Independent-founding polistine wasps build exposed, single-comb nests that are attached to the substrate by a narrow pedicel. These wasps coat the pedicel with a glandular repellent that helps protect the brood against predation by ants. In contrast, most swarm-founding wasps build nests that are broadly attached to the substrate and covered with an envelope. The envelope restricts access to the brood to a narrow entranceway, which is guarded against ants by the large worker force comprising the founding swarm. The combination of enclosed nest and founding by swarms suggests that these wasps rely on active defense to keep ants from the brood and do not use chemical defense. We show this to be the case forPolybia sericea, a swarm-founding species for which we found no evidence of an ant repellent applied to the nest.