Based on descriptions and habitats in American Beetles, I have concluded this is Boris unicolor. The genus is distinguished by a sharply constricted head behind the eyes. Of the two species in the bugguide region, the other lives as larvae primarily in fire-killed conifers, and these individuals were found in conifers that had died from causes other than fire. Moreover, the coloration fits the species name.

I kept waiting for this beetle to grown completely black before posting photos. Ha! I should have refreshed my memory from American Beetles. Borids are supposed to be brown :-)

One more of my original larvae collected March 18, 2006 has pupated. The others have died at various stages, one at the prepupal stage.