A few years ago, I read up on mason bees and what makes them reproduce in your orchard. Turns out that making mud available may be just as important as having "housing" for them. When they don't have mud, they will fly away until they can find some to pack their eggs in and then make their nest in that area. That is why they are called mason bees. I started putting out little plastic tubs of mud for them. Then I found that the tubs would fill up completely with water on a rainy day and I would be drowning mason bees, when they couldn't get out. I ended up drilling a small hole, about 1 or 2 inches up from the bottom, to keep some water in but keep the top of the mud above it. I also try to put a stick in each one, so they can climb out if needed. Then I realized that over time, the mud would be covered with leaves, moss, and pine needles. The mason bees had lost access to the mud! It was too hard to clean the tubs, so I figured I needed a quicker way. I decided to use the tubs that are wide enough to simply brush my hand across the top, removing the debris and ridding the moss. Then I found out that the tubs were drying out at times. Now in the spring, when it's dry, I will go out with a long necked watering can and make sure there's enough moisture in there. Just recently, I have become a lot more concerned about the amount of plastic in our bodies. It is flowing in our bloodstreams and getting stuck in our livers and kidneys (it's not just the turtles). I don't want to damage my health, nor the health of the wildlife. I have been raiding the family recycling bin for cans that are wider than normal, so I can fit my hand in them and sweep off the debris and moss. Then I drill them. It's a gradual process, but it makes for happier bees, more fruit and a more ecological orchard.
In my yard I am "blessed" with plenty of clay, some on the surface and some deeper down. I just take a spade and jump on it till it is all the way down, then wiggle it until I've got a crack in the ground about an inch wide. This seems to be enough space for the bees to access the mud/clay while also not letting too much moisture out. If it is is warm and dry for several days, I will pour a bit of water in the crack at night (to avoid drowning anybody).
In the past I've also dug a hole a foot in diameter until I hit clay, which they also like but those dry out faster. Also in the winter they fill with leaves and I often step in them accidentally. https://photos.app.goo.gl/5bjW.....xc8oxeBCNA
I agree that it is a difficulty having to haul mud for long distances and how retention of mason bees would have it's negative impact on local populations. I have retainer wall of cement blocks which raise the earth of border shrubs. Since my blocks are not cemented I suppose they find proper access to the right kind of thing in all the adjoining natural tunnels that are not occupied by our spiders.
Another hardship for all bees during rainy days is that the pollens get washed off the flowering fruit trees. There must possibly be recommended options of differing flowering types of shrubs etc to solve this kind of shortage during wet periods.
Before the rains of the last couple days I pruned large branches from my later flowering plums and purposely placed those near my mason bee homes which are also sheltered together near the homes front door. Honeybees, any bees are not flying today, we even had hail. But in honeybees they always have spare stashes of pollen stored in honycomb for days like today to keep up with the non-ceasing egg laying activity of the queen. Mason bees are hindered in that area. I have had both. I will wait and see what happens next.
I have mason bees in my window sills! They seem to like one area to get mud. They are very particular.
I put out a water heater drip pan with a mound of soil in it. It collects rainwater and the soil stays wet. Bees can get dry soil at the top of the mound or wetter mud down low. If they land in water they are able to swim and climb to dry ground.
Another thing I do is dig a hole under the eave of my chicken shelter roof, rainwater keeps the bottom of the hole wet and drier dirt towards the top.
I have come to realize that typical tuna cans are too short and they dry out too quickly. They are wide enough to fit my hand in nicely to scrape off the moss and leaves easily, so the bees can get to the mud. I've been switching them out for the taller, regular sized and larger cans. I can still put the hole 1 1/2 inch from the bottom, to retain some water in dry times. The height provides more shade but also more mud, and the bottom is still wet, so they remain moist much longer. A lot of the time, it feels like we're just getting a little bit better at many things over many years, and after a few decades, we're astonished at what we've picked up over that time.
Like davem, I too am "blessed" with plenty of clay. This northern part of our property is mapped as "silty clay loam." However, when the property was developed, all the "good" topsoil was pushed off to the side of the yard by Bulldozer Jockeys who know a lot about moving dirt, but don't have a clue about soils. Much of the yard is scraped down to clay that I'm sure would make some nice pots, once they're put in the kiln....
Because I like to cultivate various species of plants that are "aquatic" or "emergent aquatics," I've purchased some cheap plastic "Kiddie Pools" to set plant pots into, or to partially fill with native soil. If I use some soil, I put it in somewhat "high" toward one edge, and low toward the other. That way, there is a dryish "shore side" and a saturated, inundated "pond side". Of course, as noted above, one needs to make sure that a certain amount of water remains so that some of the soil is wet or at least moist.
If there's a decent amount of clay present in the wet soil that is available, bees and wasps who like to use clay architecturally will find it a bonanza, and will visit it repeatedly for supplies. Quite some years ago now, I was very pleased to find a Potter Wasp repeatedly visiting one of these "Kiddie Pools" that I was sitting next to at the edge of the yard. The quality of the clay ball that it could gather was to it's satisfaction and it made repeated trips.
On the other hand, one should be sufficiently active and observant around the yard so that when a bee or wasp is found floating in the water, you can lift it out on your finger-tip and let it dry-out its wings and recuperate until it can fly off... whether or not they give a "Thank you." I find most of the rescues to be courteously thankful.
We use an extra clay pot (which sits inside a large plastic bucket). The adjustment we have made is to put the clay/dirt in the pot at a slant. This way even if there is very heavy rain there will be some dirt exposed beside the miniature reservoir, and later int he season it takes longer to dry out to where they can't easily get/make mud.
I set up drip irrigation. Dig a hole in a clay muddy area and use a hose timer to make sure it stays wet. Bees actually burrow into the mud to collect it. Dozens of bees at a time. I think, perhaps, that as with birds, a water source is a better attractor than food.