I wondered if anyone had come across this before. In my greenhouse I heard a strange scratchy noise coming from a pile of polystyrene plant trays. On investigation I could see the back end of a smallish bee, the rest of its body being inside a cavity that the bee was excavating. Every 30 seconds or so it would back up and kick out the debris. I returned to the scene some hours later and the bee had gone but there was a decent tunnel chewed out. The following day it happened again in a different part of the tray (and no, I couldn't say whether or not it was the same bee!) - Has anyone got any idea what the bees might have been doing?

Thanks for any input.