Answer: Whiteflies are tenacious because they reproduce rapidly in warm temperatures. Outdoors, cold will eventually kill them off but indoor plants can be infested all year around. A strong blast of water from the hose often works, but if that's impractical, try a soapy water spray, using a hand held spritzer. Spray underneath leaves where they hang out. Do this daily if you notice insects.
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.
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.
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 is really no effective chemical spray for whiteflies but researchers are working on it. I'm afraid you'll just have to keep after the problem on a regular basis until you get them under control.
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