These are middle instar caterpillars found together on white pine in the Great Black Swamp of Millis Massachusetts. They have only recently differentiated in color - so I believe they are both the same species and probably X. badicollis. I will note here on any further development.

Definitely Xestia badicollis or similar. As final instar caterpillars they all left the needles and huddled together at the bottom of their container. They venture to the pine to feed only by night.