Mason bees (Osmia spp.) are mostly-native, solitary bees that actively forage for nectar and pollen during a few brief weeks in April and May here in Southeastern Pennsylvania. The adults began emerging this week during the few warm, dry days that interrupted our cool spring weather.
Since the bees require tunnels for nesting but can’t excavate their own nests the way carpenter bees do, you can attract them to your garden by providing nesting blocks, bundles of bamboo, or commercially-available cardboard or paper tubes. The holes should be sized as close to 5/16 of an inch as possible. Since next year’s bees will be developing inside the tunnels for an entire year, it’s best to place the nesting block or bundle in a location protected from excessive rain or snow, such as under a porch roof.
If you’re a creative type, you can already imagine the wide variety of mason bee nest structures that would provide the basic requirement of dry, 5/16-inch diameter tunnels. My friend Keith Snyder has elevated mason-bee home construction to an art form, as proven by his latest creation shown below (which I was lucky enough to receive as an early birthday gift this year!).
In a couple weeks, dozens of recently-emerged and mated female bees should be busy laying eggs in these tubes. I’ll try to post photos!