Actually, that's not a bee, but a predatory fly. There's a big group of little flies known as "hover flies" that all closely resemble bees (but have no stingers and so are completely harmless). As adults they eat pollen & nectar, just like bees, so you often see them near flowers (especially flowers with exposed stamens full of pollen like this rose), but the babies (the maggots) are fierce predators of pests like aphids. That fly is a good guy. :)