Stem borer (Coleoptera: Erotylidae) in Ambrosia

I discovered a larva in its tunnel in the main stem of the host in mid-October. A 15-cm section of the lower stem tunneled by the larva had been so completely cleared of internal tissue, and compacted frass piled up so densely in a tunnel just above this point, that the hollowed portion transmitted significantly more light when backlit by the slanting autumn sun than the portion with the frass just above it. Thanks in no small part to the brightness and low angle of the sun, this was a rare case in which a stem borer's excavations of internal tissue in a lower stem were externally visible. When I dissected the stem, I noticed that the walls of the larva's tunnel possessed a slightly rough but not ragged texture. The active larva was a bright orange color, and it overwintered in the stem. Near where I observed this larva, Hatfield (2019) reared Acropteroxys gracilis from stems of the same host, and this seems like my larva's most likely identity.