Blue-banded Bee (female)

Last year I spotted one of these bees in my garden, once. This year I find them all over the garden and sometimes 2 or 3 together on one bush. They seem to be very territorial and chase each other away when this happens. They have discovered the wealth of pollen in my garden, and must be breeding somewhere.
This one is a female, with 4 blue stripes. The males have 5.
http://www.projectnoah.org/spottings/17035428