of parental investment and sex allocation. Investment has usually been estimated by the likely costs of offspring provisioning, ignoring other aspects of parental care. Using three experimental treatments, we estimated the costs of egg-laying and provisioning separately under field conditions in a digger wasp Ammophila pubescens. In one treatment, we increased the provisioning effort required per offspring by removing alternate prey items as they were brought to the nest. In two other treatments, we reduced parental effort by either preventing females from provisioning alternate nests or preventing them from both ovipositing and provisioning. Our results indicate that both egg-laying and provisioning represent significant costs of reproduction, expressed as differences in productivity but not survival. A trade-off-based model suggests that other components of parental care such as nest initiation may also represent significant costs. Costs of egg production and nest initiation are probably similar for male and female offspring, so that taking them into account leads to a less male-biased expected sex ratio. Mothers compensated only partially for prey removal in terms of the total provisions they gave to individual offspring.