I think they are leaf-cutter bees. It's cooler and overcast today so the only bee on the plant is the dead one. I looked at it with a magnifying glass and although it didn't look like it had the hairs on the underside, it looks very much like the ones on google images. And, right next to my garden is a stand of alder saplings that show clear signs of leaf-cutting. Mystery solved, I think. Sorry about veering off topic.
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