Bombus lapidarius

What appears to be a Red-Tailed Bumblebee (Bombus lapidarius) feeding on Coreopsis flowers in the American Prairie at Wakehurst this afternoon.

It seems to me to have a distinctly yellowish 'tail' - but given the overall dark colouring of the insect, I'm at a loss to identify it as anything other than Bombus lapidarius. Maybe it's just the colouration of the flower that's affecting my perception of the 'tail' colour.