The Red-belted bumble, Bombus rufocinctus, is the species of bumble in Washington state that show the greatest diversity of color patterns. If you check Bumble Bees of the Western United States, by Koch et al, you will see that not all red-belted bumbles even show red! This is one of my most challenging species to identify. Catching one to look at under a hand lens, to confirm cheek shape may be helpful.
To quote from that reference guide: “similar to many color patterns shown by both western and eastern NA bumble bees…small bodied, short-haired, short face.” emphasis added.
Field ID Tips
I think that maybe identifying Red-belted bumble bees may be easier than I have made it out to be. They have been the species “most mysterious” from a confident field ID perspective–since there is so much variability in their color patterns.
Having said that, T2 usually shows some level of yellow, and at least as I know so far, the color morphs in Washington are all of the “shows some red” group, rather than the forms where the red is black. So, T2 with yellow, some red on abdomen.
What else. Small bodied bee, hair length relatively short and even. Very short cheek–this is one time where being able to capture an individual, cool it briefly and example the cheek ratio can be very helpful for a confidential identification.
The next element is process of elimination from other possible bumble bees. Within Washington, there are four possible bumble species that might cause confusion: the Black tail bumble bee, the Two form bumble bee, the Hunt bumble bee and the Forest bumble bee.
The Red-belted can be distinguished from the Black tail bumble bee, B. melanopygus because the Red-belted’s scutum is yellow, while the Black tail scutum is cloudy.
The Red-belted can be separated from the Two form bumble bee, B. bifarious, because the Red-belted lacks the black inverted triangle.
The Red-belted bumble is primarily found in eastern parts of Washington. The Blue mountains are a good place to look for them.