My Mason Bees will only nest in tubes of 5/16 to 3/8 in diameter and only
in a bright area, in the orchard or under an eave - they won't go into a
garage or shop where it is dark. The mud plugs they make are the color of
our red clay. Mason bees are mild-mannered and have never stung me.
The mud daubers prefer to make vertical tubes on a surface or fill larger
tubes and holes and they prefer a dark area - their mud is ashy gray. The
smaller ones are docile but the larger blue-black ones sting with very
little provication.