What is a yellow bee with red butt?

The Bee is a bumble bee Bombus lapidarius

Bombus lapidarius is probably the most easily recognised species with its black body and bright orange tail. The workers have the same colouring as the queen, but they are much smaller; some of the early workers are no bigger than house flies. The males (below) have similar colouring, but with more yellow hair. Although its body is as long as that of B. terrestris it is not as heavily built.