[DrakeJ]

Last summer was our first year putting out various design bees "houses"both tubes and holes) and we got a good number of filled nests. We waited till late Sept to collect cocoons(here in N central MN hard freezes already) and we were surprised that around a third of the larva looked fully grown but had NOT cocooned. Is this normal? Chambers had been sealed off all summer. Anything we can do to help them fully develop this summer?

 

[Akademee]

How much sun do your mason bee hives get? It is possible that they got too warm and killed the larva.

 

[DrakeJ]

I don't feel the got over heated, if anything they might have not got enough warmth to fully develop. Should houses be in sun or shade? All were oriented with the open tubes facing south.

 

[Hops Brewster]

Drake, you don't show your location so my answer refers to blue mason bees here in the west. Eastern or red mason bees may be different; in nature blue bees build their nests in holes in dead trees in forested areas. There will be mixed sun and shade. They usually orient their nests to generally face southeast to east, sometimes south. there are several competing or predator species, as well as diseases that can kill MBs at various stages of development.

Keep them out of direct sunlight in full summer. Process the cocoons in late autumn, early winter.