Thanks for your input.
I confess to ignorance of Tiarella, but for context, Ownbey and Morley's 1991 Vascular Plants of Minnesota lists it (only) as introduced and naturalized at one location in another east central Minnesota county.
Do the stout upright hairs differentiate Mitella/Tiarella?

I don't think so; the leaves of the two often seem identical when there aren't inflorescences present to check. I presume you're right about Mitella; would it be M. nuda then? Interestingly, P. mitellae was originally described from specimens reared from M. nuda in Alberta, and the frass particles were described as being closely spaced as in your photo. In Iowa and New England, I've reared P. mitellae from M. diphylla, and on this host the frass particles are consistently very widely spaced, which I've sometimes found helpful in distinguishing M. diphylla from T. stolonifera (=T. cordifolia, until recently), because the latter often has leaf mines of P. tigris, which have closely spaced frass particles like this. It's surprising that the the same species produces such different frass patterns on two different Mitella spp., but it seems to be true.

as I also searched the more recent results of the Minnesota County Biological Survey and got no hits for Tiarella (or foamflower). As to which Mitella it is likely to be, I was a bit late for flowers and didn't see a stem. My vintage Gleason and Cronquist characterizes diphylla as rich woods and nuda as bogs and wet woods/usually with moss. This site is indubitably wet and mossy.
Thanks for your help!