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Wasps of the genusGasteruptionare predator-inquilines of bees nesting in cavities in wood, stems, galls, and vertical soil surfaces. During studies of hymenopterans associated with reed galls caused by flies of the genusLiparawe recorded three species. We provide the evidence that a rare European speciesGasteruptionphragmiticolais a specialized predator-inquiline of an equally rare wetland beeHylaeuspectoralis.Gasteruptionnigrescensis a predator-inquiline of bees of the family Megachilidae, using the common beeHoplitisleucomelanaas the main host.Gasteruptionassectatoris a less specialized predator-inquiline of multiple groups of bees. The first two species,G.phragmiticolaandG.nigrescens, are usually present at well-preserved reed beds associated with meadows rich in flowering plants whileG.assectatordoes not prefer any specific habitat. Mature larvae ofGasteruptiontypically have a dorsoventrally flattened body and lateral lobes, small head with only slightly sclerotized mouthparts and tridentate mandibles. The larvae of these three species differ only slightly from one another, at most by the sclerotization of the mouthparts, colour, and position of setae on the head capsule and the shape of the mandible. As previous descriptions are insufficient, we provide first detailed descriptions with figures.

Aulacus flavus Smith, sp. n. from Brazil and Aulacus subflavus Smith, sp. n. from Brazil, French Guiana, and Guyana are described. These two species are separated from other Aulacus by the enlarged genae, flat mesoscutum, enlarged pronotum, and presence of small tubercles on the mesoprescutum, axillae, and mesoscutellum.

Aulacus machaerophorus sp. n. is described from females collected in Sapporo, Hokkaido, Japan. This new species has some interesting characteristics such as a very short ovipositor, absence of a ventral lobe and ovipositor guide on hind coxa, and smooth and shiny axillula, and is considered to be closely related to Nearctic species, A. schiffi. A key to three Japanese species of Aulacus is given.