A group of insect that keeps a close eye on the bee hotels are the Carrot wasps (Gasteruptiidae), of which several species can be found.
The wasps in this post belong to the former Gasteruption assectator agg. group. The species in this group have a short ovipositor, a smooth appearance and a short nose .
They are kleptoparasites and the females use the ovipositor to lay eggs in the brood of bees. This wasp is a parasite of at least Ridge-Saddled Carpenter Bee (Heriades truncorum) .
The wasp will approach a nest cautiously and will flatten itself when the bee is still present in the nest and comes out of it, which is remarkable to observe.
Another female searching for a suitable host nest in the bee hotel.
She will smell if a nest is in use by positioning herself in front of the entrance and pointing her antennae towards it…
…or in it as this other female is showing.
I think these Carrot wasps are very beautiful and as described here they tickle my SciFi imagination 👽😄.
References1 Kees van Achterberg, 2013. "de nederlandse hongerwespen (hymenoptera: evanioidea: gasteruptiidae)"