The conventional wisdom about nesting blocks is that you take a 5/16-inch bit and drill holes that are roughly 5-10 inches long. This will attract orchard mason bees, which is typically what people are trying to do with nesting blocks. But recently my whole attitude toward these blocks was changed by Michael Burgett, Emeritus Professor of Entomology at Oregon State University.
He showed me the nesting blocks at the Oak Creek Center for Urban Horticulture and I was amazed. Not only were the holes all different sizes, but they were tiny . . . and they were full of bees. The trick, he said, was to start with a 1/4-inch bit and work down from there and not to worry about the holes being too small. Not only that, he said the tiniest holes would fill first.
Intrigued, I went back to my room that night and ordered a series of bits to be sent to my home. I decided on 1/16, 3/32, 1/8, 11/64, 7/32, and 1/4-inch diameters. (This doesn’t seem so cryptic if you think of it as 4/64, 6/64, 8/64, 11/64, 14/64, and 16/64.) All of them are 6 inches long except the 1/4-inch, which is 12 inches long. The narrower holes don’t have to be so deep, so you can use shorter bits for those.
When the bits came I made a small experimental nesting block. I drilled about 7 inches deep with the 1/4-inch bit and 4 to 5 inches with the others. Then I added a little rain roof and hung it up. I honestly didn’t think it was going to work—after all, it was already past mason bee season and I hadn’t seen many other bees around.
Nothing happened for a week, but then one day I noticed one of the smallest holes was sealed up with a glittery resin-like substance with wood splinters mixed in. I was elated.
Now that it’s summer, many of the holes are full, and there’s no doubt that the smaller ones went first. I’ve watched the tiniest little bees disappear into holes I can barely see. More bees sealed their nests with resin, then others began using mud. Some of the seals look like chewed leaves and some like sand. It is awesome. Right now I’m getting what I call summer mason bees, a species of Osmia that is active in summer instead of spring. The summer Osmia are using the 1/4-inch holes, even though the spring Osmia used 5/16-inch holes in my other nesting blocks.
As a result of all the tiny holes, I’ve seen species I never even noticed before. It’s been both fun and educational, and I run out there every morning to see what’s hanging around. In fact, it is so much fun I fully intend to spend the cold nights of winter designing nest blocks and drilling holes.