As per the new Owlet Caterpillars of Eastern NA book, page 500, this is Hadena ectypa, Campion Coronet. This is good to know because one larva died and I don't yet know if the other one will make it. (It did.)

Lep larva feeding on the fruits of Starry campion, Silene stellata. Found by L Crim. (Although M Hayne found the first one.)
HOSTS database lists Hadena ectypa as feeding on this plant.

In 2005 I worked for Iowa DNR Prairie Seed Harvest. In an effort to diversify our production prairie plants I tried to collect seeds of this plant but never found a single seed, all fruits and seeds were gone, assumed eaten. So it was with delight when Moni Hayne found a first larva feeding and Lloyd Crim found a second larva feeding.
BugGuide gathering_2011