I haven’t planted petunias for several years–not since 2014, I don’t think. There’s a reason for that. Every time I do, my container starts looking really good–and then all of a sudden all the blooms are gone from the petunias.
If I am lucky, I might start to see this. That at least tells me that the worms have arrived.
But I am not one to treat a container of annuals with insecticide, even if the recommended treatment is BT. That kills caterpillars but of course butterfly larva are caterpillars too. So I would just as soon uproot the petunias once they start looking ratty.
What does this critter look like?
He’s right there in the middle right of the photo, a green caterpillar about a quarter inch long. They can be difficult to see because of course they are the same color as the petunia stem.
They also eat annual geraniums and calibrachoa so this planter will need a refresh shortly.
But summer is very short-lived around here so I can refresh my planter with late season plants shortly.
Gardendaze 
My zonal geraniums got them annually, but they grew like weeds anyway, so the damage was not much of a problem.
You would think that these newfangled petunias could outgrow them, but they can’t–at least not in my climate.
Karla