My experience is that Petunias attract a large variety of insects, both beneficial and pests. Why are those moths, that are probably just after the nectar Petunias produce, such a problem since they do no harm to the plants?
Which caterpillars are on the Petunias?
Which Bayer pesticide were you using? Keep in mind that most everything Bayer makes is a very broad spectrum poison that will kill off as many beneficial insect as pests. Keep in mind also that these broad spectrum poisons are also killing off the pollinators we need for much of our food.

we do not treat plants proactively ...

we wait until there is a problem... fully ID the problem.. and then react properly ...

its what is called.. INTEGRATED PEST MANAGEMENT ...

you start by saying.. you had good luck ...

if that is true... then i dont understand why a little late season damage is all that big a deal ...

soooo .. perhaps.. its your expectation of a perfect disney/marth stewart garden.. that is the root of your problem ...

let the bugs have a few plants ... who cares ... and when they get to ugly.. rip them out.. and cover the spot with mulch.. and call the season over.. for those plants .... and perhaps go buy a few fall mums.. to fill in the spots ... now that is IPM ... lol...

ken

Let me rephrase the problem.

My petunias grow very well, until mid- to late-season, when the moths lay eggs and the caterpillars arrive and decimate the plant: holes, munched up leaves, caterpillar waste, etc. The plants then begin to wither slowly in response to it. I wouldn't call this a "little late season damage" at all.

I've had limited luck with the Bayer pesticide. I just looked and I had already tossed out the empty bottle, but it's a thick white liquid that you mix with water, then systemically apply to the plant via the soil.

I'm thinking proactive treatment may be better, since the pesticide will already be present and thus perhaps the larvae won't survive to maturity.

I don't care about random bugs, just as long as they leave my plants alone :-)

Systemics aren't particularly effective against chewing insects. I suggest that you consider Dipel or Thuricide, which are two trade names for Bacillus thuringiensis Kurstaki, a naturally occurring bacterium strain that only effects caterpillars. There are probably other brands, too.

Bacillus products have been around for many many years and are used not only by organic growers but by those who aren't reluctant to use chemicals. It's very effective.

Apply the product at the very first sign of caterpillar damage.....which will take some observation on your part. Think how small the caterpillars must be when they first hatch from those tiny eggs. That's when the Btk will be most effective. If you wait until the plant is half eaten, it's too late.

Btk will be readily available at any decent garden supply outlet or any online seller of organic pesticides. Be sure to read the label....you want Bacillus thuringiensis var. Kurstaki. There are other Bacillus strains available for the control of other pests.

One of the most common caterpillars to be destructive to petunias is the tobacco budworm but others can be culprits, too.

Good luck, I don't plant petunias anymore, but I know how quickly they can be munched down to nothing.