Answer: The most common pest of petunias is the budworm caterpillar. This pest is the larvae of a moth; the eggs are laid on the flower buds and when they hatch the larva gorges on the insides of the flower bud. These caterpillars are difficult to control but Bacillus thuringensis (B.T.) and Bifenthrin are reasonably effective insecticides. If left unchecked, the presence of the caterpillars will cause your petunias to stop blooming. Bt is a biological control and harmless to pets, humans and most other insects. I don't know about cactus oil - I've never heard of it being used as an insecticide. Instead, I'd go with the tried and true Bt (sold as Dipel).
Best wishes with your petunias!
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