An interesting series of photos was recently moved to BugGuide’s “Frass” section (a place where rejected submissions are held temporarily before being deleted forever), so I thought I would salvage them and post them here so they can continue to be pondered.
Tom Klein is one of a few BugGuide.net contributors who routinely posts photos of galls, leaf mines, and other such insect signs. According to his profile, all of these come from Pheasant Branch Conservancy in Middleton, Wisconsin, where he volunteers with the Friends of Pheasant Branch. Back in January, Tom posted this photo of a mud nest on a rock, 5.5 cm across:
I believe it is the work of a mason wasp (Vespidae: Eumeninae, likely Ancistrocerus) like the nest I discussed here. But Tom returned in May to find two dead metallic green bees (Halictidae: Agapostemon) inside it.
This certainly isn’t an Agapostemon nest. These bees are ground nesters; also, there are two bees in one nest cell, whereas there should only be one per cell if they had actually developed in this nest.
A photo of one of the bees removed from the nest removes any doubt that they are in fact Agapostemon:
One last photo shows more of the internal structure of the nest:
Tom noted: “There was an exit hole in that nest cell [the one containing the two bees], but apparently no pupal shell, and no apparent feeding activity on the bees.” Assuming that the bees were in fact put there by a wasp provisioning her nest, this would indicate that either she never got around to laying an egg in this cell, or the egg never hatched. As Tom suggested, the brown capsule-shaped object in the upper left of the second photo is the cocoon of whatever developed in one of the neighboring cells of this nest. Though I think the nest was built by a mason wasp, it has evidently been inhabited by something else since then, since eumenines provision their nests with paralyzed caterpillars or beetle larvae, not adult bees. The only wasps I can think of that provision their nests with bees are beewolves (Crabronidae: Philanthus), but they, like Agapostemon, excavate their nests in the ground. Interestingly, according to the BugGuide guide page, many beewolves specialize in sweat bees (Halictidae), which include Agapostemon. I have no firsthand experience with beewolves, but I wonder if one would ever use preexisting cavities such as exit holes in a mason wasp nest, or burrow into a mud nest as if it were burrowing into the soil. Anyone have another hypothesis?