The bee in the photo is a leafcutter or leafcutting bee in the family Megachilidae. This is the same family that contains the mason bees, and like mason bees, female leafcutters carry pollen on their abdomens rather than on their legs.
A telltale sign of a leafcutter is a slight arch in her back, which you can see in the photo above. Most bees seem to bend in the other direction, and whenever I see a leafcutter she reminds me of a little gymnast saluting the judges before her performance.
Also like mason bees, leafcutters commonly live in hollow reeds or abandoned beetle holes, and they are perfectly happy to reside in man-made bee blocks. The one in the photo is most probably an alfalfa leafcutting bee, Megachile rotundata, an introduced species that was brought to North America to pollinate alfalfa—a crop in which honey bees do a poor job.
Not surprisingly, leafcutters cut leaves. The female cuts small round disks from leaves and petals and uses these to line her nest. If you look at a bee block, holes containing mason bees will be sealed off with mud or gravel, but the holes filled with leafcutters will be finished with green disks or sometimes with colorful petal parts.
Gardeners are often mortified to see the leaves of their prize roses shot though with holes, but the bees do no permanent harm to the plant. Although the leaves look less than perfect, their presence lets you know you have a host of excellent pollinators working close by.
Alfalfa fields are commonly populated with leafcutter domiciles, in which thousands of nests are constructed. Each hole is the nest of one solitary female. Although strictly solitary, they don’t mind living in large communities.
The bees are extremely gentle. Last summer I stood in some of these domiciles, taking photos at the height of alfalfa pollination in eastern Washington. The air was thick with bees, the domiciles smelled richly of alfalfa, and the sound of wings was deafening. But I wore no protective clothing and I was never warned, chased, or stung—they are just too busy to be bothered with anything but raising their young.
Unfortunately, these vast leafcutter bee communities promote disease, especially chalkbrood. Like honey bees, new bees have to be raised elsewhere and shipped in to replace those that die of leafcutter diseases. Those that live separately and away from agricultural areas are less likely to be infected.
If you want to attract leafcutters to your garden, you can provide wood blocks or reeds with holes that are about 1/4-inch in diameter—slightly smaller than those used for mason bees. If you use a variety of hole sizes in your pollinator housing, you can attract a mixture of different species which are less likely to pass diseases among themselves.