Answer: Without seeing the pest, it's difficult to suggest a control. It could have been most anything! It could have been lace bugs. If so, insecticidal soap will control them so scout your plants on a daily basis and spray the instant you see the little fellas.
Yellowing leaves on your impatiens could be a sign of over watering or a fungal disease. There are several fungal diseases that can attack impatiens. A wilt disease works its way up from the base of the plant, causing leaves and branches to wilt and die. A leaf spot disease causes circular brown spots on the leaves. Stem rots attack impatiens stems at or near the soil level. Foliage turns yellow, wilts, and dies.
There is no practical treatment for these problems in inexpensive annual plants which die at the end of the season anyway. The best strategy is to promptly remove and discard badly infected plants with their soil in the trash to prevent the spread of the fungus to other plants. Pinch off the yellowing leaves to remove them from the plant and try to keep water off the foliage when you water your impatiens.
Best wishes with your flowers!
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