America has 4,500 to 5,000 species of native bees. I don't think anyone has a definitive count, and then issues like DNA or over looked species confuse everyone. Darn you science! Anyhow, it's safe to say we at least have 4,500 species of excellent pollinators out there. So why then do farmers mostly use Honey Bees, Apis mellifera, as our primary pollinator?
There are a number of reasons for this and it's ripe full of controversy. The main reason Honey Bees are here in America is to pollinate foreign crops. The Xerces Society's view on honey bees seems to be we're better off without them. The pollination needs of farmers can easily be handled by our natives bees.They're argument is naive bees have evolved with certain flowers and are ideal at pollinating specific crops. The trouble is with 4,500+ species who can say the right pollinator for the job is even in the neighborhood?
Native Bees probably aren't used more in agriculture because they can't be controlled as easily as Honey Bees. Unlike our native bees, Apis mellifera maintains hives of 20,000 to 80,000 bees all year. Social native bees die off almost entirely, only the young queens produced in the fall survive the winter and start new hives in the spring. Wasp and Bumblebee hives have to start over every year. The Native Solitary bees though have an advantage here.
Solitary bees actually make up about 90% of the native bee population. The bees are only active for 6 to 4 weeks of the year, and depending on the species this can be any time of year. During the time the species is active females spend the entire time pollinating flowers near their nest. They'are somewhat social in that they love to nest next to other bees of their species. Despite being solitary it's possable to have an entire field, hill, or forest filled with these bees. Because so many of them nest next to one another it's just as good as having a hive of Honey Bees. The trouble here is land.
Farms are typically just a monocrop and perhaps some forest land acting as a border to help prevent erosion and other land issues. Monocrops are AWFUL! Even a forest full of native maple trees is bad. The reason is because it's a fradgile echosystem. There just isn't enough diversity around to keep things under control and farmrs have to resort to insectacides at some point or another. There are a few exceptions to this but they're not the norm. A single crop of anything is only blooming at one time of year. Honey Bees are ideal pollinators here because their hives can be carted around the country and propped up at a specific crop that blooms as a specific time, and repeat. Only a few Native Bees could really work under such limited foraging conditions.
What's worse is that it could be said the Honey Bees are stealing food from the natives. When you're a solitary bees that's only active when apple trees are in bloom in early spring, foraging can be extremely limited when you have to compete with a hive of 80,000 honey bees. Honey Bees happily fly up to 6 miles away from their home for food where as the Native bees usually don't go more than half a mile.
Don't get me wrong Honey Bees are still needed. A number of US crops actually aren't native and in many cases the Honey Bee is ideal for pollinating them. It's likely that this was the main reason why Honey Bees were used in the first place. Package that with the face that we can control the genetics of their hives and maintain them all year and it makes them all the more appealing to Beekeepers. To an extent this has also been done with Bumblebees but it's nothing compared to Honey Bees. You can find dozens of books completely devoted to the single species of Apis mellifera. And despite not being native they're the leading sources of pollination in the US.
The alternatives for farmers are to follow some of The Xerces Societies's plans. Some sound more outrageous than others but they've found farmers who sware by them.
1) Devote as much as 30% of the land to native forests.
This is a huge one to consider. But if it will provide 100% pollination year after year as well a more balanced echosystem it could be well worth it.
2) Limit the use of insectacides.
They say 90% of the insectacides sprayed are only to get the last 10% of quality. Most insects are not out to get you in any way. Solitary Bees are timid compared to everything else out there anyway.
3) Plant a cover crop to provide year round blooming.
This one fixes all the foraging needs your native bees might need and it will help their populations grow year after year. If there are any fears of the cover crop competing with the main crop then simply mow it over until the main crop is done.
4) Provide Nesting Where Able.
This is a big one. The forest itself adds to this but they're only foraging a half mile from their nest. Considering the natural home of tube nesting mason bees is the resulting hole of a particular sized grub in a dead tree branch, it might be useful to just drill some holes in a nesting block to speed things along. Bumblebees actually nest in former rodent burrows, and thatch nests. So stop mowing the grass and let the mice run wild.
More details can be found on their site.