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Are we raising extra-large mason bees?

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 originally—and 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.


Some of the paper straws I used. They come in different diameters, but you have to read the fine print to find the info. I got different colors so I can easily tell the sizes apart.
I measured the straws and cut them to the right length for the nest.
I slit the tubes to make them easier to open later. They pop back into shape after they are cut.
I put the slit straws into larger ones so they maintain their size.
I used the cut pieces as collars for those with too much free space around the tube opening.
Here is a larger straw with a collar in place.
Here is a Kinsman nest being filled with straw inserts.
Within five minutes of putting it outside, the masons were inspecting it.

*This post contains affiliate links.



I very much look forward to how this turns out. I recently started trying out mason bees houses around our house for the native mason and leafcutter bees and I’ve read a lot about cell regression in the honey bees. Considering the bees were already showing an interest in the smaller holes it’s promising that you’re on to something!

Cory Stanley-Stahr

Hi Rusty,

I’m actually surprised that the bees are choosing those smaller sizes. Bosch and Kemp summarized the earlier work done on Osmia lignaria, much of which was done by Bohart, in their book which is available for free as a pdf, In that book, they state “Optimal cavity dimensions for BOBs are 19/64 inches (7.5 mm) in diameter and 6 inches (15 cm) long.”

This is a summary of a lot of research, including research similar to what you propose. Usually when tubes are too small, the sex ratio of the progeny leans favorably toward male. Also, when resources such as pollen are limited, females tend to produce more male progeny. And, unlike honey bees, solitary bees can have a spectrum of sizes – larger tubes result in larger bees – both male and female, and smaller tubes create smaller bees.

I’m curious to see the outcome of your experiment. Is your area experiencing a water shortage, similar to California’s, which would influence resources?

I wonder if it is a regional difference. There are three areas of the country that are best known for producing the solitary bees you mention, and Washington/Oregon is one of them. I’m sure you’ll keep us posted. I’m interested in the outcome.



There is actually more to this story. A few years back, maybe 5 or 6 years ago, I purchased mason bees in tubes from a local supplier. They hatched with pollen mites which annoyed me no end. I burned the empty tubes and decided not to encourage nesting of the infested bees. So I rebuilt all the pollinator housing I had and used a maximum hole size of 1/4-inch (~6.4 mm) to discourage Osmia lignaria but encourage other species.

Instead of discouraging them, they proliferated, and within a year or two, I saw no more hairy-footed mites. Now in spring, I have clouds of O. lignaria in my yard. I routinely catch, inspect for mites, and release, but I seldom find mites. I don’t know the ratio of males to females, but this year my husband has been digging fence posts, and the muddy excavations are loaded with females.

Now this is a completely anecdotal observation from which I draw no conclusions, but it is interesting. Since that time 5 or 6 years ago, I have not used holes larger than 1/4 inch, but the number of bees has increased dramatically. That said, there is plenty of natural habitat here where they can nest if they want, including lots of elderberry canes, and maybe that is where some are coming from. Still, they are filling the 1/4-inch nesting holes.

So, like I said, no conclusions but many questions.

As for weather, it was drier last summer than usual, but the previous several years were normal. I appreciate any other information you care to send my way. Thanks.

Susan McElroy

What fun! I crafted a pollinator “hotel” last year with bamboo from my yard, all the spent foxglove branches, paper tubes, some rolled up corragated cardboard, some poorly-drilled logs, and a gourd that bumble bees used for a nest last year. I’m not much of a crafter, and the whole thing—set into an old cabinet standing against a south-facing wall—is pretty tacky. I call it the Motel 6 of pollinator hotels. Still, I am so tickled to find mason bees there, and some bees that seem a bit larger and all dark gray/brown. I’m inspired to make more hotels!



I like the image of the “Motel 6” of pollinator housing!

Glen B

My mason bees are given tunnels with diameters anywhere from 1/4″ to 1/2″. In addition I provide a limited amount of housing that resembles shingles, a 5/16″ gap made between two pieces of board such as lath, with the front open and the sides enclosed. I have equal success with 5/16″ and 3/8″, with more bees packed into the bigger tubes — they build the cells sort of on their side. With bigger diameter tunnels they may gather a larger amount of pollen per egg / cell. Although in general females are significantly larger than males, in my population I find size variation based on tunnel size and presumably pollen store provided, with some smaller females the same size as larger males. Still larger tunnels and open lath is not as popular but is used; at some point they build a favorite size cell by gathering more (even much more) mud. Two different years in an “emergence tray” they have built several cells in a small pile in the corners, that resemble a small piles of mud. (

At this point I hold off placing any smaller tunnels for other bees, until the Mason bees are almost done, because they will otherwise use up all the smaller tunnels. I believe that an overpopulation of mason bees can adversely impact the supply of housing for later / smaller bees. If I breed bigger mason bees, perhaps it is less likely they crowd out smaller bees.

Glen Buschmann
Olympia, WA USA



You say, “At this point I hold off placing any smaller tunnels for other bees, until the Mason bees are almost done, because they will otherwise use up all the smaller tunnels.” So you concur that O. lignaria will, indeed, use the smaller tunnels? Why do you think they will do that if, as some suggest, a smaller tunnel yields more males? I’m confused about what I’m seeing v.s. what the experts say I should be seeing. Do you think our local O. lignaria are smaller than other populations?

Glen Buschmann

Rusty, sorry to take a while posting my reply. I didn’t have a short answer. While my comments are about housing for mason bees, (Osmia lignaria in this case), the remarks doubtless overlap for other tunnel nesters.

If I provide my mason bees with an excess of tunnels of all sizes, their first choice is probably 5/16″ and 3/8″ diameter. I rarely offer them 1/4” inch for a couple of reasons. I know they use the small holes up to a point, but I have also opened small tunnels and found them only partially used and the ends capped off with mud. Reports I’ve read suggest that, because males are smaller, the smaller holes tend to be males. I have not read the original research, (nor done any myself), so at this point I’m not sure this is true — did the researchers determine the gender based on cocoon size, or did they actually open each cocoon? The temptations of commercial success with mason bees has come with some sloppy reading and writing including both me and some of my sources.

Each female can probably lay about 30 eggs and 1/3 to 1/2 of them will be female. I don’t for certain know if that number is correct (see above comment), although it seems plausible; a photo I include shows about 25 cells in a nest. I do know that when I tried to provide as much housing as the bees could use, after a few years I became overwhelmed with way too many mason bees. Now days I provide only as many holes as I want, not as many as they can occupy.

This deliberate limit of housing creates reproductive pressures that push them to accept housing that is less than perfect, but obviously within their tolerances — blocks of nicely prepared tunnels are just a blip of time in their eons of reproductive experience.

With WIDE tunnels they must gather more mud, build an entire cell rather than just cap a tunnel. I like pushing them into using these bigger chambers because it slows them down — I don’t want a five or ten fold increase — AND it provides a vivid demonstration of their engineering genius. An extreme example is seen in this photo I took a couple of years ago,, of a nest of about 25 cells built in rolled tarpaper. (It is also an example of pests of mason bees, a different story).

For the human observer, SKINNY tunnels are structurally less interesting than oversized tunnels; for the bees these skinny tunnels also present a challenge but different. They need to carry far less mud to build a cell in a skinny tunnel, but they may not have room in which to turn around. In these cases they back in down the tunnels, scrape from their abdomen their gathered pollen, crawl back out and then turn around and go face in to shape the pollen into larval food pellets.

While I have seen size variation in males and females, I don’t know how broadly this can be adjusted on the spot — can a larvae interpret its food supply correctly, grow dwarf or jumbo and become an adult that is reproductively successful? Certainly one sees size variation in other bees — bumblebees tend to be bigger as the season moves along and the colony work is assumed by the workers. But this is where I don’t know how much they (or we) can control size.

I am confident enough in the reproductive success of Mason bees that, up to a point, I tolerate (some would say encourage) damaging pests. Frankly I find parasitic wasps at least as fascinating as the bees they are feeding on.




There is a fascinating chapter on bumble bee size in Dave Goulson’s book, A Sting in the Tale. See Chapter 13, “Does Size Matter?”


Hi Rusty, I seem to remember you wrote a post in the past about the lack of evidence that smaller cell sizes produced by honey bees making their own comb helps combat varroa.

Have you changed your mind about this, or do you think a small cell size is better for honey bee health in other ways?

Sorry to ask an off-topic question!



I knew someone would ask this! No, I haven’t changed my mind about small cells and varroa mites, but I do think natural cell size has other biological advantages. Somewhere is a post about this, but I will have to hunt around because I can’t remember where it is.


Sorry to be the one asking the awkward question! No need to hunt around for the post on my account, you’ve confirmed that it’s not for anti-varroa mite reasons that you favour smaller cells.



My solitary bee nesters have recently been ‘discovered’ by ants and there are ants going in and out of tubes at the same time as bees are exploring them.

Do you know if ants are a problem in these situations? Should I try to block their access to the trees where the nesters are placed?




I would try to discourage the ants if you can. I’m sure the bees will be put off by their presence.


¿Do you have a reference for ‘larger foundation produces larger bees’?



Here are a few of the original ones:

Grout, Roy A. 1931. A biometrical study of the influence of size of brood cell upon the size and variability of the honeybee (Apis mellifera L). M.S. Thesis, Iowa State College.

Betts, A.D. 1932. The influence of cell size. Bee World, Jan. 1934, pgs. 2-5.

Baudoux, U. 1933. The influence of cell size. Bee World, Vol. XIV, No. 4, pp. 37-41.


I specifically use small-cell to speed up the regression of the bees because they will build cell sizes that fit their bodies. So if you have a larger bee, it will build a larger cell. I have very small bees compared to my beekeeping friends. But though I don’t think it will stop the battle against the varroa mite, I do think it’s biologically wrong to have a bee that is larger than nature intended. Animals that are larger, or smaller, than they are meant to be, end up with with a variety of health issues in addition to those that may be experienced by the “normal” sized animals.

So many opinions within beekeeping….



I agree. I think natural cell size is best, just the way nature intended.


Rusty, thanks for sharing such an interesting idea, I look forward to the result! On the notion of smaller cells for honey bees, I saw a lecture by Tom Seeley in Pennsylvania recently. He presented a study of feral honey bee hives in the Arnot Forest near Ithaca, NY. Of 11 hives he studied (surviving with varroa, I might add), he found the natural comb size averaged 5.3 mm, about the same as standard foundation. I thought it an interesting data point.



That is an interesting number. I wonder if those feral bees were derived from swarms raised on commercial foundation. I usually hear a number closer to 5.1 for ferals, but I suppose that could change with time, location, and origin of the bees.


Yes, I thought so too. Seeley tested this, though, and found that the (2009) bees did indeed derive from the 1970’s population he originally found in the same area. Of course, the adaptation to mites resulted in a great loss in diversity, from around 15 main queen lines down to only a couple (if I recall).

Debbie Evans

What great pictures!! Thank you so much 🙂



I’m just getting started with solitary bees and would like to know when I should put out the empty tubes. Is it possible to be too early?




No, you can’t be too early. I leave mine out all year.

Glen Buschmann

As Rusty suggests, there are diverse bees who use tubes throughout the growing season; you may miss one species and catch the next one.

Weather resistance and pest control are reasons to move tubes in and out of dry storage — paper tubes, (if you use them) can get soggy exposed to winter weather, parasitic wasps can sometimes be lessened with a change of location.