so.....now that I have the parasitic larvae in some of the cells, how can I manage the blocks to reduce/eliminate the infestation, and preserve as many of the mason bees as possible? If location matters, I am in the Puget Sound lowlands.
Thanks in advance to all of you!

At this point it is probably too late to do anything. Some of these parasites eat the host egg immediately on hatching. You would have to read about the biology of your species to find out if it works that way.

do these wasps only penetrate from the side, through the wood, or through the mud plug, too? From the side, any idea how deep the ovipositor can reach?

If I could id the infected cells, I could clean those out. If the entire blocks are gone, I could do a soak of some type to kill 'em all....maybe a bleach solution as I use for the mite treatment?

With 9 blocks, I should've thought about the fact I was creating a food-rich environment for something.....

I can't tell you any more than is on the page linked from our info page: http://www.nhm.ac.uk/research-curation/research/projects/chalcidoids/leucospidae.html.

Concentrating prey can be good or bad depending on whether the predators can similarly concentrate. If the density of predators is low, they can't eat the entire herd of prey until they have a generation or two to build up their own numbers. See "predator satiation hypothesis."

Thanks for the link! from the page,

".....The adults have well adapted protractile mouthparts to lick nectar from shallow and medium-deep blossoms, and may be encountered either on such flowers or near the nesting sites of their hosts."....

Does this mean Leucospidae make good pollinators, too?

and if not, would freezing the blocks be an alternative to a bleach soak?