She's a solitary Andrena mining bee, gathering pollen to take to her underground nest. She'll lay an egg on each pollen ball that will feed her larvae until they emerge as adults next spring. On this sunny day, all of the pollinator groups were active on this Plum.
As an observer, it's an amazing scene of motion in a chaotic way, with many insects moving between flowers quickly or never landing. Some of the never landers are males - forming small swarms, vying for the attention of a mate. Others are parasites looking for a host. And then there are predators that keep them moving.
This is an Andrena species, told by the "vertical eyebrow", a deep grove with hairs that runs next to the eye on its face called the facial foveae. Andrena bees are near impossible to identify to species by photo, and there are 125 species here in the East. She's Andrena sp in Prunus maritima.