One of the key components of a DIY mason bee house is the perfect nesting tube. For the ideal male-to-female ratio (about 1:3), you want it to be about six inches deep. The shallower it is, the fewer female eggs will be laid.
Make your mason nesting tubes too narrow, and they'll be too cramped; too wide, and they'll take too much effort for your bees to protect. We recommend a diameter of about 3/8 in, or 10 mm. A diameter as small as a 1/4 in or 3/16 in will also do; you just might attract fewer bee species.
While you can order mason bee nesting tubes online, paper straws are an inexpensive and more readily accessible alternative. If you want to achieve the recommended 3/8 in diameter, use boba or smoothie straws. If you can't get your hands on them, it's okay to size down to jumbo or standard straws.
While size is an important consideration, here are a few other things to keep in mind:
Don't reuse straws. Bees don't do well with mold and bacteria. Just as you wouldn't share a used straw with somebody, you shouldn't share one with your bees.
Plastic straws are a no-go, even if you're trying to recycle. They aren't breathable, which can present a number of issues, and they're less attractive to the bees.
The sturdier, the better. Since your mason bee house is going to be outside, you want to build it with materials that will hold up in the weather.
Compost the straws when you're done with them.
In a pinch? Check out our tutorial on making nesting tubes with magazines.