Solitary mason bees and wasps are also important pollinators in our area, and they can be encouraged to nest in your yard with just a little effort. Red and black mason wasps are the most common pollinator in my yard, and they seem to take particular interest in my blueberries, blackberries, and Mexican Mint Marigold.
Awhile back I noticed a big pile of scrap fence lumber dumped in the yard next door (vacant house). I had been meaning to build some mason bee houses, so I brought some of it home. Unfortunately, it wasn't untreated wood, but I figured that since it had been sitting out for so long, it would probably be Ok. The mason wasps and bees don't eat or chew the wood, so hopefully it won't keep them from nesting in it.
I took the 4x4 fence posts, and cut out approximately 1-foot lengths at roughly 45-degree angles. Then I drilled several holes in the face with a 5/16" drill bit, and tacked on a little roof cut from an old fence slat. I made four of these houses, and hung them from the fence in different places around the yard. Hopefully I'll see some nesting activity this spring.