There are a number of black bumblebees with pale bands on the West Coast, and they can be tricky to tell apart. It turns out that they are part of a well established Mullerian mimicry ring, in which unrelated species start to look more and more alike because they annoy potential predators in similar ways. Mimicry is handy for discouraging predators, but unfortunately it also works on naturalists.
I will write more about the mimicry ring at some point, but it turns out that there is one easy way to tell the Western bumblebee apart from all of the more common species (Bombus vosnesenskii, B. californicus, B. caliginosis, and B. vandykei.)
The common bees all have a yellow band near the end of their abdomen. By “band”, I mean that it’s a yellow segment with black segments on either side. If you look at the back of the bee and see “black, yellow, black”, it’s not a Western Bumblebee.
The Western Bumblebee has a white tip on its abdomen. The last two or three segments are white (not yellow), and there’s no black behind them. Instead of ending in “black, yellow, black”, Western bumblebees end in “black, white”. It’s very distinctive: some of them can look like they’ve been dipped in white paint.
There is one possible exception to the rule, and it’s even more rare than the Western Bumblebee. If you’re near the Oregon-California border and you see a bee that ends in “black, white” (actually “black, pale yellow”) but instead of a broad white tail there’s just a tiny bit of yellow at the end, you may have seen a Franklin’s Bumblebee (B. franklini). If so, let the Xerces Society (and me!) know, as soon as possible. Franklin’s Bumblebee is seriously endangered, and may already be extinct.
Places where This Rule Doesn’t Work
The black-bee-with-pale-band mimicry ring is mostly a West Coast phenomenon, limited to California, Oregon, Washington, and southern B.C. East of that area, the most common bumblebees have a lot more yellow on them, and the local populations of B. occidentalis are much more variable. They may have more yellow bands, and the tail may be yellow instead of white. I’ll try to collect pictures for a quick guide to more eastern populations later this year.
Conversely, at the northern end of its range, B. occidentalis isn’t the only bee with a white tail. In Alaska and the Canadian Rockies, it shares space with B. cryptarum (which is a close relative) and B. jonellus (which isn’t.) Again, I’ll post a quick ID page when I have the photos to make one.