Three days ago I got to witness something I hadn't seen before: the mason bees awoke from their winter's inactivity and emerged to enjoy the spring weather. The air around the house we had put up was filled with gentle little bees, checking out the fresh air and freedom.
If you look closely, you'll see a black dot to the right of the wire the house is hanging from - that's a mason bee flying. There's a second one down in the center, near the bottom, checking out one of the tubes....
When we first put up the house, Greg thought we should order mason bees to be sure there were some to start up a population. I was hoping that our landscape had enough healthy, natural habitat for us to attract mason bees that were already here. With bugguide.net saying that there were 150 species "in our area" (presumably the continental U.S.), I was hoping we could attract native species, rather than introduce a species that might not belong here.
In this case, it turns out that I was right. While we've never been even close to 100% occupancy, our bee house has definitely become a home.
The reading I've done on mason bees says that the outer sections of the tubes have males that develop in them, while the females develop in the deeper cells - the female mason bee can apparently determine the sex of the offspring that will emerge from her eggs. This makes sense in several ways, biologically. First, any predation by woodpeckers will take out males, rather than females. Since the mason bees aren't monogamous, the males that remain can fertilize more than one female, ensuring another generation.
Secondly, this causes the males to hatch/break out first. They stay around the nest, waiting for newly emerging females and mating with them as soon as they emerge. Another biological strategy for ensuring the continuation of the species.
Away from human-provided bee houses, mason bees use tunnels made by wood-eating beetles or they use hollow plant stems.
As with all solitary bees and wasps, the females will not sting unless actually handled roughly or trapped, for example between clothing and skin. Once the female stings, she dies and is unable to lay more eggs, so there is no biological incentive for her to be aggressive.
Mason bees and other solitary bees are excellent pollinators. If you haven't added a bee house to your yard, why don't you give it a try?