Watching Ancistrocerus wasps nest-making in my garden bee-hotel over recent years, I had assumed that they collected mud from damp areas (in a similar way to Osmia bicornis). Last year, I was able to follow a wasp from the bee-hotel to its "collecting point" and saw that it was collecting dry soil particles. This year I witnessed the same thing, with an Ancistrocerus wasp repeatedly returning to the same point. I was able to get a series of photographs from one visit showing her collecting individual soil granules (the soil was bone dry) and then flying off with a wet mud-ball.
I am a scientist working on the biological control of invasive social wasps in New Zealand. Using molecular techniques we have been able to trace the origins of one of the species, Vespula vulgaris, to the southern third of England (from Cambridge south).