Answer: It sounds as though your plants are suffering the effects of the insecticide you applied. Many plants are sensitive to certain chemicals and, unless the label specifically lists the plants as being tolerant to applications of the product, it's not only risky, but also illegal, to use it on them. Also, it is extremely important to dilute pesticides according to the instructions on the label.
The pansies are probably beyond help, but because they grow from underground bulbs, there may be hope for the ranunculus. I'd cut the foliage back and wait to see if they produce new foliage. I'd also cut back the dead foliage on the dahlias and remove them from the garden. It's possible the tubers still have some stored energy and can produce new foliage.
Before using any pesticide, identify just what is damaging plants. (In this case, it sounds like you didn't see damage, just some bugs.) Try the simplest and least toxic pest control methods first -- for example, hosing the plants down with plain water next time you encounter insects on your flowers. Water won't have such dire consequences!
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