I agree with John Spalding's assessment of building a nesting
block for blue orchard mason bees in treated lumber.  That is, don't do
it!  I have absolutely no scientific data for this OPINION other than a
greenie aversion to treated lumber.  And all a nesting block is, is a
block of wood with the proper sized holes drilled it it for the mason
bees.  The nicer models come with straws close to the same size as the
drilled holes to slip in prior to nest building and pull out afterwards
for easy cleaning.
 
If you're building your own nesting box, there's not more than an hour
at a drill press involved, and untreated wood will last a few years at
least, so I don't see the need for treated wood in this case.  Yes, even
this greenie can see a need for treated lumber in some applications.
This just doesn't strike me as one of those applications.  I'd use
untreated lumber and rebuild it when rot sets in.
 
Additionally, I wouldn't use treated lumber on any hive parts that
contact my honey bees, so I'd extend the same courtesy to the mason
bees.
 
Aaron Morris - I think, therefore I bee!