How to make an awesome nesting block

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.

Rusty
HoneyBeeSuite

Nesting-block-in-tree
Experimental nesting block. I drilled into both ends of the wood.
Osmia-looking-out
Some holes are sealed with mud. © Rusty Burlew
Holes-sealed-with-resin
Some holes are sealed with a shiny resin. The one on the left is 1/16-inch. The other two are 3/32-inch. © Rusty Burlew
Osmia-on-nesting-block
A female Osmia at the entrance to her nesting tunnel. This is a 1/4-inch hole. © Rusty Burlew

Comments

Tom
Reply

Nice post, as usual. I have already made 6. One for me and some for my kids and grandkids. In Ellensburg we may get different nesters than you West of the Mountains folks.

Rusty
Reply

Tom,

Did you just make them? Or did you know all this and make them before?

Donna
Reply

Do the holes have to go into the end cut side or the flat smooth side? Or does it matter? I ask because your photos look like they are in the cut side of the block.

Rusty
Reply

Donna,

It doesn’t matter; I’ve seen it done both ways. I did it on the cut end because that was the shape of my piece of scrap wood.

Mark
Reply

That is so cool. Do you know if it makes any difference whether these are installed in full sun or partial to full shade?

Rusty
Reply

Mark,

Everything I know about native bees says they like full sun. In fact, bees love deserts. For the greatest variety and diversity of bees, that’s where to look. So, I’d say put them in the sun . . . they are not “shady characters!”

Rusty
Reply

Patrick,

The diseases you mention, and many others, are specific to certain varieties of bees. Crown Bees deals mostly with mason bees, and mason bees are managed for agricultural purposes much like honey bees. The more bees are managed, the more diseases they seem to acquire. So mason bees are highly affected by pollen mites and alfalfa leafcutters are plagued by chalkbrood. But in a situation where you have many types of bees in random populations, disease build-up is much less of a problem.

If these bees didn’t use a hole you drilled, they would use a hole that a beetle or a woodpecker drilled . . . not much different. And who is there out in the wild to disinfect cocoons in the fall? The point is, with a variety of hole sizes and a random selection of bees, you are not building up a monoculture. A green sweat bee may use the hole one year, and a lassioglossum may use it the next year. A solitary wasp may use it the third year. Since they are not affected by the same parasites and diseases, those problems do not have a chance to build up. It is the randomness of the system that protects it from monoculture-induced problems.

Still, if one is uncomfortable with that, you can put the drilled blocks inside a hatching box in the spring. A hatching box is just a large box with a small hole in it. The bees hatch and leave the box through the hole, but they do not re-use the drilled nest blocks because they will not seek out a nesting tunnel in a dark space. Once they leave the hatching box, they go and find a new hole to use for a nest. With this system, you drill new wood every year so disease organisms cannot build up. You can do this with managed bees as well.

In many ways I think it is sad that people are managing monocultures of bumble bees, mason bees, and leafcutters. As soon as we started to do it, the diseases began to crop up. You’d think we would have learned our lesson after the honey bee fiasco.

Patrick
Reply

That’s good to know and makes a lot of sense. Glad to learn there’s a way of providing habitat to a variety of insects without doing more harm than good.

I still have some reservations about drilled blocks. by not being able to harvest cocoons, healthy bees are prone to being infected as they burrow out of the nesting channel and encounter infected larvae. Still, the benefits would hopefully outweigh the drawbacks.

Rusty
Reply

Patrick,

I just have a mental block against harvesting and disinfecting cocoons. It seems so artificial, and nothing close to that would happen in nature. I like to keep as near to “natural” beekeeping as I can, but sterilizing cocoons is the antithesis of natural. That’s one reason I no longer bring in mason bees. Whatever finds my bee habitat on their own can live there. If no one chooses it, that’s fine too. As I’ve written before, the only hairy-footed mites I have ever seen here where I live came not from the mason bee houses, but from the drain holes below my windows. I can’t very well take apart my window frames and sterilize whatever decides to live in there.

Rusty
Reply

One more thing, your “go to source for authoritative information on solitary bees” deals well with mason bees but pretty much ignores the other 20,000 species. There’s much, much more to solitary bees than just masons.

Glen Buschmann
Reply

Rusty –

Like you I am not a strong believer in harvesting cocoons. I do open some to see in on their world and assess their successes, and washing these cocoons allows me to see who is inside. But I believe that the strongest advocates of the technique have developed a marketing tool in a very marginal business — when rewriting my bibliography of sources and information on mason bees this year I had to completely revise my list of businesses from just a few years ago because both cocoons and housing are overpriced, not really based on good science, and the resulting businesses ultimately fail. Drilling blocks are easy but pests are not unique to only a few bees; in time, drilled blocks will be filled with many different years worth of different bees. It becomes hard tracking who’s who, and pests inevitably occupy and multiply in old tunnels.

Mason bees (Osmia lignaria) are a great entry bee and teaching tool, and one thing they have taught me works with all the varied solitary tunnel nesting bees: use naturally occurring botanical tunnels — plants with hollow stems. Mason bees can use a pretty big diameter hole — I’ve use stems bigger than 3/8″ inner diameter with great success. That size is way too big for other bees. But plant stems come in all sizes and it is just as easy finding very small plant stems. Some bees respond to cut pithy stems (such as blackberries and roses), and drill their own tunnels. (And the majority of bees nest in the ground, not in blocks and tubes; each little mound of dirt pushing up in a sidewalk crack is almost certainly a bee.)

I like observing the different predators that come with mason bees, but I also like being able to discard old housing before it becomes totally infested with pests and diseases. Cocoon management was a response to population crashes for mason bees housed in old wood blocks, and while it has gone over the top in the other direction, the basic premise that bees thrive in clean housing is sound. For most bees, what that really comes down to is providing a clean but messy (varied) habitat.

By the way my “go to” authority is the Xerces Society.
Glen Buschmann Olympia, WA
olypollinators.blogspot.com

Rusty
Reply

Glen,

Okay, this is a great comment and I agree, agree, agree. My thinking on Osmia lignaria has evolved over the years, as well as my thinking on cavity-nesters in general. My position now is that bee blocks and the like are primarily for humans, not bees. As such, they are an excellent teaching and learning tool, so in that regard, they are a good thing. But all those bees will do fine (or better) without them.

Your comments regarding suppliers are spot on. I, too, believe they have developed a marketing strategy based on all the things you must do (and buy) for your mason bees. In truth, masons bees (and I use that term loosely) need what all other bees need: flowers, water, nesting materials, and a place to build. We can provide all that without nesting blocks. And whenever you allow for diverse populations, you decrease the spread of disease. Nothing is more attractive to any predator or pathogen than a monoculture.

But I like drilled blocks because they offer me an opportunity to learn and see. That said, I let the bees emerge in a dark hatching box in the spring. Afterward, I use last year’s blocks for firewood. Holes are cheap to drill, so why not? Is it a perfect solution? No, but I’m not reusing nesting holes and I’m keeping my hole sizes varied to avoid mono-populations. Furthermore, I only keep a few blocks.

When people ask how to attract pollinators to their gardens, I talk about providing bare soil. As you point out, with over 70% of all species being ground-nesters, that is the place to start . . . that will yield the most bees and the most pollination service. If pollination is your goal, it is almost silly to start with mason bees.

On the other hand (and there’s always another) if a bee block or straw cluster will cause someone to become aware of bees for the first time, then, by all means, we should encourage their use. Education is key to helping the native bees and people have a hard time getting excited about stuff they can’t see. Even if some bees end up with mites or fungal disease, something important has taken place. Maybe that person is the one who someday makes a discovery that helps all bees. Who knows?

But if we make it too complicated—the holes have to be a certain size and length, the tunnels lined, the cocoons harvested, sanitized, and refrigerated, etc—people will be put off by it and never even try it. And if people don’t learn about bees, they won’t care.

So in the end, I suppose I’m a fence-sitter.

By the way, my pick for go-to authority is John Ascher, Research Scientist and Bee Database Senior Project Manager, Department of Invertebrate Zoology, American Museum of Natural History .

Glen Buschmann
Reply

Rusty,

Thanks for the expanded comments.

A few years ago I bought a block of smaller diameter holes that is designed to be opened. I like it and yet … I get several different bees, and SOME are on a two year plan—a full year (plus fall before and spring after) in their tunnels before they emerge. It is these rascals (and the ones that use resin and cement the blocks together) that have led me into nests that are individual tunnels, like hand-rolled straws or botanicals.

Glen

Rusty
Reply

Glen,

I recently had an interesting conversation with Doug Walsh at the WSU Experiment Station in Prosser about the two-year cycle. The theory is that, should there be an extraordinarily bad year in which the bees cannot survive, there will be enough on the two-year cycle to carry the species through. Kinda cool, I think.

Don
Reply

I have an electric arc welder and these bees fill the plug-in sockets where I plug the connectors in with their nests. I had to move it away from the outside door.

Rusty
Reply

Don,

I’ve heard they like the keyholes in locks, too.

Glen Buschmann
Reply

More odd places I have known:
Holes in my power mower engine.
Slots in my snow shovel handle.
Under a sign on a wall.
In a metal emergence tray one inch deep.
Under the neighbors wall siding.
And yes, keyholes (skeleton keys).

Glen Buschmann.

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