I watched the bee on the right patrolling a patch of hay-grass laced with a few knapweed. Periodically stopping about 2 to 3 feet from a flower, it would hover around the flower from several angles, as if it were looking for something. If it saw another bee, like the one on the left, it would dart at that bee and knock it off the flower. Usually, the other bee would just leave. Sometimes, the other bee would continue nectaring, and the first bee would attack it again. Eventually, the second bee would simply leave, and the first bee would go on to the next flower, presumably looking for more bees to harass.

I was wondering what kind of bee this is and why it seems to be harassing other bees. Or is it some sort of mating thing? because the only kind of bee it attacked was like the one on the left.