I was initially annoyed when this insect refused to get out of the way of my shot. I wanted a picture of the Portulaca flower, not some stupid fly. But it turned out to be a lot more interesting than I gave it credit for, because it's not a fly (it's a bee) and because it's metallic green, as if it were covered in slut glitter or something.
Consultation of the Audobon Society Field Guide to North American Insects and Spiders (I have several of the Audobon Field Guides, which I loved when I was a kid, and would save my allowance to buy) tells me that this is most likely a Virescent Green Metallic Bee (Agapostemon virescens). These can also be found in most of the Eastern U.S. and Southeastern Canada, plus, inexplicably, there is a population in the Pacific Northwest, from Oregon to British Columbia. (The Discover Life website resolves this by saying that the Pacific Northwest population is continuous with those in Eastern North America, and basically has these living everywhere in the U.S. and Canada except for a region in the south central U.S.: Oklahoma, Texas, Arizona, Colorado, Kansas. Other websites say no, they're totally in Texas too. So I have no idea.)
The colonies are founded by a single female, who digs branching tunnels in bare soil or vertical banks, and then provisions the end of each tunnel with a ball of nectar-dampened pollen for the young females to eat until they can start collecting food too. The entrance to the tunnels is generally guarded by a single bee, who more or less plugs the entrance with its head unless someone wants in or out. (This seems like a sucky job, since if the colony has any kind of population at all, there's always going to be somebody wanting in or out. Got to be rough on the head, over time.) The guard also has to prioritize: bees returning with pollen are given right of way over bees wanting to exit.
I can't tell if these are considered important pollinators or not; since they don't produce honey, it's hard to find people who care about them one way or the other. The Discover Life site lists various plants they're known to pollinate, and some of them are familiar: Rosa, Verbena, Gaillardia, Oenothera, Geranium, Rudbeckia, Lobelia, Solidago, etc. Whether they're important pollinators or not, I think they're cool. I mean, it takes something kinda special for a bee species to look like it belongs in Liberace's closet. [Make your own Liberace / closet jokes here.]