Answer: It's difficult to recommend a control without knowing what the insect is. Also, spraying a chemical that may be inapproriate for that insect can do more harm than good, killing off beneficial insects that may control the bad ones. I suggest that you take some samples of the insect and any damage that they are doing to the plant to a local nursery for diagnosis.
Your description sounds like it could be whiteflies, which are tiny, often on the undersides of leaves. If you tap the plant, they will rise up in a "flurry" of white. I always start with the simplest method first, and if that isn't successful, move on from there. A strong blast of water from the hose often works. Spray underneath leaves where they hang out. Do this daily if you notice insects.
Whiteflies are attracted to the color yellow. You can purchase or make yellow "sticky" traps from yellow cardboard smeared with petroleum jelly. They fly to it and get stuck.
Soapy water sprays are another possibility. Use 1 teaspoon to 2 tablespoons of liquid detergent soap per gallon of water. Use regular, not concentrated soap. Don't use soaps with lemon, as the citric acid can burn plants. Start with the lower amount and work up as needed. Spray as often as needed. As with any spray you might wish to test it on a few leaves first before you treat all your plants. Spray early in the morning before the sun heats up. Next on my list would be an insecticidal soap spray.
The insecticidal soaps are made from plant-derived fatty acids and target soft-bodied insects. There's really no way you can target the bad guys without fallout on the good guys. If you can regularly monitor and tolerate some damage to your plants, over time Mother Nature strikes a balance, with the beneficials coming in to control the bad guys. Healthy vigorous plants will withstand insect attacks best, and it's really the best thing you can do to prevent insect problems. I hope this information helps!
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