Except for natural bamboo tubes, it seems that most commercial tunnels sold for pollinator housing have an inside diameter of about 7 to 8 mm for orchard mason bees (Osmia lignaria), 6 mm for blueberry bees (Osmia ribifloris), and 5 mm for both alfalfa leafcutting bees (Megachile rotundata) and raspberry bees (Osmia aglaia).
I don’t know where these numbers came from originallyand that is what makes me suspicious of them. Everyone who cites these measurements got them from someone else, who got them from someone else. Much like that statistic that claims one-third of our food is pollinated by bees, everyone says it but no one can confirm it.
All I know for sure is this: when I give my Osmia lignaria a variety of tunnel sizes, they pick ones that are smaller than the recommended sizes. For example, they always choose a 5 or 6 mm hole over the much larger 7 or 8 mm holes. The summer masons and leafcutters seem to prefer a hole smaller than the recommended 5 mm, generally choosing a 4 mm hole.
If you’ve been around honey bee keepers for a while, you know there is heated debate about the size of artificial foundation. On average, the cells in most foundation are quite a bit larger than the cells found in natural combs, and larger foundation produces larger bees. Many people, myself included, think that a natural cell size is best for overall honey bee health.
Now I’m wondering if we are creating artificially large Osmia and alfalfa leafcutting bees by providing housing that is a little bit wider than that found in nature. Furthermore, we know that wild species raised in artificial nests are falling victim to ever more diseases and parasites, just like honey bees. Are any of these ailments more apt to appear in larger bees raised in larger tunnels?
Just as an experiment, I purchased a pair of these: Kinsman Giant Solitary Bee Nester with 60 Tubes. The tubes inside are about 7 mm in diameter. Next I bought a supply of Paper Straws ranging from about 4.5 to 7 mm inside diameter. I made collars for the paper straws so they wouldn’t float around inside the larger tubes.
When that was done, I set the first Kinsman nest on top of another pollinator unit behind my house, just to get an idea of how to fasten them together. I went to get some hardware, but by the time I got back, the mason bees were already investigating the smaller holes. I decided it was too late to move it, so I just tied it on with survey tape. I will set out the second nest in about six weeks when the summer masons and leafcutters are flying.
My plan is to remove the cocoons in the fall and look for differences between those in large tubes vs those in smaller tubes: pollen mites, mummies, parasites, whatever I can find. This is by no means a controlled experiment, but just a look-see to decide if there is something to study in the future. I will keep you posted.
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