I found a chunk of dried mud with some larvae embedded in it. It was in our mason bee box, but way too big for mason bees. What could it be?
The chunk of mud looks like a mud dauber nest with several mud dauber pupae inside. The female wasp captures spiders, paralyzes them with her venom and then stuff several of them into each cell. She then lays one egg in each, and after hatching, those larvae feed on the paralyzed prey.
They are not a pest or nuisance to humans. Mud daubers are not aggressive. They provision their nests with spiders. They are not interested in the mason bees - it's just the location suits them as well.