I report for the first time the parasitization of overwintering gynes of Parapolybia species by the entomogenous nematode Sphaerularia sp. (Tylenchidae). The nematode was found in 42% of the overwintering wasp clusters examined but occurred in only 6% of individual wasps. The prevalence of parasitic nematodes among group hibernating hymenoptera is briefly discussed and compared with that in solitary hibernating hymenopterans.

Wasps of the genus Eustenogaster van der Vecht, 1969, with 17 species currently recognized, are distributed from the Indian subcontinent in the west to the Philippines, Sulawesi Island and Java Island in the east. Two new species of hover wasp genus Eustenogaster (E. multifolia sp. nov., E. sumatraensis sp. nov.) are described from specimens collected in Sumatra Island. The female of E. vietnamensis occurring in Vietnam are described for the first time. The lectotypes of Paravespa eva Bell, 1936 and Ischnogaster ornatifrons Cameron, 1902 are designated. The new taxonomic status is proposed for Stenogaster eximioides Dover and Rao, 1922 as a good (=valid) species of Eustenogaster. The synonymy of Ischnogaster ornatifrons Cameron, 1902 with Eustenogaster micans (de Saussure, 1852) has been confirmed. A revised key to species and a taxonomic and distributional checklist of all the species of Eustenogaster are provided.

Thatched roofs are artificial environments that serve as nesting sites for a variety of insects, including tube‐nesting wasps, but they have been declining drastically in recent years. In this study, we investigated the nesting habits of a eumenine wasp, Symmorphus apiciornatus, nesting in a group at thatched roofs in a northern part of Ibaraki Prefecture, central Japan. Genetic diversity and genetic structure in the area were also investigated. Sex ratios at most study sites were not biased. Males emerged first and waited for females to mate around the natal nests, suggesting a high possibility of breeding between the wasps having emerged at a given thatched roof under the recent situation where there has been a drastic reduction in the number of houses with a thatched roof. Nevertheless, the genetic diversity in a given local population or a thatched roof was relatively high and significant genetic differentiation among thatched roofs was not detected.