September 2020 Natural Mystery Answered

Our September Natural Mystery was an entomology puzzle more than a tracking question—and the answers showcased the broad and deep knowledge of the naturalists in our community. Congratulations to Bill Kass, Catherine Zimmer, Eric Vehe, Kirsten Welge, and Mike Holz who correctly identified this species, and carefully distinguished it from its closest look-alike. This is a Brown Belted Bumble Bee, Bombus griseocollis.To explain, I’ll let Bill Kass start us off: "The first thing to note is that the thorax is mostly yellow with a black spot in the middle. This eliminates several species that have a lot of black between their wings. The abdomen is mostly black except for the 2nd abdominal band which is not even yellow all the way around. If it was all the way around the abdomen and with the black spot on the middle of its back, this would most likely be a Common Eastern Bumble Bee,… but it’s not."

Mike Holtz points out similar features, and also guides us to a useful resource:

"Looking at the Guide to Minnesota Bumble Bees, the bee in the picture appears to have a black spot in the center of the thorax. That, in combination with the rusty brown color directly above the black of the abdomen point to Bombus griseocollis, the Brown-belted bumble bee."

The closest look-alike to the Brown Belted Bumble Bee is the Rusty Patch Bumble Bee, Bombus affinis. As Eric Vehe explains:

"Bombus affinis (Rusty-patched Bumble Bee) has a prominent rusty patch with multiple black segments in the lower abdomen, but it also has yellow between the rusty patch and the lower black segments, which does not appear in this photo. While the angle makes it slightly hard to see detail of it, it also has a central black spot on the back of the thorax that appears relatively round. Bombus affinis has a wider patch that tapers and extends both laterally and down to the base of the thorax."

Kirsten Welge explains the difference this way:

"Rusty patched bumble bees are most commonly confused with brown-belted bumble bees. The rusty patch on Bobbus affinis, which is an area of rust-colored hairs, is on the front of the second abdominal segment with yellow hairs on the sides and towards the back of the segment. The second abdominal segment of the brown belted bumble bee, Bombus griseocollus, has brown hairs towards the front and black hairs on the sides and back."

And Catherine Zimmer notes even more succinctly: "Not the Rusty-patch bumble bee because the brownish patch is not surrounded by yellow."

Eric Vehe also rules out two other native bumblebees that have reddish brown on their abdomens:

"While Bombus ternarius (Tricolored Bumble Bee) and Bombus rufocinctus (Red-belted Bumble Bee) also have distinct rusty abdominal segments, they both have two abdominal segments with that color rather than one and also have significantly less black abdominal segments in the lower abdomen."

Finally, Kirsten Welge points out: "The slender, black hind leg appearance points to no pollen basket on hind leg - indicating this is likely a male."

I haven’t verified this yet, but I plan to go back and have a look!

Congratulations again to Bill, Catherine, Eric, Kirsten, and Mike. Thank you for sharing your knowledge and your process with all of us!

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