These beautiful and very small chalcid wasps can regularly be found visiting the bee hotels. There are a number of similar looking species. This is Monodontomerus obscurus [1,2,3], (see also: here).
Another common species in this gender, M. aeneus, is difficult to separate from this species. The main differences are located on the hind part of the scutellum, the frenulum.
Its sculpture is striped in M. obscurus [2,3] although apparently there can be ambiguous specimen . Here the stripes are just visible on this enlarged picture.
Another character is the length of the scapus (ls) that is equal to the length measured from the center of the torulus (antennal implant) to the apical margin of the clypeus (lc) .
An important character is the row of pits on the apical rim of the frenulum, which is interrupted or not [2,3]. Unfortunately that is not clearly visible on the picture.
References1 Nederlands Soortenregister
2 Grissell, E.E.. (2007). Torymidae (Hymenoptera: Chalcidoidea) associated with bees (Apoidea), with a list of chalcidoid bee parasitoids. Journal of Hymenoptera Research. 16. 234-265.
3 Zerova, Marina & Seyogina, Lyudmila. (2002). A revision of Old World Monodontomerus (Hymenoptera, Chalcidoidea, Torymidae), Kiev, Ukraine, Institute of Zoology of NANU, Nauka-Servis Publisher, 1-74 pp. (in English language).