Answer: In my experience, it is unusual to find caterpillars munching on petunias, so you might want to verify which type of caterpillar it is before you decide if you need to do any thing -- it might be a larval stage butterfly! (A good basic book with photos is the Color Handbook of Garden Insects by Anna Carr, ISBN 0-87857-460-3.)
However, since they are destroying your plant, you might be able to hand pick them. Since it sounds like the infestation is quite severe, you might also wish to fight them with Bacillus Thuringiensis (Bt.) used according to the label instructions. When using Bt, be aware that it can take a few days to see results -- it is effective but not until the pests ingest some.
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