Gasteruption erythrostoma  is a rare species but is every year present on the bee hotels in the garden. Often two female and less frequently one male can be found.
This year, in 2020, up till now (June) I’ve seen five females and two males active simultaneously.
Remarkable is also the amount of Chelostoma rapunculi bees that are active in the garden and on the bee hotels in comparison to previous years. This bee enjoys the preference of G. erythrostomum . Many C. rapunculi females are atively nesting in the bee hotels and fill up nest hole after nest hole.
Even an unpopular commercial insect hotel that i received as a present and which is located in a less favourable but not bad spot is actively used by the females.
They nest in a small compartment with horizontally stacked reeds. I’ve not been able to observe whether they enter the reeds or nest between or behind them. They enter pretty deep into the compartment because the first layers of reeds are not accessible from the sides as they are blocked by the compartment walls.
Around the hotel male roam waiting for a chance to mate with the females.
Here as well the presence of the bees has not gone unnoticed by the G. erythrostomum wasps.
The females G. erythrostomum hang in front of the bee hotels and keep a close eye out for an opportunity. It is pretty clear where the bees nest by the number of concentrated female wasps hovering in close proximity to each other.
I has directly observed twice that a G. erythrostomum female entered a C. rapunculi nest hole with a bare ovipositor, I assume to lay an egg.
Here another female is staking out a C. rapunculi nest. Apparently the resident female had an eye on the wasp as well, because she suddenly appeared at the entrance, scaring away the wasp. After some time the bee returned inside and shortly the wasp appeared again. In the end the wasp left without entering the nest because the bee wisely decided not to leave the nest.
References1 Nederlands Soortenregister
2 Kees van Achterberg, 2013. "de nederlandse hongerwespen (hymenoptera: evanioidea: gasteruptiidae)"