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!