One source says that sawfly larvae feed on white pine. Apparently they can come in all different colors. One reference said "Sawfly larvae caterpillars are variously colored but generally feed in groups on the needles. Some sawfly larvae will flex or rear back in unison when disturbed."

Here is a link that might be useful: Sawfly larvae (from Chattahoochee Nature Center)

I had something similar, but different infest CA Christmas Trees. Killed 'em. They were fond also of slash pine, but not loblolly or LL (thankfully).

Yes that's it! Nasty little things