I have a mason bee next box hanging in my garden which they do use.

This year, in addition to the mason bees, there is something else nesting in the holes. I assume whatever it is has something further back in the tube and then plugs the hole not with mud but with folded over grass blades. They are a few inches long and extend into the hole, fold over the diameter of the hole and then stick back out, effectively a U shape for each blade. Sometimes they are twisted a bit.

Any ideas what bugs these are?

Charles