(BG) variation - Four-toothed Mason Wasp (Monobia quadridens) - presumably taking the caterpillar back to its nest for larvae food
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D&M MD 9/2/11 - looks a bit like a Bald-faced Hornet, but different
UPDATE 8/19/15:
http://bugguide.net/node/view/1124650/bgimage
The two spots made me nervous.
Eventually I located the .PDF file for "Identification Atlas of the Vespidae (Hymenoptera, Aculeata) of the northeastern Nearctic region".
It's the second item down in the "References" links here:
https://en.wikipedia.org/wiki/Monobia_quadridens
After "Species Recognition" (page 222) it talks about variation, and there it says, "The following ivory markings are sometimes present: (...) small spots on dorsal surface of propodeum (especially in males), (...).
"Propodeum" got on my last nerve, but gritted my teeth anyway and looked it up. I think I got lucky.