(SG) When I attended Pollination Guelph's annual symposium this spring, I learned that some bees will use coreplast to nest in. I added a coreplast "room" to our insect hotel, and sure enough, someone has moved in. Not sure what species this, but it uses clay to seal its nest chambers, so I'm guessing mason wasp. Note the lack of interest in the plant stems below (and elsewhere in the hotel). Also, we lined the holes in that wood block badly, so bees are ignoring it. Elsewhere they are happy to use holes in wood.