This group contains some of my favorite species to encounter in the field. A couple of them can pose a bit of an ID challenge.
The Two form Bumble shows an inverted black triangle on her scutellum.
T1 = yellow/white
T2 = black
T3 = black
T4 = yellow/white
T5 = black
This form of the California bumble bee looks a lot like the next two species, except this species, the California bumble bee has a BLACK face.
The Obscure bumble bee has a yellow face, scutum and T4. Stergites S4 and S5 have some white hairs.
On the van Dyke bumble, T3 shows yellow, in contrast to the Two form bumble bee, where T3 is black.