Answer: You didn't indicate whether the whitefly infestation was on indoor plants, in a greenhouse, or outdoors. If they're feeding on plants out in the garden, you can discourage them by hosing off your plants, making sure to hit both the top and the undersides of leaves. There are insecticidal soaps that you can spray, but when treating with insecticides remember that you may be killing benefical predacious insects as well as the pesty ones. You can make traps to use, both indoors and out, by taking yellow paper or cardboard (or spray-painting index cards), smearing petroleum jelly on the yellow surface, and suspending the traps among the infested plants. The yellow color attracts the insects and the sticky surface captures them. Give it a try.
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