Here's a bit on Carpenter Bees, and "Robber" technique.
Apparently, size matters despite what some've been told. Carpenter Bees
Because of their size, Carpenter bees do tend to use the "robber" technique more so than most. particularly, with long tubular flowers, like your petunias.
I'm curious, do you see many Hummingbirds with these flowers?
I get a few at mine.
It's very interesting, isn't it? I had no idea that this happened with bees! No, I have not noticed hummingbirds at the Mexican Petunia. They seem to prefer the red salvia.