I think they must be sawfly larvae, probably Macremphytus testaceus, the dogwood sawfly. I don't know why they'd be on your petunias, though.
Here's a link to some pictures of M. testaceus larvae - some look just like yours.

Hopefully, Larry Gene will see this, and he'll come up with a definitive ID.

Sherry

Here is a link that might be useful: Sawfly Larvae

That surely looks like the dogwood sawfly larvae to me, Sherry. The yellow underside clinches it. I wonder if these petunias are planted near (or under) one of the dogwood host plants. Sawflies are so very host specific, in general; I can't find any citing that mentions alternative host plants for the dogwood sawfly.

The white wooly instar is relatively early on in the larva's life cycle...too early for it to be leaving the food source on purpose to seek its overwintering structure. This little guy should still be actively feeding.

Yes, there is a redtwig dogwood in the same pot! Thanks.

I have green caterpillars eating my 2 baskets of Petunias this year. Consequently, the baskets don't look too good. Anybody know what commonly uses Petunias as a host plant? I could try to get a pic if it's helpful.

Terrene, the list of green caterpillars that might be found munching away on petunias could be quite a handful. The first that comes to mind is the pesky tobacco/geranium/petunia budworm. You might wish to look that one up to compare.

Carol.....lol!