Unfortunately, it is not always possible to keep bees wherever one lives. Because of this, if bees are still desired, solitary bees are an option. There exist solitary bees that are native to most regions of the world. In this classification of bees, there is no queen bee, and therefore every female is fertile. There are no worker bees for these species, as essentially laying females act as both a worker and queen in and of her self. Solitary bees typically don’t produce honey or beeswax on a harvestable scale.
Solitary bees are especially important as pollinators as many feral social bees are in danger, due to dwindling numbers and parasites.
Bee Types 
Mason Bees 
The term "mason bee" describes a solitary bee that lays their egg within a hole and separates that egg with mud, resin, leaf bits, pebbles, or chewed up vegetation/mud. These solitary bees typically have life spans of about 6 weeks and are active only a part of the season. The bees emerge from cocoons when the weather reaches a specific temperature keyed to that bee's instinctual requirements.
Mason bees will complete their life-cycle entirely on their own. However, most species of mason bees will nest close together, to increase breeding diversity and lower predation rates. This gregarious behavior of the bees makes it possible to keep mason bees in some capacity.
Blue Orchard Bees 
The blue orchard bee, a spring mason bee, is opportunistic when it comes to its home and nesting site. Blue orchard bees lay their eggs in holes which they do not bore themselves. Rather they depend on preexisting holes to lay eggs. When preparing a nest site, the female bee will place a dab of mud at the end of the nest. The bee then begins to place up to twenty-thirty loads of nectar and pollen at the end of the tube. Once sufficient food has been collected, an egg is laid and she then seals that segment of the nest with a thin mud plug. This process is repeated for the length of the hole until she places an extra thick mud plug at the end to prevent pests from entering her nesting eggs.
Blue orchard bees are fantastic pollinators of early spring plants and fruit trees. Blue orchard bees are high-speed flyers, and will pollinate many flowers in a short period of time. The bee’s high rate of activity, even during poorer weather, makes it an attractive pollinator for early crops. Much like honey bees, blue orchard bees will gather nectar in their honey sack as they forage. Any collected nectar is eaten directly by the foraging bee or saved as provisions for its nest. However, unlike honey bees, blue orchard bees do not have modified hind legs to store and carry pollen, rather pollen is collected within rows of stiff hairs that are located beneath the abdomen. The pollen tends to fall off on every flower they touch. As a result, they can pollinate about 2,000 flowers a day in comparison to the honey bee's 15 flowers a day.
The blue orchard bee may appear black in appearance, but is actually a dark metallic greenish blue color. Because these bees look strikingly similar to common flies, be sure to identify them before dismissing them. The female is about half an inch in length, the smaller male is more slender and of a stouter length. Males can be distinguished from females because of their long antennae and the existence of a light colored hair on the front of their head.
There are two subspecies, Osmia lignaria propinqua (west of the Rockies) and Osmia lignaria lignaria (east of the Rockies).
See also 
Hornfaced Bee 
Introduced to the US in 1984 as a gift from Japan. Dr. Batra sent the cocoons of the Hornfaced Bee throughout the US. This was before there were US restrictions on introducing invasive species. As a result, the hornfaced bee, or the Japanese Orchard Bee (JOB's) are now a naturalized insect in the US.
There are many orchards using this pollinator, predominantly in the Wisconsin and NE states of the US.
It is extremely similar in characteristics to the Blue Orchard Bee, or BOB.
Leaf Cutter Bees 
Carpenter Bees 
Attracting Solitary Bees 
Typically, most plants which are considered good for honeybees is considered proper for solitary bees. Most of these will be to late in the lifespan for masonbees, as they live only about one month as active pollinators. That is especially during early blooming fruit. Different species of mason bees will pollinate plants. For example, the blue orchard bee exists only in the spring and pollinates fruit trees or virtually anything blooming at that period. Other mason bees, like the leaf cutter bee which flies at 85 degrees pollinates plants later in the season like alfalfa. Solitary bees often tend to start forage earlier in the morning (and earlier in the spring !!!), and thereby may pollinate some plants better. Some plants may be visited more often or more efficiently by differing types of bees, therefore a bee variety means better pollination.
Building Solitary Bee Houses 
A solitary bee house for blue orchard or hornfaced bees can easily be built from scraps of non-treated wood (not cedar) by drilling a number of holes so as they don’t come though the other side. Each of the holes is what the solitary bees will make their nests in. Each individual hole is a separate nest for each individual female bee.
- Blue orchard bees seem to prefer 5/16 in (8mm) in diameter holes. Holes should be drilled between 3 to 10 inches (7—25cm) deep for best results. The deeper the holes, the more females may be created.
- Caution: these drilled holes are easy to build, but actually are bad for the solitary bee. Pests (pollen mites, chalkbrood, etc.) also increase which results in the entire hole not opening up after a second season of use. The uninformed gardener sees a mudded-over entrance and assumes their solitary bees are fine. In reality, these holes over 2-3 years tend to not open up and are instead, mason bee cemeteries. Using paper straws, reeds, or wood trays are already better for producing more solitary bees. Placing straw inserts into the drilled holes is an option, but at times the pollen within the hole is so wet, it cements the insert to the wood... again making the hole useless. Using plastic straws traps the moisture within the straw resulting in moldy cocoons. Basically one must know that any kind of artificial nesting device must be really weathered before bees are going to accept it. Whatever the material is, factory smells and tastes must be gone before using.
- Other non-specific holes may attract solitary wasps. Smaller 1/8" holes attrack aphid wasps which help the gardener reduce aphids without using chemicals or pesticides.
- Put houses facing south to southeast to allow morning light to warm the houses and encourage foraging. Placing the house under an overhang helps the holes stay dry. A height of 5-7 feet (2m) above ground is preferred so that the gardener can watch the activity. Any height is fine, but on the ground increases the likelihood of rodent interference.
In Flanders, Belgium (Europe) modular ceramic and rubber breeding systems are developed by the founder of these, Leo Aerts. Easy to maintain and very sustainable. Also suggested to use with BEE-O-Smell (Olfactorius) for starting populations. The properties of ceramic and rubber makes it possible to focus even on ground-nesting bees as Andrena and some Colletes, which are both also excellent pollinators. Especially usefull in hillside cultures of any fruit or veggies. Modules can easy withstand being half digged in. Experiments are running.
Buying Bees 
Some solitary bees (Blue Orchard and Japanese Hornfaced) may be purchased locally at some garden centers or nurseries. If they can’t be found locally, they can also be found from online retailers and mail order catalogs. When purchasing mason bees through the mail they may arrive as cocoons within a cardboard tube. If purchased in February or later, ensure they are transported with cooling packs as you don't want your bees emerging mid-transit. The cocoons should be kept in your crisper drawer in the refrigerator until the outside weather is about 55 degrees F (12 degrees Celsius) and pollen is available.
- Crown Bees
- Sells blue orchard bees, hornfaced bees, and supporting supplies. Detailed "how to" instructions to help the gardener or orchard manager succeed. A very ethical company.
- Bee Diverse
- Sells Mason Bees to select regions, along with books and other associated supplies.
- Raintree Nursery
- Sells Mason Bees and mason bee houses to the US.
- A citizen science project created by the University of Florida in order to track the nesting preference, biodiversity and distributions of solitary bees and wasps. Many resources are made available for participants and casual observers.
- Government of British Columbia’s Blue Orchard Mason Bee Page
- USDA’s Bee Biology and Systematic Library – How to build a nesting block
- USDA Agricultural Research Service- Building a Nesting Block
- The word 'Stouter' refers more to width than length. A better wording is needed here.
- Sihag, R.C.,1991. Methods of domiciling and bee keeping with alfalfa pollinating sub-tropical megachilid bees. Korean J.Apic., 6(2), 81-88.
- Sihag, R.C. 1993. Management of some sub-tropical megachilid bees for pollination of alfalfa (Medicago sativa L.) and pigeonpea (Cajanus cajan(L) Millsp). In: Pollination in Tropics, (Eds. G.K.Veeresh, R.Uma Shankar and K. Ganeshaiah). Pub. IUSSI- Indian Chapter, Bangalore, pp: 229-232.
- Sihag, R.C. 1995. Pollination Biology: Environmental Factors and Pollination. Rajendra Scientific Publishers, Hisar, 195p.
- Sihag, R.C.1995. Pollination Biology: Pollination, Plant Reproduction and Crop Seed Production. Rajendra Scientific Publishers, Hisar, 210p.
- Sihag, R.C.1997. Pollination Biology: Basic and Applied Principles. Rajendra Scientific Publishers, Hisar, 215p.
- Sihag, R.C. 1995. Management of sub-tropical solitary bees for pollination. In: Pollination of Cultivated Plants in the Tropics, (Ed. D.W.Roubik).F.A.O.Rome, pp: 157-160.