Probably the best known method of attracting solitary bees to a garden or balcony are mason bee nest blocks. These are simply wooden blocks, logs or slices
of a treetrunk with drilled holes. The holes can vary from 1 up to 12 mm, though holes ranging from 3 to 8 will probably get most visitors. The most suitable
wood is hardwood, but make sure it comes from a suistainable source. Destroying the habitat of countless animals and plants to provide a couple of our
animals with a nestingplace is not a great way to help nature... Hardwood is so suitable because it will last longer outside, but also because the drilled
holes will have a smoother surface inside. European wood that's suitable is Chestnut, False Acasia and Oak for example. I usually make the holes as deep as
possible, depending on the length of the drill. But something around 10 cm will do.
My first nestblock was made of a piece of wood I got from my mom's husband, Ton (see also his website (in Dutch)). The wood originally came from the roof of a danceschool in Deventer (the Netherlands) that had collapsed. The thick beams were used by Ton to make bookshelves, but when he and my mom did some redecorating, I got a couple of the beams. I'm not sure about all sizes of the holes are anymore, but there are certainly 8, 6 and 3 mm holes. The 'block' hung on the wall of our balcony in the Nijmegen city centre for a couple of years, even though that location was far from ideal. Bee blocks should always be put up in the sunniest possible places, the sunniest spot on our balcony however only recieved a bit of evening sun. Nevertheless, a tiny bee species found it after only a week or two and nested in a couple of the 3 mm holes. The very distinct yellow spots on the bee's head, combined with it's size and it's method of closing the hole (using a substance produced by glands of the bee, forming a thin skinlike layer) meant it could only be a Hylaeus species. Unfortunately, I only have one picture of the actual bee, but I might be able to take new pictures if/when they find the blocks in our current garden.
The most common solitary bee species here in the Netherlands is the Red Mason Bee (Osmia rufa). These fly from around april to june. After moving my first bee block from the city center wall to the sunny garage wall of our new home, Red Mason Bees quickly found it.
Other bee species that I've seen nesting in my bee blocks are Hornfaced Bees (Osmia cornuta) and Heriades Truncorum. Other insects might use bee blocks to nest in as well. I've seen the very common Cuckoo Wasp Chrysis ignata and a as yet unidentified yellow and black wasp species.
Another possibilitie for creating nestplaces for bees is to make a bundle of hollow twigs. I've used Bramble and Common Elder twigs. Both of these species' twigs are filled with a kind of foam. It's not necessary to (try to) remove this, the bees can do this themselves. I've just cut longer twigs into pieces about 20 cm long and bundled them together, see also the picture in the list to the right.
Building a loam wall is slightly more work, but can attract different species to your garden. I filled a crate with loam which I then pressed down thoroughly. I've read somewhere the loam should then be allowed to dry, but I didn't do that. I just put the crate on one of it's sides, creating a vertical loam wall. I've covered the top of the crate, the 'roof' of the wall, with some roofing material to keep it dry as much as possible.
Another option is to just keep some dead wood laying around in the garden. Dead wood will attract beetles and woodwasps who's larvae will attract parasitic wasp species. That may sound negative, but it's all part of the amazing diversity of nature and it's very interesting to learn more about. I've seen, photographed and filmed a beautiful ichneumon wasp laying eggs in the rotting wood of an old treetrunk in our back garden (see first picture below). It's interesting think about how all this works, how these wasps can 'smell' where their prey are inside the wood and how they can use their ovipositor to deposite an egg right on top of it. A wasp species called Dolichomitus imperator is an example that can show this very well. I've seen this species a couple of times in the nearby Dal Palland, a small valley surrounded by forest and home to a lot of dead wood, either incorporated in the different styles of layed hedges and other types of 'fences', or just in small piles of treetrunks and branches. Dolichomitus imperator is a large wasp (20 to 30 mm, not counting the ovipositor) with an even larger ovipositor. It searches for larvae of beetles in dead wood, with a preferance for laying oak wood. The sense of smell is located in the tips of the antenna and is used to locate the beetle larvae. When she finds one, she positions her antenna so that they are lined up right beside eachother. She then aims her abdomen straight up and pushes the thin ovipositor down with the ovipositor-sheath to right between the antenna. When the ovipositor is settled in the wood the sheath is pushed back and the 'drilling' begins. The ovipositor consists of three parts, two of which take turns in moving downward, so wrenching their way into the wood. When the larvae is reached it is stunned, after which an egg is layed onto it. The egg is pushed through the ovipositor for that, during which it changed shape as the ovipositor is too thin for the egg. Once the egg is out of the ovipositor it regains it's natural shape. The Dolichomitus-larva lives off the beetle larva, which stays alive. The beetle larva eventually spins a coccoon to pupate, but a young Dolichomitus is what will hatch. The wasp on the first picture below seemed to lay eggs in a very similar way. A video of it can be found in the videos section.
Then of course it's important to offer the bees and wasps you're trying to attract feeding possibilities. Most bees will need nectar and/or pollen, so flowering plants are very important. In order to attract a species variety as wide as possible, make sure there are plants flowering from the beginning of spring 'till the end of summer. Use as many native and uncultivared species as these are most valuable for native insects. Native and uncultivated species have been around longest and complex relations between such plants and native animals have had time to evolve.
More pictures of bees can be found on the Bees and Wasps pages in the Pictures & Videos section.