Answer: The oils in insecticides, which make them stick, are what damage delicate flower petals. Petunias, malva, and other tissue-thin petals cannot stand up to the sprays. You can try deadheading the plants prior to spraying (new flowers will be unaffected), or you can simply avoid spraying where your petunias and other susceptible plants are growing. The problem with broad spectrum insecticides is that they kill the good guys along with the bad. So butterflies, bees, wasps, and predacious ground beetles - the helpers in the garden - will be eliminated along with the pesty insects. Instead of broad spectrum insecticides, you might want to consider using pest-specific products, such as insecticidal soap against aphids, etc.
Best wishes with your garden!
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