Ok, not a pest... but failed to find appropriate forum...

Not the best pic either... camera fought hard against focusing on the green caterpillars.

Google isn't giving much help either...
so far, seems like possible sulphur larvae.
The other options have a pronounced head shape that doesn't seem applicable.

What plant are these feeding on?

Like the title says... robinia... (black locust).

I need the fence posts.... but learning that it's a butterfly host plant... also good.