I noticed this year that the petunias are sticky--like from sap(one
they are secreting). When I dead head them the dead flowers all stick
to the sap all over my fingers. I don't know if that was always true
of petunia. I don't recall that on last year's wave petunias from
seed.
I'm curious to see what answers you get because I had a similar experience with
my wave petunias, although I don't think it was a heavy aphid infestation. I
had one that just dripped with sap, but it was healthy as hell and kept
producing flowers until November.(and I'm in zone 3)
Dora2