I think the one I have is Gamochaeta pensylvanica a.k.a. Pennsylvania Cudweed a.k.a. Pennsylvania Everlasting. I've heard this weed called Sweet Everlasting, too. Check out the photo gallery on the ISB Atlas of Vascular Plants. I'm sure you've seen this one before. It's all over the place down here, but is not native.
http://www.florida.plantatlas....
The caterpillars build nests at the top of the plants by silking several leaves together. Sometimes you can also see the frass inside. I'll try to get a picture of the nest without getting myself killed. Once you've seen one, they're pretty obvious and so it become easy to find the caterpillars.
