The 'bee hotel', made from short lengths of hogweed stem packed into a small wooden box, that we installed in the garden in spring, hasn't attracted many residents, except for some spiders and these small but ferocious-looking digger wasps Ectemnius sp..
Their eyes are massive, like out-sized designer sunglasses, and remind me of some of the Dan Dare comic aliens that I remember from childhood. They have very acute vision and are very wary of coming out of the tubes if we get close.
I'm not sure whether they are nesting in the tubes or simply visiting to catch other tiny flies that frequent them - or maybe they're doing both. On most visits they enter several tubes rather than just one, together with the spaces between tubes, so maybe they are just foraging. There are rotting cherry tree and walnut stumps in the garden where they usually nest, tunnelling into the decaying wood and provisioning their brood with small flies.
They visit the largest as well as small tubes but when they enter the smallest they have a look inside first before they exit, turn around and then enter backwards, so maybe they're laying eggs in these.
More digger wasp pictures here and a BWARS collection of ID photos here - I haven't identified this species for certain yet but it looks like E. lapidarius.