The food web of Symmorphus cristatus wasps, leaf beetle larvae (Coleoptera: Chrysomelidae) and willow (Salix spp.) trees is a model system for studying the chemical ecology of plant, herbivore and predator interactions. However, little is known about the natural history and prey use of the predatory wasps. We conducted detailed observations of nesting behavior and quantified time allocation to nest building activities in Big Pine Creek in the eastern Sierra of California. Symmorphus cristatus exhibits aggressive territorial behavior over nesting and mating sites, but does not respond aggressively to kleptoparasitism by cuckoo wasps Chrysis nitidula. We also compared prey use, nest provisioning and nest architecture of wasps at Big Pine Creek and at Sebastopol, near the central California coast. Using trap-nests we identified two new prey species for S. cristatus at Sebastopol: Plagiodera californica and Chrysomela schaefferi (Coleoptera: Chrysomelidae). There were significant differences in nest architecture for the two allopatric wasp populations and, although they do not use the same prey species, populations did not differ in the mean mass of prey provided for each offspring. Five other cavity-dwelling insect species pre-empted nest sites in Sebastopol. Prey species in Sebastopol differ in the chemistry of their larval secretion. The main component of C. schaefferi secretion is salicylaldehyde, which is derived from the willow host plants. Plagiodera californica secretion contains (epi)plagiolactone, an autogenously produced monoterpene-based defensive secretion. This is the first evidence that S. cristatus uses prey larvae that possess a monoterpene-based secretion.